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TerryE

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  1. TerryE
    In my topic Modelling the "Chunk" Heating of a Passive Slab, I discussed how I used a heat flow model to predict how my MBC WarmSlab heated by UFH + Willis heater would perform.  What I wanted to do in this post is to provide a “6 years on” retrospective of how the house and slab have performed as built based on actual data that I’ve logged during this period, and to provide some general conclusions.

    In this, I assumed 15 mm UFH pipework, but we actually used 16mm PEX-Al-PEX pipework with an internal diameter of ~13mm.  At a nominal flow rate of 1 m/s, say, my three pipe loops in parallel have an aggregate flow rate of 0.4l/s or 1.4 m³/hr.  At this flow, a 3kW (2.88 kW measured) heater will raise this stream temperature by 1.7 °C.  However, when I commissioned the system, I found setting the Gunfoss manifold pump at a high setting (roughly equivalent to this flow rate) gave a very noticeable circulation noise in the adjacent toilet, so I tried the pump on its lower settings and found that the flow was almost inaudible on lowest one with in to return delta at the manifold still only about 5°C, so I stayed with this.  The actual as measured delta for two loops of 4.9°C and the third slightly shorter loop of 4.1°C (close enough not to bother balancing the flows out).  This corresponds to an actual flow nearer to 0.4 m/s or 0.56 m³/hr by volume.  When scaled to adjust for this lower flow rate, the actual measured temperature profiles are pretty close to those modelled.

    I measured the actual Willis heater’s heat input as 2.88kW.  In analysing the actual slab heating rates, I found that this raises the overall slab temperature by some 0.45 °C / hour after the initial start up.  Plugging typical specific heat and density figures for the concrete, this is empirically equivalent to heating 25 tonne of concrete (Cmass = Ewillis/ΔT/SIconcrete = 2.88*3600/0.45/0.9 kg), or 10.6 m³ concrete by volume (23000/2400 m³). 

    In the case where the Willis provides heating for the full 7 hour off-peak window (just over 20 kWh), at the end of this heating period the flow input to the slab is +9 °C above the initial slab temperature and the flow return is +4.4 °C.  The temperature of the concrete immediately in contact with the pipe will follow this same gradient, with this temperature excess decaying radially away from the pipe centres.  By the end of this heating window at the slab surface, there is barely a noticeable difference in the measured temperature of the floor above the out and return UFH pipe runs (perhaps 1°C).  These temperatures and gradients are also comfortably within the reinforced concrete’s design parameters.  As soon as the Willis is turned off, the internal temperature gradients start to flatten and any unevenness redistributed across the slab; the rebar reinforcing has a thermal conductivity 60 × that of concrete and this accelerates this, so that within an hour or so of the heating turning off, the overall slab is left about 3.1 °C warmer than at the heating start time (actually about 10% less than this, as the slab has already started to dump heat into airspace). 

    In my original modelling topic, I mentioned that my passive slab has ~73m² of concrete 0.1m thick (~ 17½ tonne of concrete with another ~10 tonne of perimeter beams, cross bracing and steel rebar, with the UFH runs laid in 3 × ~100m long standard “doubled back” spirals (common to most UFH designs) on ~150mm centres and roughly 50 mm below the slab surface.  (Actually only 75% of the slab is covered by the UFH runs, because of the need to avoid proximity to ring beams, partition walls, areas under fitted cupboard areas, etc..) Nonetheless, this empirical 25 tonne figure is still consistent with the total volumetric 27½ total estimate if we assume that the rebar is effective at spreading heat through the wider slab over this multiple hour timescale.

    In conclusion, based on this modelling and observation:
    First recall our context: our house is near passive in class with a lot of internal specific heat capacity.  We only need about 1kW overall heater input in the coldest winter months to maintain overall heat balance, e.g. either by a resistive heater such as a Willis or an ASHP.
      IMO, there are two extreme approaches to house heating: (i) “agile” tracking of occupancy patterns so the living spaces are only heated when and where occupied; (ii) a 24×7 constant comfortable temperature everywhere within the living space.  Our warm slab design is very much optimised for this second case, and our slab supplier did a good job in designing an UFH layout to match the slab characteristics to this  The slab is covered in “doubled back” spirals with each loop using up a full 100m roll spaced on roughly 150 - 200 centres (and avoiding partition walls and cupboarded areas) so that each heats roughly 15 - 20 m² slab.  In our case three loops were enough, and there was no advantage in trying to squeeze in a fourth.  Our 3 loops will happily take up 3 kW heat input.  Circulation speeds between ⅓ - 1 m/s seem to work well, with the only real difference being the slower the flow speed, the higher the delta between in and return temperatures. The slab does just as its trade name suggests: it can be treated as a huge low temperature thermal store, but because of its extremely  high thermal inertia, one that is not rapidly responsive to heat input.  In our case, a heat input of 3 kW input will only raise the slab temperature by 1°C over a couple of hours, and radiating 1kW will drop the slab by only 1°C over roughly six hours.
      In a true passive class house, one key to heating economy is the high level of thermal insulation coupled with a substantial internal heat capacity.  Trying to drive such a house in an agile manner is a fruitless exercise, so forget the traditional having room-specific thermostat control; forget having traditional time-of-day heat profiles.  It is far easier to treat all ground-floor rooms as a single thermal zone to be kept at a roughly constant temperature.
      In my view, using a resistive heating approach (such as a Wills heater) as well as an ASHP can both work well.  In this second case something like the 5kW Panasonic Aquarea ASHP would be a good fit as it uses a modulated inverter compressor so it can heat the slab directly without needing a buffer tank.  The choice is a trade-off between running costs vs. installation costs.  In our case, switching from a Willis to this type of ASHP would save me about £600 p.a, in electricity cost, so I would really need to do the install for a net £ 3-4K to make the investment case feasible.  However I would like to defer this discussion to a separate thread because there are other issues that such an approach would need to address.
  2. TerryE

    Downloading OVO Smart Meter Data

    By TerryE,


    Design Note
    The OVO portal does have a publicly available RESTful API, but because the UI makes heavy use of Javascript (JS ) scripts to do the webpage renders and these make JSON callback to the OVO server, so it is quite simple to use your favourite scripting language to automata downloading data and aggregating it into a database or equivalent.
     
    I have written down-loaders for Python, and NodeRED (based on node.js) but I currently only maintain the latter.  In essence your script will need to do a bunch of GETs which then return the JSON data that you need to parse, and one POST upfront to establish your login credentials. This script will also need to roll forward any session cookies as these are used to maintain the session context and authentication.  I retro-engineered the request chain using a browser and web-tools window (typically Cntl+-Shift+I).  (Hint: if you have two displays, then detach the debug window and move it to the second display, to keep things simple.  Also set the "Persist logs" option to be able to track everything. Below is the result of my latest analysis, which was a useful exercise since OVO have updated their framework.  My NodeRED flow looks like this and I've posted the JS code as a Gist: NodeRED Function to Download OVO Smart-meter Readings.

     
    The first two request are to https://my.ovoenergy.com to initiate the authorized session context.
    GET https://my.ovoenergy.com/login to initiate logon.  On a real browser this downloads bunch of JS scripts that display the logon page and process the "Log in" button.  These can all be ignores except the session cookies. POST https://my.ovoenergy.com/api/v2/auth/login. Post content is a JSON object with items; username, password, rememberme (boolean). Once authorized, the remaining request are https://smartpaymapi.ovoenergy.com.
    GET /first-login/api/bootstrap/v2/.  This JSON object lists off the accounts used by the user (typically one).  So either parse this or just use your known ACCOUNT.  For a standard log on, the welcome page also generates a whole load of non-usage related requests, but the key one is: GET /orex/api/contracts/ACCOUNT. This JSON object lists off details of your contracts both electric and gas.  Note that my smart meter is classed as ineligible for SMETS1 ot SMETS2 but is classed as "already smart"; I am not sure what this means but this could effect the format of the following requests for your analysis. You can now loop through requesting daily or half-hourly data:
    GET /usage/api/daily/ACCOUNT?date=YYYY-MM.  This returns a JSON object with electricity.data[i] containing the values for day i+1 of the given month.  The consumption cost, and rates can be used to calculate the standing charge, peak and off-peak use and cost. (If are up to solving a simultaneous equation. 🤣)  This will work for any ACCOUNT that you've logged onto with the session. GET /usage?datePeriod=daily&date=YYYY-MM-DD&unit=kwh.  This returns a JSON object with electricity.data[i] where i = 0..47 being the energy reading for that half hour slot.  Note that for can only get energy and not cost data on a half-hourly basis.  You can also use the next and prev parameters to chain fetches for a date range.  In this case, the account context is maintain in a session cookie rather than a request parameter.  I need to check whether this is set by the daily breakdown request. I assume that you can also do gas readings the same way.
  3. TerryE

    Heating the Slab – an overview

    By TerryE,


    Heating
    We have a passive-class house where the net heating requirement to keep the house warm in the coldest winter months is approximately 1kW.  The only heating system for doing this an underfloor heating (UFH) system base on 3 ~100m UFH loops buried in our passive slab.  That's it; no upper floor systems; no towel rails; nothing.  The reason for this is that our timber framed house is super insulated and air tight so there is very little temperature variation throughout the house, but that's all been covered in earlier posts.  What I want to do in this post is to provide a simple explanation of how I am going to heat my house and how this works so that John (@joe90) and other forum members understand my approach.
     
    This basic heating strategy was first evangelised by Jeremy Harris (@JSHarris), but variants have been adopted by other forum members and their consistent experience is that it works and works effectively for this class of passive house.  However, what I am doing is a slight variation on Jeremy's approach:
    I am using the slab itself as my main heat store, so no buffer tank. I will be heating it by circulating warm water through the UFH loops and this water will be heated by a simple small inline 3kW electrical heater element. The heating charge will normally be done as a "chunk" once per day during the E7 cheap rate period to take advantage of low tariff rates. However, I am also including in the design provision for the later addition of an ASHP, should the heating data collected over the first year show that there is a 10-year payback in doing this. As I said, Jeremy's approach has been well documented by him in his blog and by others.  He has recently described that his system settles down into a repeating pattern over the colder winter months winter where his heating comes on for a few hours once a day in the early morning, and the heat in the slab is topped up during this period.  This is broadly what I call "chunk heating": unlike a traditional house central heating system which is turning on and off pretty continually, the heat losses in our type of house are so small and the house has such a high thermal inertia that you can heat the it practically with a single daily top-up to the slab; this heat then "trickle feeds" into the house over the day.  Yes, there is a slight residual ripple on the temperature in the house, but this is less than a 1°C undulation over the entire day and so this isn't really perceptible to the occupants. 
     
    I am adopting this same approach, but shifting my heating period earlier so that it ends at the same time as the E7 low rate tariff ends.
     
    The main difference in my implementation is that I am heating the slab directly without a buffer tank. I wanted to get my head around this before committing to this decision,  so I modelled this in some detail and covered all of this physics and modelling stuff in my Boffin's corner thread.  This modelling has persuaded me that the mechanisms and dynamics of heating are pretty simple, and so in this post I want to cut out all of the equations and stuff (with one exception) and focus on describing what happens in plain terms.
     
    First, I am using a small 3 kW electric element to heat the water circulating in the UFH loops (the same type is used as a hot tank immersion heating element).  Just like an electric shower this heats the water stream a step in temperature. Sorry I am a boffin, so I will call this temperature change ∆T.  (BTW, the triangle is just the Greek letter D and is short for difference; blame Isaac Newton for that one.)  Just like an electric shower, double the power and ∆T doubles; double the flow rate and ∆T halves, and if I do the sums for a typical flow throw my UFH loops, and for a 3 kW heater then ∆T works out at about 1.6°C for my system -- a lot less than a typical gas-boiler fed UFH installation, but my heater is puny in comparison.
     
    So if I start pumping 3kW of heat into my slab, then the system settles down after about 10mins and the heat output is pretty much the same along the entire  3 × 100m runs of UFH pipe, pipe work, that is each 1m of pipe dumps about 10W of heat into the concrete.  This lifts the temperature of the concrete, and at the same time cools the water in the pipe pretty steadily along its length so it comes out at 1.6°C cooler than it went in.  But cooler or hotter than what?
     
    The heat flows radially away from the UFH pipe creating a thermal gradient. [Boffin bit warning, and the only one] this gradient is pretty close to what is known as the steady state radial solution to the 1-D heat equation, which has a formula Tr = Tp - A.log(r/rp). where T is the temperature and r is the distance from the pipe centre, with the p subscript relating to the pipe/concrete interface. The A term is a function of the amount of heat flow.
     
    The main thing to note here is the general shape of this gradient: the temperature of the water ends up roughly 4-5°C hotter than the slab average for this sort of 10W/m value, and the temperature in the concrete falls away rapidly as you moving away from the pipe towards the average slab temperature.  Since the volume of concrete goes as r2, the actual proportion of the concrete more than 1°C hotter than slab average temperature is small.  So the overall effect of the heating is to slowly lift the average slab temperature.  There is also a general heat gradient along the water in the pipe but once you get more than a few cm from the pipe centre the concrete is all within 1°C or so of the slab average.  There are also local hot regions around the UFH pipes up to 5°C or so hotter than the overall average slab temperature.  However, this is factors less than you will get with a conventional UFH system.
     
    A key difference of Jeremy's approach is that the water continues to recirculate after the heating is turned off, and now the water flow acts to redistribute the heat rapidly along the pipe levelling the previous 1.6°C gradient; at the same time (without the heat being pumped from the UFH pipe) this central warmer region rapidly flattens out as the heat flows outward, and within an hour or so hardly any heat variation remains and the entire slab is within ½°C of the slab average temperature.  A good analogy here is pouring water into a bucket: the surface level steadily rises as you pour it in and the surface itself is a bit churned up by the act of pouring, but as soon as you stop pouring, it rapidly levels out to flat surface.
     
    OK in a real slab this is also complicated by the deep elements (the unheated ring beams in my slab are over a third of the total volume) and the heat does flow into these largely thanks to the high thermal conductivity of the rebar.  But overall, the slab is acting as a heat battery soaking up the power that you pump in.  The trick is not to put a somewhat arbitrary limit of the maximum input water temperature (say 25°C) as this will limit the amount of power that you can apply.  This heat gets quickly spread uniformly throughout the slab.
     
    By the end of the heating period, the slab is 2°C (or whatever) warmer than the room temperature, and is starting to transfer heat into the room fabric at ~15W/m² whilst itself slowly cooling.  This is more than the external heat losses in the house, so this heat both warms the air and the rest of the wall fabric.  This creates a very slow rise and fall in the room temperature over the course of the day -- of roughly 1°C.  But so long as you put in enough heat each night, the overall house temperature remains stable.
     
    So how much is "enough" heat?  In my case I use a very simple strategy.  I am using the UFH circulation temperature at midnight as my test.  If it is less than the previous night, then I add a bit more heat than last nigh and v.v.  Simple really.
  4. TerryE
    As I've previously discussed we have an MBC Passive Slab and Timber-frame, but unlike most builds, our house also has a very traditional stone cottage-style exterior because the new build sits between our current farmhouse, which dates back over 400 years and a cottage which dates back approaching 200 years, so our planners required that we use the same local quarried stone.  So a topic that often comes up is "how do we do the window / door treatment on a timber-framed house with an exterior stone / brick / blockwork skin?"  In this blog entry I want to describe how we approached and addressed these issues on our build. Whilst I make no claims about our approach being the only or the best one, Jan and I do believe that this has worked well for us; we are pleased with how it has all turned out and we don't think that we would do it differently if we were doing this all again.  So if you are in a similar situation to us, please consider this as one possible approach.
     
    There are a number of issues that we considered in designing our detailing:
    Decoupling the inner and outer skins.  In order to achieve thermal isolation of the inner passive slab, MBC also lays a separate outer ring beam for blockwork, brick and stone skinned houses.  The inner slab carries the Larson trusses of the MB twinwall frame, and the outer ring beam carries the stone skin. The inner frame is CLS; the outer stone and mortar, and these two have different expansion characteristics so you should anticipate up to 5mm, say, differential movement between the inner and outer skins.  So we decided that we should not use the window and door furniture to couple these.  Closing the gap. Even so, we still have the issue of the 50mm nominal air gap between the inner and outer skins and how we close this for weather protection and cosmetics.  Our solution to thee two points is to move the front of the windows some 45mm forward of the outer surface of the frame. The stonework then sits immediately in front of this,overlapping the window frame by some 30-40mm. Fixing the windows and doors. We have Internorm KF200 Aluclad PVC windows and I agreed a fitting profile with both MBC and ecoHaus SW who supplied the windows.  This comprised a box section (something like marine ply would do here) that framed each window opening at the top and sides as follows. there was a 10mm filling gap for fixing the windows there was a 15mm filling gap at the top ditto the windows had to sit hard at the bottom, but I inserted a 44 × 38 tanalised carrier to lift the base above the internal frame base.  This was to give adequate clearance to fit the internal cills. Protecting the windows during the build.  EcoHaus SW fitted the windows on day 8 of the the frame erection, so by day 9 we had a completely weather-tight and lockable house. The windows had to be in place before erecting the stone skin, and so needed protection from the stone erection process.  The solution that we agreed with the ecoHaus technical manager was very simple and extremely effective and one that I would suggest to anyone else doing this.  We simply covered the windows in heavy grade clear building polythene, and this served a dual purpose: It provided total protection against the muck and dust of stone erection. You need a slip surface between the aluminium cladding and the stone skin.  (Cf. the first point)  The PVC does this.  Once the stone skin was complete we simply cut around the PVC on the mortar line.  All that is then needed to achieve a total weather seal is to run a thin bead of sealer at the join.  Minimising any bridging impact.  The windows have fire-break socks around them which acts both as insulation and a gap closer.  The doors require special treatment.  Here prior to slab pour, we had the MBC team cut out 50mm deep slots at the door openings and we placed extra shuttering in to extend these out by some 40mm in front of the outer frame line.  These were rebarred and when the slab was poured, these became a 50mm deep concrete tongue that extends out to the front face of the door opening.  The doors then sit on a 30mm upstand on these tongues. The upstand acts as a thermal break, but to minimise any bridging through the tongue itself, we used FoamGlass structural bricks to isolate the tongue from the outer cill and the stone skin. If you do the 2D thermal calcs (or at least I did), the thermal capacity of the stone face overlapping the face of the windows materially mitigates the extremes of the temperature variation, and whilst there is a little uplift in the Psi-factors for the window, in absolute terms this equates to adding an extra ½m2 of glass to the house overall, and not enough to cause condensation risks   Maximising internal light. Our old farmhouse has thick stone walls with window reveals and these work well.  So we decided to ask MBC to do a similar treatment in our new build.  In short not only do they work, they work brilliantly. They let in perhaps 10-15% more light than deep squared frames and they help open out the rooms.  They are an extremely attractive feature and both Jan and I would recommend them to anyone considering using a twinwall frame.  
    Here is a picture of the slab during the pour. Note the trays for the kitchen French windows and the back door.

     
    Here are a couple diagram extracts showing the window treatment and detailing:
     


     
    and some photos of the wall in construction showing the set forward windows and the finished effect (less the porch that still has to go in.).
     

     
    and an internal shot of the kitchen window detail showing the angled reveals:

     
  5. TerryE
    When we first decided to self-build in 2014, Jan and I visited quite a few passive house builds and talked to various experts;  we soon decided that a low energy approach was broadly the way to go for our build.  One of these experts, a passive-house evangelist called Seamus O'Loughlin, emphasised that a conventional heating approach (where boiler demand is based on some central thermostat set point) doesn't work well in a passive house, because the time constants of a high-thermal capacity low energy house are a couple of orders of magnitude longer than those anticipated by conventional CH control systems.  
     
    At the time this seemed a controversial assertion, but because I have done some mathematical modelling professionally, I was able to and decided to do some time-dependent heat-flow modelling and control strategy simulation of how our designed house would behave and this very much supported this assertion.  I have already covered a lot of detail of my CH approach in previous posts and discussions, but it’s probably worth summarising some key headlines to set the context for my changes to our heating strategy: 
    We were cash-flow limited during the build phase, so had to make various cost-benefit trade-offs on our build, like most members here. I based these on a general net 10-15 year payback, and it was clear that we wouldn’t be able to achieve a true zero-input passive house largely because of design compromises owing to planning restrictions and our plot size and orientation. However, we would be able to build a low-energy house that would need generally low levels of supplemental heating for maybe 6 months a year, with overall heat losses an order of magnitude less than a conventional build, and the thermal capacity of the heated fabric be many factors more.   We decided to go all electric in the house with wet UFH embedded in the ground floor slab only.  Cost benefit trade-offs didn’t even support installing an ASHP, though I did future proof the installation to simply the later addition of one if the cost numbers changed. I decided to adopt a simple but unconventional strategy for heating the house: calculate the total heating requirement for the coming day daily at midnight; this is based on actual averages for energy use, average house temperature and forecast average external temperature for the coming 24 hrs. This allows me to dump as much of this heat into the house fabric as practical at the cheapest electricity rate, and for us this is in the 7 hour overnight off-peak window on our E7 tariff. We used to get some spill-over into peak rate top-up in the coldest months, but a year ago I added an oil-filled electric radiator on my 1st floor landing, and one in my son’s 2nd floor bedsit controlled by my Home Automation System, with these scheduled to come on in the overnight E7 window to dump extra heat in the upper floors.  This simple addition reduced the thermal layering from ground to second floor, and almost  eliminated  the need for daytime slab top-up. In practice we have roughly a 1°C daily ripple on overall winter house temperature. Because using a daily forecast computation does have some intrinsic prediction error, this can add typically less than  0.2°C day-to-day ripple on top, but any longer term drift can be corrected by the daily feedback. I have RPi3B running NodeRED attached to some digital thermometers and 4 GPIO controlled solid-state relays (SSRs) to control the time of the UFH pump and Willis heater, plus the 2 × SunAmps for DHW.  This was very cheap to implement, and basically has no monthly or annual maintenance.  With the current Electricity price hikes, we have decided:
    To trim our house temperature set-point back from 22.3°C down to 21°C   To hard limit automatic heating of the slab to the cheaper 7-hour off-peak window.  (We can still do peak by request in one hour chunks if we want to.) To use electric oil-filled radiators overnight to do any additional top-up.  I can automate this through my Home Assistant (HA) that runs on a separate RPi4 and do this using MQTT via WiFi connected powered/metered sockets. This strategy currently limits heat into the house to: 
    ~21 kWh through the slab and  ~7 kWh through the two radiators. 28 kWh is enough to maintain overall house temperatures so long as the external temperature is at ~7 °C or higher, and it clearly isn’t the case at the time of posting.  The house needs about 2½ kWh/K, so with the average daily external temperature at zero today this is 17½ kWh too little to maintain house temperatures.  The long term Dec / Jan average where we live is about 4°C, so to maintain temperatures in this case we would need an extra 7½ kWh/day. (This last year, we had 26 days where the average external temperature was 4°C or below and only 2 where  temperature was below zero or below.)
     
    So what happens when we underheat our house?  Simple: it slowly cools down, and very slowly.  For example, in the last 5 days of cold-spell, capping the heating has dropped the average house temperature from 22.3 down to 21.3°C, and given an average of -1°C for today, it will be down to our new target of 21°C by tomorrow .  At this point I will need  to add more heat or to accept that the house temperature will fall further.  I will definitely need to add another 7kWh or so extra radiative capacity for overnight topup.  We will play it by ear over the next week or so.  I can either accept that I will be paying £0.38/kWh for extra peak period top-up during these really cold spells, or let the average temperature fall a little further if we find it comfortable enough (wear a thicker jumper, etc.)
     
    This approach works well for us because our house is so insulated and it has a huge amount of thermal capacity within the heated envelope.  If we accept a small heating ripple then it really doesn’t matter that much when we heat within the day and so we can time-shift our demand to make use of the best tariff rates: currently over 85% of our electricity use is at the off-peak cheap-rate price. This latest exercise of clamping the heat output to 28 kWh when the maintain level is closer to 40 kWh underlines that the heat budget for and given day can be off by 30% or so and the net temperature drift is still on 0.1 °C or so; the time constants of the system are of the order of a week rather than days or hours.
     
    By way of a contrast my daughter lives in a pretty large but conventional 1990s house.  When her heating goes off in the evening, the living room temperature drops maybe 4-5°C within an hour.
     
  6. TerryE
    As I have discussed on earlier podcasts and various topics, I have a Willis-based configuration for heating our low energy house, and control is implemented with a dedicated Raspberry Pi using a custom NodeRED application for our underfloor heating and SunAMP-based hot water.  This system logs a lot of instrumentation temperatures every half hour and also any significant events such as turning on and off pumps and the heater.
     
    Our electricity supplier has been OVO for the last 4 years, and because we have a smart meter, the control application also includes a script to log on to the OVO Portal and download the daily usage data into the MySQL database.
     
    Because these latest energy hikes, we have decided to revisit the issue of whether it would now be cost-effective to install an ASHP in order to save on monthly electricity costs for heating.  Because I have been logging all relevant data for the past 4 years, I can base this decision on hard actuals rather than some generic planning assumptions. The next two graphs summarise these results.  The first is an analysis of our daily energy use (we have an electricity only house).  What I have done here is to aggregate the 4 years of data by calendar month and split these into three categories: 
    Underfloor Heating (34%  or ~4,000 kWh/yr).  In practice, we only heat off-peak and use the thermal mass of the floor slab and the house itself to smooth out the overall background heat levels.  As I have discussed in other topics, this results in a temperature ripple of about 1°C which is quite acceptable given the reduction in overall all heating costs.
      Other Off-peak use (25% or ~2,900 kWh/yr).  We also use a couple of small oil-filled electric heaters on the first and second floors for the 4 cold winter months.  These output roughly 1 kWh and run on a timer (actually controlled by my home automation system).  We find that 3 or 4 hours is typically enough to keep the upstairs acceptably warm in the coldest month; this also means that the UFH on-time doesn't need to run over into peak periods. Our resistive load white goods (the washing machine, dishwasher, SunAmp DHW) are timed to come on in the off-peak period. 
      General Peak Rate use (39% or ~4,500 kWh/yr). Pretty much all of our baseload and direct hands-on devices: fridges, freezers, cooking, computers lighting, etc. Note that the 2 retired (out of the 3) occupants of the house spend most of May, June, September, October abroad; hence the dip in this general use figure.  I find the annual variation on this base load a little intriguing ,and I am not sure why it is so high.  Our live-in son often has his radiator on in the evenings when he's at home, and we do spend more time indoors in the cold dark months.

    The simplest ASHP implementation would be for slab heating only and would give a CoP of ~4 (as the circulation temperature is under 35°C) hence saving perhaps 3 mWh p.a. @ 18.86p/kWh or roughly ~£560 p.a. at our currently quoted OVO night rate.  Given that we would need to use an MCS certified installer to exploit a permitted development waiver, I would expect our installation to be £10K or higher, so I still don't have a viable cost benefit case to go this route.  Yes, adding pre-heat for the SunAmps would increase this annual saving, but this would complicate the installation, and given our volume of DHW use this would in fact worsen the cost benefit case rather than improve it.
     
    Another interesting point is raised by the following graph which I pulled from a 2014 Thermal Design post. The bottom line is that thanks to entropy, pretty much all of the electrical energy that we use ultimately ends up as heat within the fabric and airspace of the house. Given this, the overall heat losses (if you take December for example) are pretty much double what we originally estimated.  The following can account for the majority of variance, but not all.
    We had to drop the U-value for the warm roof to minimise ridgeline heights keep the planners happy We added 60° reveals to our fenestration to improve overall light levels given the planners putting hard limits on our window sizes, and these some limited thermal bridging Winter solar gain is almost non-existent for our window configurations. As discussed in an earlier post, we had a cock-up in our slab design which created a thermal bridge between the inner ring beam (this supports the frame) and the outer ring beam (supporting the stone skin).  We could only partially mitigate this during slab pour. We estimated that MVHR would have a recovery efficiency of around 90%, but looking at the inlet temp vs room, I estimate the actual recovery is nearer to 80%, that is double the heat loss. We run the internal room temperatures a couple of degrees warmer than initially planned. However the house is built and well established so getting any convergence is now unlikely.  So the house performs as a low-energy one, rather than a true zero-energy one.  And we still only put ~20kWh into our slab in the coldest months.

     
  7. TerryE
    We moved into our new build mid-December 2017 in time to host an extended family Christmas.  We are now over 4 years into living in our new home.  We have lots of accumulated experience and made a few small tweaks.  However, we are delighted about how the house has turned out, and we love living here.  There were no material cock-ups, or regrets on design decisions, so we have probably fared a lot better than most new purchasers or self-builders.  Maybe a general experiences post should be on the to do list, but what I want to focus on here, and a couple of follow-ups, is a general topic that others on the forum have asked about over the years: that is how our central heating system works in practice, and how I control it.
     
    The system as currently implemented is still largely the same as when I first commissioned it, that is a now 5 year-old RPi-based custom control system directly controlling the CH and DHW subsystems.  This is a pretty minimal headless system running Node-RED, MySQL and MQTT client for control.
     
    The three material changes that I've made since moving in are:
    I have followed my son and son-in-law in using Home Assistant (HA) for general Home Automation.  My server (an RPi4 in an Argon One case) uses an attached Zigbee gateway, and I have a lot of Zigbee devices around the house: switches, relays, light sensors, etc. and I do the typical home automation stuff with these.  There are loads of YouTube videos and web articles covering how to implement HA, so please refer to these if you want to learn more.  My HA installation includes an MQTT service for use as a connection hub for these IoT devices.  I also have another RPi4 acting as an Internet-connected portal / Wireguard gateway/ file-server for caching video snippets from my PoE security cameras.  Note that none of my IoT devices directly access the internet, and the only in-bound access into my LAN is via Wireguard tunnelled VPN, and my HTTPS-only blog. All other ports are blocked at the router.
      Before moving in, we assumed a target internal temperature of 20°C.  In practice, we have found this too cold for our (fairly inactive OAP) preference and so we have settled on a minimum control threshold of 22.3°C.  As you will see below, because we largely heat during the E7 off-peak window the actual room temperatures have a ~1°C cycle over the day, so the average temperature is about 22.8°C.  This hike of 2.8°C increases the number of net heating days since my design heating calcs and the increased delta against external temperatures in turn increases our forecast heating requirement by roughly 18% over our initial 2017 heating estimate.
      Because our UFH is only in the ground-floor slab, we found that our upper floors were typically 1-2°C cooler than the ground floor in the winter months. We also need more than the 7 off-peak hours of heating in the coldest months, so I have added an electric oil-filled radiator on our 1st-floor landing; HA controls this through a Zigbee smart plug that also reports back on actual energy drawn during the on-time.  HA uses MQTT to pass the actual daily energy draw back to the CH control system.  This radiator provides enough upper-floor top-up heat, and does so using cheap rate electricity.   Note that all servers are directly connected to my Ethernet switch, and the CH/DHW system has all of its critical sensors and output controls directly attached.  It can continue to control the CH and DHW subsystems even if the HA system or Internet is offline.  There is also no direct user interface to the system, with all logging data is exported to MQTT, and key CH/CHW set-points and configuration are imported likewise. This integration with MQTT, enables user interfacing to be done through the HA Lovelace interface.  If there is sufficient interest I can do follow-up posts on some more of the "Boffins Corner" type details on these implementations, or if this turns out to be more of a discussion then it might be better to move this stuff to its own BC topic.  However, for the rest of this post I want to focus on the algorithmic and control aspects of the heating system.
     
    In terms of inputs and outputs to the control system, these are:
    There are ~20 DS18B20 1-Wire attached digital thermometers used to instrument pretty much all aspects of the CH / DHW systems.  Few are actively used in the control algorithms but were rather added for initial commission, design verification and health checking.  Some are also used to monitor and to trip alarms; for example, there is a temperature sensor on the out and return feed for each UFH pipe loop.  These were used to do the initial zone balancing. However, the average of the return feeds is used as a good estimate of the aggregate slab temperature.  One of the temperature sensors is also embedded in the central hall stud wall to act as a measure of average internal house temperature.
      There are two flow sensors on the cold feed to my 2 SunAmp DHW storage units to monitor DHW use and to help automate during-day DHW boost.
      There are 4 240V/20A SSRs used to switch the power to my (2-off) SunAmps, my (1-off) Willis heater, and my (1-off) circulation pump. These and the rest of my 240V household system were wired up and Part P certified by my electrician.  These SSRs are switched by a 5V 50mA digital input, and so can be driven from an RPi or similar. (I used a I²C attached MCP23008A multi-port driver to do this, as this can drive 5V 50mA digital inputs, but its input I²C side is compatible with RPi GPIO specs.) There are many ways to "skin this cat", but whichever you choose for your control implementation your system will need to control some 240V/12A devices and take some input temperature sensors. My preference was to directly attach all such critical sensors and outputs.
     
    My heating algorithm calculates a daily heating budget in kWh (each midnight) as a simple linear function of the delta between average local forecast temperature for the next 24 hrs and the average hall temperature for the previous 24 hrs.  This budget is then adjusted by the following to give an overall daily target which is converted in minutes of Willis on time.
    heat input from the heater mentioned above.
      a simple linear function of the delta average hall temperature and the target set-point (currently 22.3°C). This is a feedback term to compensate for systematic over or under heating. I initially calculated the 4 coefficients of the two functions using my design heating calcs and an estimate of the thermal capacity of the interior house fabric within the warm space. After collecting the first year's actual day, I then did a regression fit based on logged actual data to replace the design estimates by empirical values.  This was about a 10% adjustment, but to be quite honest the initial values gave quite stable control because of the feedback compensation.
     
    The control system runs in one of three modes:
    No heating is required. Up to 420 mins of heating is required. The start time is set so that heating ends at 7 AM, and the slab is continuously heated during this window. More than 420 mins of heating is required.  420 mins of heating is carried out in the off-peak window.  On each hour from 8 AM to 10 PM, if the hall temperature is below the set-point (22.3°C), then an N-minute heating boost is applied, where N is calculated by dividing the surplus heating into the 1-hour heating slots remaining.  
      
     
    Here are two history outputs from HA showing some of the logged results.  The LH graph is the slab temperature over the last 7 days.  The general saw-tooth is identical from my 3-D heat flow modelling discussed in my earlier topic, Modelling the "Chunk" Heating of a Passive Slab.  The 7 hr off-peak heating raises overall slab temperature by ~4-5 °C; well within UFH design tolerances, and no need for any HW buffer tank: the slab is the buffer. The RH graph is the hall temperature.  Note the days where on-hour boosts were needed.  (Also note that the CH system only updates the MQTT temperature data half-hourly, hence the stepped curves.)
     
    So the approach is fairly simple, and the system works robustly. ?  And here is a screenshot of my HA summary interface, which gives Jan the ability to control everything she needs from her mobile phone or tablet.
     

  8. TerryE
    We use 2 × SunAmp PVs for our HW system in a household of 3 people.
    According to our water bills, our consumption is about 83 ltr per person per day. Our pattern of use is pretty even across the year: more showers in the summer; an occasional shared bath in the winter.
      The year round average temperature of our rising main is 11.3 °C (Oh, the wonders of logging everything in a DB and knowing how to do SQL subqueries).
      The H/W manifold is mixed to 53°C (perhaps a little too hot for kiddies but we are an adult household).
      I estimate that ~40% of our water is run as hot. (The washing machine and dishwasher, bogs, etc. are cold fill.)
      Cranking these number into a heat calculator, this gives a total heating requirement of just under 5 kWh / day + another 1 kWh / day heat loss as the SunAmps are tight side-by-side and amazingly insulated.  (I don't separately meter the SunAmps, but a quick sanity check of my actual half-hourly electricity meter readings would indicate this figure is about 20% too high, but let us stick with this figure for estimating purposes. 
      All heating is done at cheap rate tariff ( fixed at 9.66 p / kWh inc VAT) so this costs us ~ £211 p.a.
      Using an ASHP to supply the SunAmps at 40 °C, say,  would drop this to 2.5 + 1 kWh saving us less than £100 p.a. or about £1K over 10 years. So in our case if  we decide to install an ASHP, there aren't enough savings to make it worth installing an extra pump,  a buffer tank and a two temperature ASHP to use it to (part) heat the DHW. 
     
    We will stick to Keep It Simple Stupid.
     
    A couple of caveats here: 
    I think our pattern of water use would be very different with children in the household. We have a fixed price deal until end 2022.  We are going to see a big hike in our next tariff, but I feel that this will settle down in the longer term, so I am ignoring this for now.
  9. TerryE
    Janet and I want an energy efficient house, but what does that mean in practice? The whole concept is still largely rejected by the UK building industry. In our initial research, we either found books like the House Builders Bible which are good but superficial introductions on the concepts but without serious detail or at the other extreme academic papers on micro details. There is precious little in between, and to be honest we have found far more gems of knowledge in this site. All my experience and intuition concerns living in a traditionally built house. An energy-efficient house is just a different beast entirely, so I discarded my intuition and put my trust in the physics, maths and engineering. Likewise, we only considered the views and recommendations of those who have actually lived in this type of house.

    One of the first things that I did was to build up my own version of Jeremy Harris's Heat Loss Calculator.xls (which he first developed on this GBF topic). I plugged in the numbers for our own house design, but in reality there are only a few parameters derived directly from the house geometry that drive this calculation (my numbers are in brackets):
    The internal footprint of the slab (71m²) The internal surface area of the external walls, less windows (179m²) The internal surface area of the roof, less roof windows (93m²) The area of windows (23m²) The total volume of the internal living spaces (419 m³). Each of these is multiplied by a factor derived from the design to give a heat loss per °C: a U value in the case of the first four. The last is more complex in that I had to build up a composite heat-loss based on the rate of air exchange, its unit mass and specific heat, and importantly the recovery efficiency of the MVHR.

    For a typical winter external temperature of 4°c and an internal 21°c, this gives a delta of 17°c for fabric heat losses. The slab delta is somewhat different in that the ground temperature under slab is far more constant – say 10°c at the centre of the slab and maybe 6°c at the edges in the winter raising to 15°c in the summer. Plugging in our current design values (from our frame supplier, MBC) gives the following heat loses for a typical January day:
    Slab: 97 W (9%) – 71 m² x 0.105 W/m²/K x 13° Walls: 364 W (32%) – 179 m² x 0.120 W/m²/K x 17° Roof: 171 W (15%) – 93 m² x 0.105 W/m²/K x 17° Windows: 313 W (28%) – 23 m² x 0.800 W/m²/K x 17° Air change: 180 W (16%) – 419 m³ x 0.025 W/m³/K x 17° That's 1.1 kW in total, or as I sometimes say to friends, the whole house could be heated by a single 1-bar fire. Clearly this heat loss varies according to season, so if I plug in an overall temperature profile for my location, I then get the following daily heat losses in kWhr :

    The house is reasonably balanced as a system: no single component dominates the heat losses. However, this wouldn't be the case if we dropped the heat recovery element of the MVHR, for example, as this would increase the air change heat losses by roughly 5x, becoming the majority of total heat loss. This is why the inclusion of MVHR is such an important component of energy-efficient design.

    If we look at the risks and sensitivities in this sort of calculation, then they broadly fall into two categories: failures in airtightness and thermal bridging at boundaries. (Googling these highlighted terms will give background explanations of what these are). All internal surfaces in a properly implemented energy-efficient house are within a few degrees of the internal temperature, which also means that there are no internal condensation surfaces. A serious consequence of thermal bridging is that surface temperatures can drop significantly at the bridges below the internal dew-point, causing surface condensation.

    Failures can occur with sloppy design or poor attention to detail during construction, so I believe that it is a lot more important to find a frame manufacturer / assembler who gives us confidence that these issues will be effectively addressed (as failures here could cost kilowatts of heat loss or mouldy surfaces) rather than whether the walls have a nominal U value of 0.12 or 0.14 (which varies the total heat loss by roughly 60W at most). However, this heat calculation isn't the whole story:
    We typically have three occupants in the house and being alive we radiate heat – roughly 300-400W between the three of us. The 23m² of windows can let in up to 1 kW/m² incident energy in direct sunlight. Our average daily electricity usage in our current house is 16 kWhr / day. We will cut this a bit in the new house, but this is a combination of lighting, DHW and running electrical equipment – fridge, washing machine and my live-in son's Gaming PC + Xbox. Apart from some of the DHW used (which literally goes down the plug hole), all of this energy eventually ends up as waste heat within the living environment, and therefore adds to the general heat budget – that's roughly 1kW plus the solar gain. This 24 kWhr/day or more means that we will often have a heat excess within the house. On the other hand clearly these temperatures are profile averages and there will be periods where the temperatures will be lower, and below zero for extended periods. However, even a doubling of the temperature deltas only give a running heat loss of approximately 2 kW. My overall conclusions are:
    Our overall heat budget will be in near equilibrium for large parts of the year. At most the sustain heat input requirement will be of the order of 2kW peak. We need to manage heat excess efficiently and automatically up to say 2kW. I want to expand on this last "automatically" point: in our current farmhouse with its 2ft thick walls, the house environment is sufficiently stable for a large part of the summer and autumn that we turn off all heating and leave windows ajar all day, relying on natural ventilation. We feel that warmer weather should result in periods where we can do the same in our new energy-efficient house. However, we don't want to be forced into the situation where we have to dash around the house a few times a day opening and closing curtains, blinds and windows just to keep the house at a stable temperature: in general, the house should look after itself.

    This imbalance (or a lot more on sunny days with the solar gain through windows) is a material issue and to put this in perspective consider:
    The mass of the liveable airspace in the house is some 500Kg with a heat capacity of just over 0.5 mJ/K or 0.14 kWh/K. The specific heat of the slab is less than that of air (0.75 kJ/kg K), but the mass of over 7m³ of slab concrete is significantly more (16.4 tonne) giving a heat capacity of 3.42 kWh/K and the plaster board, etc. within the walls adds perhaps another 10% to this capacity. There are various equations for the heat transfer between the slab and the air above it but a good ballpark is 10 W/m²/K – that is roughly 0.7 kW/K for the entire slab. The slab and the other fabric which sit within the thermal envelope of the house has over 20 times the heat capacity of the air inside the house. So if we were to heat the input air from the MVHR to 10°C above room temperature, this will transfer about 0.7 kWh heat into airspace of the house in one hour at 0.5 ACH – the same as running the slab at 1°C above room temperature. In extremes we can easily lift the slab temperature say 5°c to increase the heating slab effect five-fold, but at another 2x the air heating route will start to be problematic with noticeable effects on air quality and background noise if we increase the ACH to do so..

    Our initial intention was to use an integrated MVHR + ASHP(Genvax), but there was always a concern that this would be inadequate to cope with extreme cold spells, so we planned to use a supplementary closed wood burner. However the problem with any stove is the minimum output (typically 2-3 kW) which is simply far too much for the living room to cope with. So we abandoned that idea: there's no point in installing a stove that you will never use in practice.

    This analysis plus Jeremy's reasoned argument also convinced us that an active UFH slab was the way to go, for the following reasons:
    It's a relatively low cost option. It addresses a saleability risk: "I want to fit a gas boiler" With a suitable ASHP it enables active cooling as well as active heating. In circulation mode it is extremely effective at distributing the heat throughout the entire slab from any hot spots caused by direct sunshine through the windows. With this reasoning the advantages of a combined ASHP+MVHR just seemed to dissolve, and we've now decided to abandon the Genvax in favour of a standard passive MVHR system. We have still to chose the ASHP, but we are looking at a low power (say ~5 kW) monoblock inverter-based design alone the lines of Jeremy's active slab approach, but more on this in later posts as I finalise details. We also need to think about the vertical temperature gradients in the house (our hallway / landing rises through all three floors into the loft space). Hence this is only part I of the thermal design.
  10. TerryE
    I just wanted to include a brief post explaining from a self-builder perspective why we have decided not to use an Unvented Cylinder (UVC), Thermal Store (TS) or combi-boiler for our domestic hot water (DHW) in our new build.  Instead we are using 2 × SunAmp PV heat batteries heated by E7 tariff.  So why?
    We decided that we don't need gas to be installed avoiding the Gas connection charges, per day supply charge and the maintenance costs on gas appliances.  Big saving here.
    We don't have the room for a TS and we've heard too many horror stories about the problems of heat losses in a passive-house class new-build like ours, so no TS.
    We didn't want to get into all of the regulatory crap around installing and annual maintenance contracts for an UVC.  So strike this one as well.
    So what is the alternative?
     
    The SunAmp is a thermal battery with an in-built heat exchanger (a bit like a combi boiler) which can store ~5kWh of heat for delivery in water typically at 50-65°C.  Here is a simplified schematic of the store.  (Note that I've left off all of the essential safety features such as the expansion vessel pressure relief and overflows to simplify this down to the functional essentials.)

    The guts of the device are a couple of Phase Change Material (PCM) cells which act as the thermal store.  It in essence it works in one of two modes:
    Discharge Heating, where the CW supply flows through the two PCM cells and is heated to between 55-65°C and then blended with a CW mix in a TMV down to a preset output temperature. Recharge. When fed with an external electricity supply (typically PV or E7 off-peak tariff power), water is circulated internally through the cells and a 2.8kW heater to bring them up to an internally preset maximum temperature. So the SunAmps can only be charged by electricity, and there is no alternative form of heat supply.  The form-factor is very small – two SunAmps side-by-side take up (d × w × h) 530 × 600 × 740 mm.  The rectangular packaging also facilitates the use of internal vacuum pack insulation panels so the total standing heat loss is ~ 1kWh / day which is a lot less than a typical TS.
     
    The exact choice of PCM is specific to SunAmp, but the linked Wikipedia article lists the common ones with a phase change at around this 55-65°C range.  However in terms of the physics of how this all works, it is easier to describe another common PCM that we are all familiar with and which has its phase change at 0°C: water.  There are three material properties that you need to consider when looking at how a PCM works: the specific heats of the solid and liquid phases, that is how much heat you need to supply to heat 1 kg of water by 1°C and the latent heat of fusion that is how much you need to convert 1 kg of water at 0°C to ice at 0°C.  I could give you the figures but a good way to think about is that you need the same amount of heat:
    To heat ice at -158°C to ice at 0°C
    To melt ice at 0°C to water at 0°C
    To heat water at 0°C to water at 80°C.
    OK these ratios and the fusion temperature differ for different PCMs (as well other properties which reflect the long term stability of the using it in cells, etc.), but that is all the proprietary stuff (discussed in the detailed below from Andrew Bissell). Even so, the bottom line is simple: the systemic heat losses are far less than alternative solutions, and
    As to why we have chosen the 2 × SunAmp PV approach, there were 2 main drivers for us:
    5kWh isn't enough to meet our typical daily use, and 10kWh is so we will be able to charge our stores overnight at E7 rate and only need daytime top-up in exceptional circumstances.
    The pressure drop across the store in Bar is roughly 0.0142×f1.81 where f is the flow rate in ltr/min, and if you crank the numbers one store doesn't give us enough flow rate.
    Even so if we look at our planned use (I'll go into the figures in a later blog post), our household of 3 people has had an average use of 280 ltr/day averaged over the last 6 years.  Most of this is hot water -- say 80% or at an average lift of 25°C, this amounts to 5,500kgK = 6.4kWh/day or 7.4 kWh/day allowing for heat loses. This will cost us £194 p.a. at my current electricity tariff for my household's DHW.
     
    Will I really realise the payback from additionally investing in gas or ASHP based DHW systems?  I think not.
     
    PS.  Slightly amended wording to reflect the earlier comment of Andrew Bissell quoted below.
  11. TerryE
    This is the Part II roll up of a couple of earlier blog posts and forum topics which provide the groundwork and context.
    Plumbing Design – Part I
    Heating the Slab – an overview
    Modelling the "Chunk" Heating of a Passive Slab
    Another DHW / DCW / UFH design.
    in summary, so far into commissioning and early use, everything is at least achieving our expectations and the house might in fact perform better than my predictions.  The key design points that I listed in part I seem to be spot on.  I want to compare a figure that I gave in the modelling topic with a corresponding plot during commissioning and testing to underline this:
      
     
    The first graph is a theoretical model based on a few simplifications, and the second live data, warts and all, and complete with hiccups as I test and restart the control system.  The bottom line is that the slab is reacting exactly as I modelled in overall behaviour, though one of the parameters is different.
    The UFH pump at its medium setting is under half the modelled flow rate, increasing the delta temp between out and return from 2°C modelled to  5 °C measured.  However, I decided to stay with this setting because the pump is almost silent at its medium setting, and the system and its subcomponents are still operating  well within specification at a delta of 5 °C.
     
    So in my view, if you are building a house with near Passive performance (wall, and roof U values < ~0.15; windows < 1 and not too much area; well sealed warm space and MVHR; decent insulated slab), then you should expect heat losses of less than 40kWhr / day in worst winter months.  You therefore need to put roughly the same into the house.  You only need to input the net top-up, because your occupancy, normal electrical consumption and solar gains all contribute to this input; this net is going to be 1kW or less on average.  Given that a cheap and simple Willis heater can provide 3× this,  using something like a gas boiler capable of 16-20 kW is just crazy, in my view; in our case even the economic case for considering an ASHP is marginal at best.   
     
    Yes, in terms of running costs, the electricity unit cost per kW is more than that for gas, but you also have to factor in other running costs such as boiler maintenance.  In our case, the British Gas boiler maintenance contract in our old house is less than our total expected heating cost in the new house so unit price comparisons are irrelevant to us.
     
    As I commented in the Boffin's thread, you need to limit the heating going into the slab: one way (the one Jeremy currently uses) is to throttle back the heating rate right back (e.g. using a buffer tank and an accurate thermostatic blender) ; the other way is to use a chunking approach and simply heat the slab in one (or possibly two) chunks per day.  In the chunking case you instead limit the total heat injected into the slab per heating round (that is the integral of the power rather than the power itself).  Doing this might seem awfully complicated, but in practice you can let the slab physics do this maths for you.  You can use any moderate heating source that has a reasonably consistent but limited heat output; this could be an inline heater like a Willis heater or an ASHP with the flow temperature and rate at present set-point giving water at, say, 30°C. The slab itself slab acts as the buffer, so no additional buffer tank is needed.  The algorithm is simple:
    Turn on the heating at a fixed time.  This could be the start of E7 or in the window of peak power if you have PV installed. Turn it off when the average return temperature from slab reaches a specific set-point threshold. The actual set-point (which in my house is going be around 27°C in winter) does vary by season because what you are doing is control the total heat put into the slab, and it will need trimming for any specific house and heating scenario,  but it is largely self correcting for short term temperature variations in that if the house gets a little colder due to greater heat loses in a cold snap, then the slab will require more heat to  reach the set point.
     
    At the moment we are using a twice a day heating cycle.  This is settling down to ~6 hrs overnight during the E7 window about £1.50 and a couple of hours top up during the day (another £1).  This being said, we are still warming the house from a pre-commissioning temperature of around 13°C to a pre move-in target of 20°C as as you can see from the graph, we are currently increasing the house temperature by ~0.5°C / day on top of the sustain heat losses.
     
    This in itself takes a lot of energy as we have approximately
    17 tonnes of slab, 5 tonnes of plasterboard, 11 tones of wood inside the heated zone of the house and
    8½ tonnes of cellulosic filler in the insulation.  Plugging these numbers and their  Cp's, it takes roughly 25 KWhr to raise this fabric by 1°C, or 4 hrs of Willis Heater to raise it by ½°C.  So at the moment roughly half of the heat input is maintaining heat loses and the other half is slowly raising the temperature of the house fabric .  This maintenance heating element is less than the JSH spreadsheet estimated for current average outside temperatures.
     
    So another way of thinking about this is that if we do without heating for a day, then the house will drop in temperature roughly ½°C to compensate for heat losses.  The daily ripple in temperature with a single heating chunk will be less than this. 
     
    If we only heat the slab during the E7 time window, say from 2 - 7AM, then the house temperature will peak roughly 3-4 hours later late morning and then fall by maybe ½°C during the rest of the day.  I feel that a ripple of ½°C will be barely noticeable to the occupants, and given that the heating during the E7 window is effectively half price, it is better to accept a midday peak (and possibly set the target temperature half a degree higher) than to pay double for an afternoon heating top-up to reduce the ripple.  
  12. TerryE

    Plumbing Design – Part I

    By TerryE,

    I have been doing the design validation of my plumbing solution partly so I am comfortable that it is feasible and partly to write this up so that others have a model of how to approach this task.  The last time that I did anything like this was with my current house where everything apart from taps for drinking water was fed off a (non-potable) header tank in the roof space and the central heating system was a classic 2-pipe (with branches) radiator system fed from a gas boiler.
     
    Even though our new house is a generation away in technology: passive-class, airtight to better than 0.6 ACH, low-temperature UFH in slab, pressurised water system using a Hep2O manifold / radial configuration, I still approached the design by refreshing my understanding of pipe dynamics, etc. using such reference works as this excellent intro into pipework calculations: John Heartfield, Water Flowing in Pipes I  – The Theory  and useful site like the Pipe Pressure Drop Online Calculator.  The first is worth a scan if you want to get a handle on some of the sizing issues.

     
    However, the figure above shows the pressure losses for the major system components in my Domestic Water System.  Note that the pipework losses represent about 1% of the total pressure drop and this value is lost in the noise compared to some of the uncertainties on the larger ticket items.  So it really is a waste of time worrying about the pipe losses in a pressurised radial system so long as your follow the following guidelines; this is not where you need to focus your design attention.
    Configure your pipe layout as a radial system.  Try to avoid putting multiple appliances on a single pipe, except where there are strong practical reasons for doing do.  For example, our dishwasher is adjacent to our kitchen sink and is a cold fill unit T'ed off the cold to the sink. Plumb all cold and high-flow hot radial piping in 15mm Consider plumbing low-flow hot runs in 10mm, though there is a lot of simplification and little to be lost in going up to 15mm if these runs are short. If at all practical co-locate your manifolds, DHW storage, HW heating, and other directly related equipment in a single service area.  This will keep all shared pipe runs short, and associated heat looses small. Properly lag all hot piping up to and including the manifolds. Lag the cold piping as well to avoid condensation. Whilst the pressure drop on common pipework is relative small, it is well worth while plumbing this in 22mm at a minimum. Pipe noise is still a risk so where practical use swept bends rather than tight elbows, and keep track of worst case flow velocities.  Keep these under 1 m/s where at all practical and under no circumstances allow them to go above 2 m/s. User full bore valves and fittings where practical to avoid unnecessary flow restrictions. It is worth finding out the the pressure drop vs flow data on all of your system components.  You'll typically get these as a set of log-log plots or power curves on linear axes.  They are almost invariably approximated by power curve fit and therefore all of the form a.fb where f is the flow rate and a and b are pipe / device-specific constants.  So in the case of my calculations, I used the following constants to compute the PD in kPa as a function of flow rate in m/s:
     
    Name a b Int Dia 15mm HEP2O 0.00300 1.743 0.013 15mm copper 0.00243 1.742 0.0134 22mm copper 0.00038 1.728 0.0202 25mm MDPE 0.00035 1.748 0.021 28mm copper 0.00011 1.718 0.0262 SunAmp 1.42000 1.810 n/a Softener 0.27600 1.740 n/a PRV 0.04400 2.000 n/a TMV 0.10000 2.000 n/a  
    I then created a test scenario that I wanted to make sure that my system could cope with. IMO, at a minimum this should include two high-flow devices at full open setting running in parallel, but for our design I used what I considered a worst case morning scenario and that was one shower @ 10 l/min and 42°C, one shower @ 8 l/min and 42°C, and the kitchen sink @ 8 l/min 48°C.
     
    Note that if you crank the numbers using Dec/Jan water supply temperatures, this comes out at an equivalent instantaneous heat demand of 67 kW, and given that combi-boilers top out at 40 kWhr, this is well over 50% more that the largest combi- boiler could deliver.
     
    It's then just a case of doing the temperature blend and flow-rate calculations and cranking the numbers in a spreadsheet.  On my first pass through, it was very clear that attempting to satisfy this short of flow rate through a single SunAmp was just beyond its rate capacity, but luckily we had already two configured in parallel.  Even so, the parallelled SunAmps account for ~ 0.55 bar pressure drop, along with the DHW TMV.  The water softener accounts for 0.8 bar and the Honeywell pressure regulator 0.3 bar. 

    The difference in pressure drop on the 3 pipe runs (all being ~1% of the total) is negligible, but since the total is ~2.25 bar and the actual head is 3 bar, we have ample headroom to sustain this scenario.  (Actually one of the showers is on the second floor, so in this case we lose nearly another half bar getting the water up there.) If the net figures are negative then we aren't going to achieve this flow rate and we will be system limited.  But they are all positive, so we are OK, and this means that the taps or the various flow restrictors are going to have to do their work to limit the flow.
     
    So what could I do if I wasn't achieving the desired flow rates?  Basically the answer either to revise my expectation downward (after all my current house design can just about deliver half of this); to increase unit capacity by upgrading in some how (e.g. in my case doubling up on the SunAmps), or to think out of the box.
     
    The biggest single hit here is the water softener, and in fact I introduced this fairly late in the design process when I realised that having one is pretty much essential with my level of water hardness, but that's life, I guess.
     
     
  13. TerryE

    Harveys Water Softeners

    By TerryE,

    If you have a Combi boiler, or SunAmp, or pretty much any device with a built in Plate Heat Exchanger (PHE) and live in any region which has hard water (about 80% the UK population), then you will need a water softener if you want any decent life out of your plumbing installation.  As far as I can see you are down to one of two options for a direct plumbing solution: the UK Harvey twin tank system and the US Kinetico range.  All of the rest are niche suppliers, IMO.  The Harvey system seems to be viewed as the best in terms of performance and running cost; and a few of the others on the forum use one and have recommended it, so we went with this choice. 
     
    The basic Harveys internals are boxed and rebadged by a number of suppliers: Harveys, TwinTec, Fountain Softeners, but the versions currently shipping are one of four standard Harveys models under the hood: 500, 750, 1000, 1400 (which relates to the volume of water in litres per flush).  And these have a guideline maximum PPM of 320K/<model no>, so the 750 is advised for a maximum of ~ 430 ppm water hardness. This is the one that we have.  Each flush on the Harvey is 17l, so the 750 flushes 17/750 = 2¼% of the water (which incidentally is under the 4% threshold on the Water Usage calculation required for Part G approval). 
     
    So in our household we use ~250l /day and so will do 10 flushes / month @300g salt per flush or 36 Kg of salt p.a. 12 × 8 Kg block salt costs £72, so our expected annual salt cost is approximately £27.  A lot cheaper than some alternatives, and this has to be offset against reductions in soap and other consumables.
     
    Now to my big bitch. I was researching the performance data of the Harveys Softene and trying to find simple performance data on the typical pressure drop vs flow.  Talk about shifty and evasive.  You can't find this anywhere on the Harveys site; ditto the TwinTec, and the Fountain Softeners site.  I also tried an email to my named contact at the last, but got no reply one this.  There's lots of qualitative hand-waving in YouTube videos and promotional material, including from Mr Harvey himself -- all to the effect that the pressure drop is not that bad / a lot better than the competitors, etc.  Calculating this pressure drop vs flow graph isn't difficult.  These curves all approximate well to a power curve, so the tester will normally takes a few PD : flow measurements over the working range of the appliance, plot them on a loglog plot; fit a straight line; and then invert this back to the power curve.  OK, in the case of a water softener, the actual value might vary over the flush cycle so you might need to put in a upper / lower band on this curve, but all of this naaa-di-naa-daa sounds like they are hiding something to me. 
     
    So after a bit of Google searching I happened across a link to a PDF on a website run by a landlady in Reading (how Google found this, I have no idea).  This was the standard 4 page installation guide that is most of the sites, but plus a fifth "Technical Data" page: which includes this plot.  What this shows is that at a 30 l/min flow rate you are loosing 1 bar through the softener,  This equates to a couple of showers going at the same time as the kitchen sink.  However if you have power showers or the like, then by 45 l /min you are at 2 bar and with the other system loses, your water system will begin to struggle if you only have a 3 bar supply.
     
    Of course if you have an accumulator, then this will be positioned on the house side of the filter, so this will greatly mitigate this limit.
     
  14. TerryE
    (This post is a précis of a post and thread discussions that took place on the eBuild forum October last year and subsequent discussions with my builder.)
     
    Many of the self-builders active on the forum will have used or be familiar with the Passive Foundation system marketed by MBC Timberframe.  The essence of this is that the foundation is a raft slab that incorporates a ring-beam that sits inside an EPS former.  This former both acts as shuttering for the concrete pour and as insulation between the slab and the underlying hardcore base.  The slab is therefore wholly contained within the thermal insulation envelope of the house, typically giving an overall U value for the slab of around 0.1 W/mK. So far so good.

    A variant of this is where the house has an external brick, blockwork or stone skin.  In this case one approach is to pour a second outer ring-beam to carry the outer skin, and the MBC structural engineer (SE)
    Our skin is a rough-cut Cotswold-style limestone with an s.g. of around 2.5 (or 2.4 allowing for mortar); the walls are ~5m high, and the courses on average 125mm deep giving a linear loading of around 1.5 tonne/m rising to 1.8 on the 3 gables. The underlying ground is a very stiff impermeable (Oadby Member) clay, but We have some medium size tree quite near the foundations.  
    Here is a simplified diagram of this.
     
    The SE specified bridging H20 rebars at 150mm centres to couple the inner and outer ring beams structurally, so that the load of the skin is carried across onto the main ring-beam and transmitted down through the ESP300 underneath the beams.  If we assume that the load of the stone and house was carried only by the EPS300 sections of the slab, this gives an overall GBP of some 12 kPa and the 266 H20 rebars ensure that there will be minimal differential movement between the outer stone skin and the timber frame supporting everything else.  This is comfortably within the allowable bearing pressure of 120 kPa recommended in the Geo-survey report.  So this is a good structural design, but it unfortunately embeds a major thermal design flaw.
     
    If you consider the thermal cross section of the total rebar, it is pretty much the same (from a thermal perspective) as replacing the rebar and the EPS  between the two beams by solid concrete.  To be honest I along with everyone else missed this thermal design flaw when I was given a copy of the slab design to review.   The penny only dropped for me when I saw the rebars in place, and the concrete was just about to be poured.  The inner slab and ring-beam is within the insulation envelope of the house, but the outer ring-beam is at ground level and directly carries the stone skin.
    In the base design this would be fully exposed to the elements and could often drop to ~0°C or below in winter.  The 266 × 2cm diameter mild steel rebars have a total cross-section of 0.084m, and this couples a slab at roughly 21°C with a ring-beam at roughly 0°C across a 20cm gap. This is a pretty perfect thermal bridge as steel has a thermal conductivity of roughly 40 W/mK -- this means that we will lose heat at roughly 21×40×0.084/0.2 W = 350W or 8.5 kWh / day in colder winter months through these bars. This is over 3 times the design figure of 2.6 kWh for the entire slab. Here is a small extract from the slab engineer's design. I've coloured the different components and removed a lot of the structural detail which isn't relevant to this discussion, so we can focus on the issue here.

     
    We were too late to change the design fundamentally, but if left uncorrected, this flaw would result in the slab being the single largest source of heat loss (more than the walls, the roof, or the windows and doors for example). So after discussion with MBC, Hilliard their SE, and members on the eBuild forum, what we decided to do was:
    We retained the outer EPS formwork that wrapped the outer ring-beam. This still left a thermal bridge between the top of the outer ring-beam and the stone skin it was carrying.  Hilliard confirmed that a course of Perinsul Foamglas would be capable of supporting the design load of the skin and largely close the thermal bridge. However, we would then have an exposed ESP front and FoamGlass course edge which is cosmetically crap and vulnerable to rodent damage. So after discussing options with our builder we decided to cover the entire exposed EPS / FoamGlass surface with some courses in engineering brick. And when the skin was complete we would then put a perimeter path 60cm wide and 10cm (min) deep around the house on the crushed stone bed. Here is a simplified schematic that I drew up for my builder of the approach that we finally agreed on. What he did was to use two external courses of engineering bricks as an plynth in front of and on top of the FoamGlass, followed by two header courses to step back the wall line.  This engineering brick wrapper is primarily cosmetic and a weather protection as the load is actually carried down vertically through the FoamGlass onto the ring-bean,  I've also included a photo of the plinth at one of the rear French windows where you can see how it looks in practice.
          

    There is still going to be a little bridging on the diagonal between the ring-beam and the outer engineering brick layer, but my rough estimate is that this will be more like 50W rather than the 350W discussed above.  An extra 1.5kWh/day, I can live with.
  15. TerryE

    A Little Aside On Radiance

    By TerryE,

    We've sort of covered this topic buried in various earlier threads, but since I need to use this info for my heating calcs, I thought useful to cover this in a short summary post.

    Characterising the components of heat transfer across a solid / air surface really does come down to basic physics and we just need to crank the numbers into the two main factors at play in this:
    Radiation. Any surface is radiating heat but is also simultaneously absorbing similar radiation from everything in line of site. If everything is at the same temperature, then this all cancels out and no net flow of energy occurs. However even with a small imbalance in temperatures, because of the amount of radiation being transferred, results in a net energy flow. The physics depends on the Stefan–Boltzmann law, and when you crank the numbers for a surface at roughly 20°C, this works out at ~5.7 W/m²K. OK, this has to be factored by something called the emissivity which can be as low as 0.03 for a mirrored aluminium surface, but for normal painted surfaces like house walls, it's nearer to 0.9. Also remember that the whilst the area can easily be calculated for very smooth surfaces, any texturing (like clothes or carpet pile) can dramatically increase this. Nonetheless, a good general rule of thumb is to assume 5 W/m²K. Conduction. This is atoms of air bumping into the walls and transferring energy that way. Air, being a gas, is light on atoms compared to the solid wall, and so is a poor conductor, but it is also free to move and so the air region in contact with the wall is continually replaced due to any air movement. Once the temperature difference between the wall and the air is more than a couple of degrees, then the heating of the air itself generates convection and this make the heat transfer even more efficient. However in internal spaces, where there are no major drafts or temperature induced convection differences, this conduction makes a relatively small, say 30%, contribution, and radiation is the dominant component.  
    So a good overall figure for bare surfaces is ~7W/m²K and this is what I use in my active slab calculations.

    This means that when doing U-value calculations, I can treat any material/air interface as having an effective thermal resistance of its reciprocal, that is roughly 0.15 m²K/W within the R-value calculations. Note that the references often assume some level of internal air movement, and so quote a lower thermal transmittance value of 0.12 m²K/W. Also if the surface includes a reflective / foil layer then the emissivity can drop significantly (though not to the 0.03 figure that I quoted earlier unless a high-spec multi-layered material is used); a typical foil-backed plasterboard might achieve an effective emissivity of around 0.3 which is why this is often quoted as having a transmittance value of 0.4 m²K/W.

    The bible which gives all of these magic figures is the BRE Conventions for U-value calculations document and the data given therein broadly corresponds to the above.

    Incidentally the average human has a surface area of roughly 2m², so radiates 5 x 2 x (33 - 20) (=130) watts if naked in a room at 20°C. Clearly the more clothes that you wear, the less your effective surface temperature, and the less your radiant heat losses; so with a light covering and a jumper on the torso, this might drop to 100W or so. This echoes a point made in one of Jeremy's earlier posts: how cold you feel in a house relates to your overall heat loss and in still air maybe 60-80% of this total is due to radiant losses rather than conductive/convective ones. So the temperature of the wall surfaces is just as important as the air temperature in determining this comfort level. Being in a room with walls and air at 20°C can feel just as comfortable as being in a room with walls at 17°C and air at 24°C.
  16. TerryE
    One of my preconditions it to provide an accurate survey of site levels across my site. The last time that I did anything like this was just under 40 years ago as a young Lt. in the Royal Engineers when I was surveying for a road, but that was using a decent theodolite to do cut and full calcs. Nowadays you typically lasers and GPS, but I didn't want to pay a fortune for something that I could do myself, so I reverted to a variant of a technique that the Romans used and that is to use a water level. You can buy them off-the-shelf (e.g. this Handyman Faithfull Water Level 10m/33ft which looks like the first image). However, I decided to hack together my own using a couple of 18" lengths of transparent tube, a garden hose and a couple of steel rulers and black masking tape as per the second.
     
       

    Basically the technique is to leave the chair at a reference datum level and move the ladder around the site. The bottle of water on the chair is to top up the water if any is slopped out. Occasionally, I had to shift the position of the readout tube up/down on the moveable measure (to keep the water column in the transparent section). I just entered both readings in a spreadsheet with reading 1 being the moveable readout at the datum. (This is in row 2, because the headings are in row 1.) So the height formula is just (and copied from row 2 to the rest of the rows):
    =(C$2-B$2)-(C2-B2) Since my garden hose is about 40m long, I was able to cover my entire site from a single datum, and even where I didn't have line-of-sight. As you can see from the inserts, it is really easy to read out the levels to 1mm accuracy, so even if I was sloppy the entries in this spreadsheet are at most a couple of mm out for the entire site.

    The one thing that you do have to be careful about is to flush the full length of the hose through before you start to remove any bubbles / airlocks and more importantly to ensure that you don't have one end of the hose at (overnight) air temperature and the other at mains water temperature, as this density gradient can cause a systematic error of a few mm.
  17. TerryE
    We've had our second planning application in the system since mid December.  Our first application was in May and the main reason for the delay between these was negotiating with Highways over the new vehicle access and parking to our existing farmhouse.  (The existing access and parking falls within the new-build plot.) This delay was due to a combination of us being new to this and not understanding the rules to this bureaucratic game, and the highways engineer being just totally disorganised.  Very frustrating and annoying for us, but what can you do?

    We have been checking the LPA website most days.  The acceptance Notice of Decision was posted on the Thurs 12th (also the expiry date for our application).  Hurrah! On the following Monday, I rang our planner to ask a few follow up Qs, and got an answer phone message that the 13th was his last day and he'd left the LPA.  Arggggghhhh!! – I felt like I had just walked out of the front door to a building only to have the ceiling collapse behind me; not because of any loss (even though we like the guy), but more the idea of our having to change the planner that we'd been negotiating with at such a critical stage in the process.

    Having compared our NoD to others that have been recently posted on the LPA website, it seems broadly typical.  A lot of the conditions are defensive measures on the part of the LPA to give them mechanisms to enforce the application should a builder try it on.  We had six pre-commencement decisions for details to be agreed: three relating to the stone and slate of the outer skin and the access onto the highway; one on surveyed levels for the existing and proposed ground levels, floor levels, eaves and ridge heights in relation to adjacent buildings; one on the planting plan and one on an archaeological survey.

    The planting plan one was silly but easy to fix: we'd already provided one in our application, but it wasn't in the format that they liked, so we essentially had to represent the planting scheme as a marked-up site plan with lots of text boxes and arrows, rather than a figure and a couple of pages of text.

    IMO, the archaeological survey requirement is both silly and capricious.  Apparently a survey ½ mile away below the village uncovered some Roman/Iron Age finds in 2010.  So we now have to dig a survey strip across our 14.5m wide plot under the supervision of a qualified archaeologist; get him to write this up and file it before we can start our foundations.  There have been 5 full infill development with permission granted in the village since 2010; none of the other four had a survey condition, despite 3 being closer to the previous site and ours being the only one that had previously contained (Victorian / Edwardian) farm outbuildings; the others were green field / garden.  We also dozed the site down to the virgin clay 25 years ago to lay a tarmac drive so I don't think that it will yield anything more than the odd Victorian bottle shard.  However, the cost and time impact of appealing this condition will probably be more than the cost of gritting our teeth and doing it – as I said to Jan, at least we don't have Newts!!

    We have six conditions before specified phases of work; for example they want to agree the paint (which must be white or off white) that we will use for exterior paintwork.  I would class these as mostly stupid / annoying, and the LPA having a dose of Big Brother complex.  We have another 3 conditions before occupation / use.  So overall we can live with all of these, but even if we were to ignore their petty ones (e.g.  the colour of paintwork, there's no way that the LPA would realistically issue an enforcement notice (the village falls into a category the LPA classify as "modestly sustainable"), so why bother attempting this level of minutiae control?

    However, overall I think that the LPA provided a good and effective service.  Two of our neighbours strongly objected to having a house built beyond the bottom of their gardens, but the planner agreed with us that what we were proposing was the best compromise given that it provided a reasonably sized infill dwelling that would fit well within the village.  We'll just have to be careful about one of these neighbours during the build itself; I suspect that he'll be complaining to the enforcement officer about anything that he possibly can – which is a pity because I'd prefer to stay on good terms with neighbours and do a bit of give and take where practical.

    So upwards and onwards.  The main issue for us here is now to reach the necessary agreements without this taking another 3 months off our timelines.
     
  18. TerryE
    I am writing these posts for two main reasons. The first is for my benefit, in that I find that if I have got to the point where I can explain my thinking to others, then I've got to grips with the problem myself. The second is that I might just help others going down this same path, by documenting my thought processes.

    It's three months since I wrote the Part I of these three Thermal Design posts, and I concluded this by saying that I intended to adopt Jeremy Harris's UFH For A Low Temperature Slab concept. Since then I have done quite a lot of modelling to understand how the house has a system will react to changes in input and output (as discussed in my last post). Basically, the house fabric has a huge thermal inertia, so I can use average weather conditions and ignore diurnal temperature changes, and even the odd day or so of unseasonal weather. The only wild-card in the heating equation is the issue of solar gain.

    Part 2 was a bit of a mixed bag which summarised these issues, and to be honest in the process of getting to grips with some of these, I ended up pretty much rewriting chunks of this and cutting out a lot. It's take me some time to get my head around some of the secondary issues, to get to the point where I could do this post. I realised that once I approached this issue with the right mindset, then achieving this control is going to be quite straightforward. Perhaps the best place to start this is by a showing a graph of the net heat balance for my planned new house:


    This is just a summary of the heat balance output from my spreadsheet of overall heat balance by calendar month. It shows three curves (note that this these curves now reflect poorer wall U values, a shift from 0.12 to 0.16):
     
    Gross Heat Balance excluding solar gains. This is similar to the one that Jeremy calculated for his house and roughly similar to one that you might calculate for you own house if it is a near-PassivHaus specification. Note that daily household electricity usage ultimately ends up as waste heat within the fabric of the house and acts as a heating source, so this is included in the overall heat equation -- which is why this curve goes positive (up to 7kWhr/day in July/Aug) as well as negative (down to -12 Whr/day in Jan/Feb). Ditto but less slab losses. I explain why I break these loses out below. Ditto but adding in expected solar gains. These gains are just the season estimates based on the PVGIS data as I describe in my last post. My house is aligned on an S/E axis with no large south facing windows, yet even for my window configuration solar gain is quite an issue to be addressed.  
    Using an Active Slab

    So the essence of Jeremy's and my approach is to have an actively controlled slab. By this I mean that I can to define a target temperature set point for the slab itself, as measured by a sensor in the slab, and there is an automated mechanism for adding heat and dumping heat so that it's temperature can be automatically maintained 24x7 within a defined dead-band about that set point.

    The slab temperature is coupled to the room temperature: if the temperature difference between these is ΔT and the area of the slab is A, then the heat transfer between the slab and the area is linearly dependent on these, say h.A.ΔT, where h is a constant. A good ballpark for h is 7 W/m²K. This will could vary from house to house because it is dependent on floor coverings and treatments, but it is unlikely to be outside the 5-10 W/m²K range. If I crank in the numbers for my heat balance curves and slab area, a ΔT of less than 1°C will create this range of heat transfer even in the depths of winter.

    For example, if I set the slab temperature at 22°C in January, (ignoring solar gain for now), then maintaining the slab at this temperature will pump enough heat into the house to keep the room temperature at roughly 21°C. Likewise if I maintain the slab at 20° in the summer by dumping heat, then this will dump enough heat out of the house to keep the room temperature at roughly 21°C. I am using the heat flow between the slab and the house's internal air to keep the overall house temperature near a desired temperature. The whole house is treated here as a single zone for temperature control purposes. I am not trying to separate out individual rooms or even floors.

    Yes we will lose heat directly into the ground through the slab, but this doesn't directly factor into the temperature at which I need to set my slab.

    This type of slab control is two sided: I need to be able to dump heat as well as add it. However, the average rates at which I need to add or dump heat are pretty low -- never more than ± 0.5kW. Because the slab itself has a significant thermal capacity (~ 2kWhr per degree), I have a lot of freedom in how this heat adjustment is timed, if I am willing to tolerate a degree or so temperature variation during the day.

    I am planning to use a monoblock inverter ASHP for heating broadly the same as Jeremy discusses in his topic that I referenced above. One variation to Jeremy's solution that I am still assessing is to add an external buried ground loop as an extra controlled zone on my UFH. As the ground is varies in the range 8-12°C (depending on season) at 2m depth, dumping 400W or so from a slab at ~20°C doesn't take a large loop size or flow rate. (I will cover these details in a separate thread.)

    Dealing with Solar Gain

    This is a difficult issue to get to grips with. As tried to discuss in an eBuild topic, Rfc -- U F Controlled Slab + Control For A zero energy home (ZEH) on this, I can get a pretty good estimate of the average season gain for my window configuration and this is not trivial. Some (less than half) of the sunshine will end up on a ground floor and directly heat the slab. Having the slab circulation pump on (e.g. even 20 mins every hour if not otherwise heating) will effectively redistribute this heat across the entire slab, cooling the sun-facing rooms and heating the sun-shaded rooms. Once in the slab, these heat deltas will simple get factored into the overall slab control.

    In terms of overall temperature control, it is the major proportion -- that where the sunshine lands on other than the slab -- and the heat from which then gets transferred into the room air is more problematical. However, in my case I have a simple remedy which will work for most of the year -- which is to leave my MVHR set in summer bypass mode. With this, I can define a trip threshold for exhaust air temperature (say 23°C) above which, the MVHR automatically starts to bypass the heat exchanger dumping hot air and drawing in cold air. This is an automated heat dump which in my case can effectively dump up to 1.5kW in winter though a lot less in summer.

    The TBD issue is whether I also factor predicted solar gains into my slab temperature offsets, and this is one that I can't really model effectively, and (to be honest) not one where I think that I need to. I am not directly controlling the heat input into the slab, only its temperature set-point. If the air temperature does rise above the slab set-point then heat will flow into the slab from the air. My system won't demand extra heat input (and heat that I pay for).

    I want to avoid the scenario where my system is actively heating the slab at the same time as the the MVHR is in active bypass mode, but accepting a 3°C offset should effectively eliminate this situation.

    High summer is going to be the period that I need to consider more. This is when I will almost certainly need to dump quite a lot of heat, but the MVHR heat dump is going to be the least effective and this is where I might need to factor solar gain into the slab setpoint, but this is a case of refinement on my real house.

    I am lucky in that my house plan and orientation means that solar gain isn't going to be a major problem for me.
  19. TerryE
    I discussed my overall static thermal design of my house in Part I.   In this second post, I wanted to discuss the dynamic characteristics of its design – that is how the house will respond over time as external temperatures vary, but I've decided to break this into two parts leaving how I propose to control the internal temperature to a later post.   Again, this content is a self-developed analysis, because I have yet to find any decent design guides to build upon – the Internet either seems to be full of qualitative hand-waving at one extreme or the reader requires degree-level maths/engineering to understand the content at the other.  In my view any intelligent reader with at least O level Maths (or GCSE A-C in new money) should be able to understand the general principles here and make informed quantitative decisions relating their application in design.  So in this post, I will try to apply this to my own design.  I will accept any constructive feedback and will update this accordingly.

    I look at rates of heat loss and heat gains in steady state when doing the static design – that is I assume that nothing varies of time.  Such heat losses are typically a summation of factors which relate to an intrinsic property of the house fabric known as the Thermal Resistance; this relates to the extent to which a given material resists a heat flow when a temperature difference exists across it, though conventionally its inverse, thermal conductance is usually quoted in the case of sheet materials and is usually referred to as the U value, measured in W/m²K (where K refers to degrees Kevin; the same as per °C when measuring temperature deltas).

    By way of an example, let's say that I want to maintain my house at a target temperature of 20°C.  If the external temperature is 10°C then the U value calculations that I showed in Part I give a gross heat loss of 0.65 kW for the house fabric including ventilation, with this increasing by 68 W for every extra °C in delta temperature.  However, I need to offset this loss by the intrinsic heat inputs to house which includes most of the electricity usage, roughly 0.7 kW (as I discuss below).  On top of this we have 3 warm human bodies of the occupants (0.3 kW say), and any solar gain.  However, let's assume that it is grey overcast weather so I can ignore this last factor, and so this all gives a rough gross heat input of 1 kW into the house, or if I take off the heat losses a net heat gain of roughly 0.35 kW.

    Given this heat excess, the house will slowly warm up and will only stop warming up when the heat losses and heat input are in balance, that is when the temperature has risen by (350/68)°C at roughly 25°C internal temperature.   Maybe in reality, we'd have started opening windows to cool down, but my point is that we might need such manual intervention to prevent this scenario occurring even in spring or autumn.   This might be difficult for the reader to accept, but the problem with relying on our experience is that we've all been brought up in poorly insulated housing stock which has a net heat deficit for at least 3 seasons a year.

    If I look at the trend for typical U values (in W/m²K) for house walls over built over recent decades: the 60s – 1.8; by the 80s – 1.0; the 90s – 0.5, 2010 to 0.35.  The target figure for my house is 0.12 and it will also have MVHR, so its overall heat losses are over 15 times smaller than the house that I grew up in.  So we are used to heating systems (usually automatic central heating) to top up the heat in the house when the temperature falls below a threshold, but the heat losses are so small with this type of energy-efficient design that the intrinsic heating can frequently exceed the heat losses.
     
    With a near PassivHaus type design, any house will be in a state of heat excess at least half the time.  If we want a controlled comfortable living environment, then knowing how we will dump excess heat is just as important as generating heat.
     
    Understanding our heat inputs

    Because the whole house design is nearer equilibrium, I need to have a good understanding of what our likely heat inputs are.  These broadly fall into three categories:
    Electricity.   I have been tracking our electricity use in our current house for over 7 years.  We have Economy 7, and our daily usage is currently 10kWhr (day rate) and 6 kWhr (night rate).  We don't use electricity for space heating (but we do top up our DHW using E7) and all of our lighting is energy efficient.  Nonetheless, this usage still rises by perhaps 2+2 kWhr in the winter quarter.  We have the usual array of electrical equipment as well as using an overnight timer-based cold-fill dishwasher and washing machine.   We are now retired, so there isn't a noticeable weekday / weekend usage pattern.   However, our live-in son is the main consumer of electricity with his games PC, Xbox, etc. running a lot of the time.  (The only noticeable dip in usage is when he goes on holiday!) Apart from the small amount of DHW that goes down the plug hole, this electrical energy all ultimately ends up warming the air or the fabric of the house.  So this average heat input in our case varies from roughly 650W in the summer to 850W in the winter. Body heat.   This varies according to body mass and activity levels but is as low as 70W when sleeping, maybe 100W when sedentary and up to double this if moderately active.  So a reasonable estimate for 3 lazy house occupants is 300W.   Small but worth including in the totals. Solar gain.   The energy in direct sunlight is roughly 1kW/m² directly facing the sun.   If you want to estimate the overall solar gain, then you can use the PVGIS calculator.   It's main purpose is to predict PV array outputs, but it also gives you in one of its output columns Hd, the average daily sum of global irradiation per square meter, by calendar month.   You can plug in your location and the area (of glass) and orientation of the windows on each wall to get an average solar irradiation.   Your need to allow for any shading and the fact that windows have roughly 70% transmittance at the sun's colour temperature.   Here is the predicted plot for my windows.
     
     
    I feel that it is quite difficult for us to vary these without investment or material lifestyle changes.  Of course we will look at the energy efficiency of new appliances, PCs, etc. for the new house, but given that we've eliminated most of the "low hanging fruit" over the last 5 years, I think that we will have a comparable run rate in the new house.  If you are doing this same exercise, then I recommend that you do your own, albeit similar analysis, as your usage and patterns might well be very different.

    The approach that I discussed for solar gains is useful to determine ballpark values to feed into your overall heating calculations.   Yes, it is intermittent and unpredictable the UK and the actuals could vary significantly from this on a daily basis.   However when I compare this graph with ones that Damon published for his actual intra-day energy collected (Earth Notes: Grid-tie PV Power) and look at the spread against his predicted (which I did by analysing his CSVs), the actual daily figures nearly always vary between 0 to 2x this predicted value with a 1σ of roughly 0.4x so this still a reasonable approximation for heat calculation purposes.

    The Role of Thermal Inertia

    What this analysis doesn't tell me is how quickly the house will react to this net heat excess and how I can control this.  To do this I need to consider the impacts of a second material property usually called the thermal inertia or mass (but more strictly is the Thermal Capacity) and which relates to its ability to store heat energy.   It is normally quoted in J/KgK for bulk materials, but since I prefer to calculate everything based on lengths, areas and volumes, I find it easier just to multiple this figure by the density of the material to give a per unit volume equivalent, that is in J/m³K.   You can look these coefficients for standard materials up on the Internet (e.g. by Googling "thermal capacity for concrete", etc. I also find the Engineering Toolbox site a useful resource for this).

    These properties can very dramatically for different materials: for example EPS has a high thermal resistance but low volumetric capacity; in contrast, concrete has a poor resistance but reasonably high volumetric capacity.  Thermal resistance is linked to flow of heat, that is power or the rate of energy usage – measured in watts (W), whereas the capacity is linked to total energy stored – measured in joules (J) so any equations linking the two involved multiplying in a time factor.

    In my example, if I assume that the excess heat is transferred to the internal air in the first instance and from this to the slab (as discussed in this post), the slab will exchange heat with the air at roughly 0.7 kW per °C temperature difference between the slab and the room air.  So once the overall air temperature is roughly half a degree warmer than the slab, the air and the slab will rise in temperature together, lock-stepped at this offset.   The slab is the major component in the thermal capacity of the house, and concrete has a thermal capacity of 1.9 mJ/m³K, so a 0.35 kW excess will raise the slab 1°C in 7 × 1,900,000 / 350 secs or in roughly 10 hours, so it will take days rather than hours to reach this 25°C endpoint.

    The ratio of the thermal capacity to thermal conductivity for a given body, say one m² of external frame and wall has the units (J/K) / (W/K) which is in seconds when you do the cancelling out.   This is known as the thermal time constant of the body (often referred to by the Greek t, tau or τ).   This has a well defined meaning (as link explains) for homogeneous materials, but you can think of this much as the half life is used in radio-active materials: in this case its a measure of how quickly a change is temperature at one side starts to be reflected at the far side.

    The τ value of my house walls is about 16 days.   What this means in practice is that I don't need to worry about the daily variation in external temperatures or even the odd really cold or hot day when calculating expected heat losses or gains.   The fabric of the house smooths out all of this sort of variation long before it is felt internally.   (I discuss this further in my later post Modelling Thermal Lag).  
  20. TerryE

    Modelling Thermal Lag

    By TerryE,

    I was really unhappy with my discussion of this on my last blog entry, so I want to get a better quantitative understanding of how this would impact my house design, so I decided to write a simple 1-D explicit form finite mesh simulation which could be used to explore various wall and roof profiles. I initially intended to do this as a spreadsheet so that others could have a play with their own designs but without needing to get into code and programs, but stability issues means that the mesh size was just too small for this to be usable, so I ended up coding this up as a program.  (Save this wall-thermals_c.txt as a C file.)  The source is fully documented inline so I won't repeat this here. It's simple to compile and run on any system with a C compiler, e.g. Linux / Mac / Windows with MinGW. If it is useful and there is a consensus for an alternative language then I'll port it.
     
    If anyone is interested, then I can post some sample graphic analysis, and also reflect any comments later. But some general observations:
     
    The thermal lag through my walls is measured in days not hours. The wall acts as a huge filter to remove even daily fluctuations, so don't worry about sizing heating for worst night-time conditions for ~0.14 U-value or less walls. You need to think of time averaging any external temperatures over days not hours. You really don't want to think about saving heating, when you go away on holiday. I looked at how long it took to heat the walls up on a typical February day from cold limiting the heating to 6kW. The walls just sucked that heat in and it took over a week to get the to the point where they were still warming towards a steady-state thermal profile but at double the base heating load!  
    These are a couple of profiles for a daily 0 - 10°C external cycle. Note that I will have an external stone skin which does nothing in terms U-values, but does act as huge thermal damper of heat variations. There are significant heat flows in the walls, but the second plot show the "one sigma" for these as a function of depth in the wall. There is bugger all ripple getting through to the internal walls. (Windows have no thermal lag so if you have large areas of window in your house then this won't be the case.)
     
     
    I still want to do a bit more playing to look at the effects of solar gain, etc., but anyway if you can read C (the code should be understandable to anyone who knows Fortran / Basic, etc.) or the code documentation, then have a browse and tell my what you think.
  21. TerryE

    Traffic Survey Blues

    By TerryE,

    The latest turn in our planning journey has occurred.  As part of pre-planning we send an early draft of our submission tried to consult with LPA Highways department and got this reply:
     
     
    We will have two drives and access onto the road from our plot which is being divided onto two: the existing drive which will serve the new house at the bottom of our garden, and a new drive will be added in the reduced plot left with the old house.  We initially planned to have turning provision in both hard standings, but on the LPA case officer's recommendation we removed the turning area from the new house (as we like everyone else on the street reverse out, and trimmed the turning area for the new house a little) -- he was very keen to retain as much of the existing stone walls and planting as possible.  He also dismissed the idea of needing a speed survey during his site visit, since it was clear to him that that traffic wouldn't get anywhere near 30mph.
     
    So despite the DaS clearly stating
    that the new drive has a turning head and that we will instate the splays required by the LPA that on the LPAs advice the additional turning head had been removed from the existing drive to maximise green planting we had the following response from the Highways officer:
     
     
    So they hadn't bothered to read the DaS; had invented a mythical pre-application advice, and were playing hard-ball (see my highlight).  I now had to find a course between the conflicting advice of the case officer and the Highways people who seemed to want us to demolish our boundary stone walls and turn the garden into a car park.  After a quick chat with the case office, he then confirmed by email:
     
     
    But at least, he informally confirmed that we'd cleared all other pending issues, but that we should still withdraw our application until we could resubmit complete with traffic survey   And so after discussing options for survey companies with a couple of eBuilders, I selected a few more local ones through Google (I found the best search term was Highways speed survey TA22).  I talked to / swapped emails with 4 of them.  Their general advice was consistent: a radar gun based survey would be impractical because the traffic down the road is so low that it would take hours or days to get the vehicle numbers needed to make the survey meaningful; a 7-day loop on the road survey was both cheaper and more appropriate in these circumstances.  Two only offered gun surveys and the others were in the £250 + VAT range.  In the end I chose PCC Traffic Information Consultancy because they were based reasonably near to us and their engineer was very helpful and knowledgeable. He also asked us to check a couple of issues with the local Highways.
     
    So I then phoned the Highways and during the conversation, the Local Highways Engineer casually dropped into the conversation that there wasn't really any hurry because they don't accept surveys taken during school and other holiday periods, so another two months hold-up!
     
    We had our next wobble with Highways, because the just wouldn't agree about our lines of sight at our proposed new access. The Highways officer said that it was only X metres to critical north side, but we measured Y on the ground.  We eventually tracked this discrepancy down to the fact that our TA had used the OS 1:1200 map for his road layout, but which ever OS bod had digitised the road in the 1970s or 1980s from the aerial photo had taken a shadow-line from some trees as the line of the road, and hence the road (as shown on the map) bent by an extra metre just past our house.  Not much, but enough to shift the minimum stopping distance that we would require by over 5 metres.  In the end, the survey came back supporting our claim that our proposed drive was within the Highways guidelines (luckily thanks to the 5m adjustment), and we can now prepare a second application for submission. 
     
    End of panic, and a lot of hassle and delays.
     
  22. TerryE

    The Plot And Its Context

    By TerryE,

    As I discuss in my first post, we have a large garden that is side-on to our road, and it is large enough for us to divide off a strip at the far end from our farmhouse to act as the plot for our new build, whilst leaving an acceptable plot for the farmhouse.  However it is just large enough to do this, and neither plot is going to be generous.  We therefore need to balance where we position the new boundary so as to give the new house a sufficient plot, whilst leaving the existing house with a plot that won't impact its saleability.


     
    The site plan shows a plot of some 26 x 14.5m with the new house 11.5m long sitting in the middle of this.  This is where the garage currently is and between our neighbour's extension (which is largely obscured by a plum tree in her garden) and the plum tree in ours.  We have little freedom in the build orientation or placement given the overall floor space (and which I discuss below).  The plot is within the centre of the village and parallel to the road, so this means that we have quite a few neighbours who could be impacted by the new development – ten in all, though only five have gardens that border the new development (if we include the farmhouse itself).  So we need to be sensitive to the views of our neighbours views and the planners in positioning the new build.

    I've orientated the site plan so the adjacent road and access onto it is at the bottom.  Our farmhouse is to the right; this was first built in circa 1680 and added to in later centuries.  A friend owns the cottage to the left (visible in the photo); this and most of the adjacent houses on the road date to the mid 1800s or earlier and are built in the locally quarried stone (excepting one 1990s in-fill).  On the opposite side of the plot from the road is an estate build in the 1970s on what was originally the farm's land.  Three of these estate houses back onto the plot, so these neighbours will have the new-build at the bottom of their garden.  The main axis of the house is aligned with the adjacent cottage extension, with the trees in its garden breaking the visual line between the cottage and the new build.  The small mature plum will be retained within the remaining farmhouse garden to right, near the boundary and centred on the gable, will also soften the visual impact of the house from the farmhouse.  To the front we have an old stone wall and hedge and along the road boundary with some damsons in the hedge.  There is a 10ft high laurel hedge to the rear.


     
    The planners want the build to be sympathetic to the local style, that is faced with local stone, a slate roof and having a general cottage style in terms of doors, windows, etc.  We agree that this will help the house to sit well in the street context.  We can't have windows in either gable in order to avoid overlooking issues, and so the house has a very “front and back” feel to it.  Also the fore and aft positioning of the house is pretty much dictated by the aesthetics of the axis alignment and the need to balance distance from the rear neighbours and having adequate off-road parking to the front.  Overall, we feel that our proposed position for the new-build sits well within the plot.

    We've had our first round of planning application, which we unfortunately had to withdraw after Highway Agency comments (to do a traffic survey during school term-time and to address some details with the new vehicle access to the farmhouse plot – which I've already discussed in a separate eBuild topic).  However, the overall the LPA feedback in regard to the new-build itself and it's positioning within the plot was very positive.
  23. TerryE

    Our Design Drivers

    By TerryE,

    We've live in a 300 year-old farmhouse with lots of beams, wobbly walls and character -- and the odd draught. It's a large family house and, after 30 years living it, we feel that it is now time for a change. We aren't interested in a Grand Design; we want a modest design that is a good balance of function, of being practical and cost-effectiveness. Given this, our main drivers in selecting our design were:
    Comfort and space. We want a smaller, cosy house with minimal running costs, low maintenance, and energy-efficient; with ample room for ourselves and a bedsit-style bedroom for a son who lives with us; with enough space to accommodate our two other children and their partners when they visit. We estimate we need an internal floorspace of roughly 200m² to achieve this. Constraints on external footprint. Our plot is 14.5m wide, and our house is aligned across the plot because of the overall placement considerations. Minimum clearances to the left and right give us an external length of 11.5m. The LPA also requires that the overall style of the house fits in with the street scene, so must be faced with local stone and have real slate roof. It can have a maximum depth of 6.5m (1m less than we initially planned), though the planner also suggest we could have a rear gable, but again plot constraints (minimum rear separations) limit this to 2m. There are also constraints on the ridge height. I know that we all bitch about LPAs, but in our case our planner is a nice guy and has been very helpful. Yes he has laid down restrictions on how we can use the site, but to be perfectly honest on reflection we agree on nearly all that he's suggested, and the rest was so marginal that it was easier just to go along with him. Our build has to be pleasing to the eye for ourselves and our neighbours; it has to fit in well. For example, we didn't like the idea of restricting the depth and adding the gable, but we found that this works really well when we planned it out and the house is more interesting to look at than a boring Monopoly house shape.  We have really pushed the plot to it's sensible limits in terms of the house footprint and envelope, and now we have fill that envelop to meet our needs.
    External wall thickness. We have to balance a desire for high thermal performance and the impact of a deep wall cross section on the internal living space. When you are working outside-in you realise just how much of the slab footprint is taken up by the external walls: at a U-value of 0.15, the walls already account for nearly 20%. Usable living space has a value to us, so it's just not cost-effective to thicken the walls to drop the U-value further. We also have to go for a timber-framed build as we can't achieve this scale of U-value and have a cut stone skin any other way. The stone skin is a nominal 10cm deep but can be 25% deeper and stone needs backing or a controlled stand-off; we are using SureCav to mitigate the width impact of the stone skin, but a safe overall budget for this is still 17.5cm. A high-spec frame can achieve 0.15 with an overall wall depth of 40cm (though we may still have to go up to 45cm when we finalise the profile). Use of loft space. We aren't going to achieve our target living space on two floors, so we will incorporate the loft space within the liveable environment. However, ridge-height considerations mean that we will have to limit our roof pitch to 45° and we will have ceiling heights less than we would have preferred (2.4m on ground floor and 2.3 on first). Even so, the loft pitch profile will still be tight, so we can't class this loft as full living space; however, it will still prove very useful floor space for two main functions: (a) this give us a good space for a MVHR / equipment / storage room; plus (b) an occasional-use / guest bedroom. General layout. Our thinking is to keep the overall layout simple. The window-less gable constraint gives the house an internally feeling very similar to that of a double fronted terraced layout. We will have entrance hall and utility block in the centre of the house with the living room and kitchen/dinning area straddling this on the ground floor; bedrooms straddling it on the first. The bulk of the services are contained within this utility block. We want a fairly clean modern styling internally, and our main extravagance is that the hall space will be fairly large with the centre void carrying up into the roof space and floating staircases to the 1st and loft floors. Apart from the visual impact and sense of space, this also has a very pragmatic benefit of making it a lot easier to get bulky furniture and equipment onto the upper floors.

    Aligning all of the toilets, bathrooms, utility services on all three floors into a single block significantly simplifies pipe runs, etc. The only "outriders" are the hot and cold feed to the kitchen.  Another design decision was to make the internal fore-and-aft walls which divide the central hall/utility block from the rest of the living space aligned on all floors and load bearing. This eliminates the need for internal structural steel, and (since the maximum floor spans are now ~3.5m) reduces the depth of the inter-floor voids and giving more volume to living space.
      Thermal design. This is very important to us. We are aiming for an overall zero-carbon design, albeit it with some careful payback constraints. This bullet merits it own post which I will do next.
    So the final layout will look something like this.


    Note that we've since decided to abandon the fireplace in the living room (though we'll leave the flue capped and in place to help buyer-proof the build), and the 1st floor void will be carried into the loft.
  24. TerryE
    Thirty years ago my company wanted to relocate my group to Milton Keynes. I was working in the West End, and Jan and I were living in Croydon at the time. We had just started our family, so the opportunity for a paid relocation out of a terraced house in suburbia into a larger family home in the country was just too good to miss. We ended up buying a somewhat run-down farmhouse in a village between Milton Keynes and Northampton. Jan said: “Think of the potential!”; to which I replied “Think of the work; this is going to be a 10-year project!”. Well, it took us nearer to 20 years to finish the place. We were always cash limited, so we did nearly all the work ourselves, but it proved a beautiful home to raise our family: we had lots of room and a large garden. Today, two of our kids have 'flown the coop' and set up their own homes with their partners; one briefly flew, and then returned.

      
     
    A large rambling 300-year old farmhouse might look beautiful, but it is now far too big for us; the house and garden are high-maintenance, and it is expensive to heat in the winter. This burden is only going to get worse as we get older, so it is now time for us to downsize. But where and to what? We have friends locally, and we like area. We want a smaller house, an energy-efficient low-maintenance one, but also with enough space to include a bed-sit for our son, and to be able to put our other two kids up when they regularly visit for their mum's excellent cooking and free booze. However, when we started looking at local properties, the choice wasn't to our taste or they were just too expensive for what you get.

    Our garden has enough space at the opposite end from the house for an infill development – it's now probably the only such plot remaining in the village as all of the other plots, the two timber yards, the bus business, the garage, the DIY place and even the old Methodist Chapel have all been bought up and developed. We have been approached a few times over the years by builders offering to buy the end of the garden, so we thought: why not do this ourselves? We could build a new house for our own use and then split the plot. So at the beginning of this year we started exploring costs and options, and the more that we looked at this, the more compelling the case became.

    We discovered this site during our searches. We have found it – and especially Jeremy's The House at Mill Orchard blog – extremely informative, so we felt that we should also follow his example and write up our experience for the benefit of others who are considering the same path.

    I will cover how our requirements and site have constrained our design choices in my next post.
     
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