TerryE

Great minds ... Jan thinks I am mad, but tolerates this one. We use maybe 100ltr in a typical bath and maybe 120 lr in a full one, but let's stick with the 100 figure. We have our baths reasonable hot, let's say 45°C; our rising main is currently at 7°C and bathroom ambient at 21°C so the Δt riser -> bath is 38°C and ambient -> bath 14 °C. By letting the bath cool to ambient we lose roughly 37% heat to the drain and 63% to space heating. The specific heat of water is ~ 4.2 kJ/kg/K so cranking the numbers, we have 4.2×100×24/3600 kWh = 2.8 kWh going into space heating and 1.6 kWh going down the plug hole. In our case we have a low energy house with electric resistive heating and 2.8 kWh is a non-trivial % of the daily heating requirement so this is definitely worth doing. Incidentally: We have SunAmps heated overnight for storage so this bath costs about 80p on our tariff of which 50p is offset against space heating, so the effective cost is 30p (plus the 0.1m³ water). We have a double bath and use this feature (though usually serially these days 😉) so the cost per person is 15p. We have softened water, so bathing generates very little scum and the bath is easy to wipe clean, even if the water is at room temp.

Yup, I talked to the MD of MBC about this. The timber lengths were all PNC cut so were pretty much spot on to the drawings. The prob seemed to be a few of the internal framing uprights were not machine place, so some laterals we being hand trimmed to fit by the assembly crews. They've since fixed this process.

In principle you can work out the transfer function of any control system by putting a impulse or a step change in an otherwise constant input. In practice, doing this can prove almost impossible as the system can be non-linear. For example, most new builds use dot and dab fixed plasterboard on an inner leaf of blockwork. The blockwork usually leaks air like a sieve and you get convective circulation behind the PB which kills any nominal U-values. This convection is non-linear and also very dependent on external wind, etc. One way of estimating spot U values is to use a single pixel FLIR meter to measure external wall temperatures. If the ∆t from internal room temp is x °C then you will be losing 7-10 W/m² into the wall depending on the amount of surface convective flow. The ∆t wall surface to external will allow you to estimate external U-values, and more importantly find any thermal breaches.

MBC used a small number of "regular" subcontract teams who understood the quality issues. I was always amazed by how hard they worked and to a standard that is pretty much unknown in the general UK building industry. We fitted the Internorm windows and doors on the day after the frame was finished. (There was supposed to be a 7 day slack contingency but this all got used up by slippages in the erection team's previous build + 1 day on ours, so the contingency got eaten up.) We had a 3 month lead time on the windows so we ordered the windows according the detailed frame drawings. The window supplier was only willing to do this in the case of TFs using factory assembled cassettes. Their normal practice for conventional block and brick builds was to measure actual openings in the erected skin. Hence our TF was weather tight and lockable less than 2 weeks after there was just a bare slab; whereas convention builds can be exposed to the elements for months. I double-checked all opening dimensions during frame erection and a couple were off by ~1cm. However we had our windows in lined boxes (see this blog entry for a detailed explanation), so in these cases i got the framing crew to pack out between the frame and the liner box. The internorm fitting crew had no issues at all, and as a result they finished fitting a day early because they used none of their contingency.

Back to the OP topic, we've averaged 80% off-peak electricity use over this last cold spell. This time last year it was typically just under 70%. We still have ~12 kWh Peak use per day, so that's around £820 p.a. excess over off-peak rates at current OVO tariffs. With battery prices at around £120 / kWh (and still falling steadily), that's around £1½K for 12 kWh + inverter control, then you have to wonder when battery-based time-shifting of usage will become cost effective.

In our case the air-tightness barrier was done by the MBC crew. The main barrier was an inner "green" OSB3 sheeting (that is with a green gas-tight plasticised surface). These sheets were but-jointed and taped on the TF verticals, with the service cavity spacer battens nailed to these verticals. Having the service cavity in front of the barrier allows all cabling and pipe runs to be run in the cavity and pretty much eliminates the need to breach it for 1st fit. A second MBC crew did the cellulosic fill and completed all taping up around fenestration, etc., then coordinated with the independent tester who did the air-tightness test. So: The whole frame was airtight by design, with design detailing aimed at avoiding unnecessary breaches. Specialist crews who understood air-tightness requirements were used at key stages in the build. The rest of the trades (eg. electrics, plaster boarding out) had a simple set of rules to work to, and these didn't inconvenience their work, so were easy to follow.

One of the things that we did on our build was to ban all trades from penetrating the external airtightness membrane. I did all of the internal joinery / carpentry, kitchen and white goods fitting myself, and Jan and I did all of the plumbing between us. We had electrics, BT, satellite and a propane backup rings, but we discussed all breeches needed with the tradesmen and we put in the necessary conduit / ducting ourselves, doing all of the air-tightness sealing. It was then just a matter of adding foam fill and silicon caps to all ducts before doing the air-tightness test. Incidentally, we got 0.6 on the first run, and I had forgotten to do the final window adjustment. (The tester was amazed.) About a month later, I went around on a really windy day and found that ~5 windows and one door needed the seal stops adjusting, so I suspect that our current actual is a bit better than the 0.6 test result.

Some post build improvements try to make a house airtight without bothering with MVHR. Big mistake IMO, as the net result is a stuffy / smelly environment. The V in MVHR is very important for occupants' comfort: having a fresh environment without bleeding heat loss. I did my own version of J's spreadsheet, but the results are very much the same. All of the material losses are the big ticket items and something like the PHPP calculator just adds a huge amount of complexity on immaterial detail IMO. The devil is in the detail, especially in avoiding thermal bridging flaws in design, and quality cockups in construction e.g. insulation missing or badly installed; lack of attention to detail in air-tightness sealing. I am delighted with the overall characteristics of my MBC house: the build is performant by design and in detail of construction: e.g the passive slab with UFH in the slab; the larson strut twin-wall with cellulosic filler; ... the whole build was low energy by design, and in construction detail. MBC has some worthy competitors, and there are other build approaches, so it's very much your choice. However, I would emphasise that you are better off with a construction approach that is low energy by design rather than a convention build with some attempts to tweak it to get this goal as an afterthought. Also having the high thermal capacity internal to the insulated envelope (e.g. the warm slab approach) gives an extremely stable environment. We've just had our 5th anniversary since moving in and overall and we remain delighted at how well our house performs as a lived-in environment and how it has met all of our expectations.

Our windows are Internorm continental-style. All open inwards for easy cleaning and tilt so like @Conor, none open outwards.

One thing that I have noticed is that we lose maybe 10-20% of our slab heat because of a partial thermal bridge that I discussed in an earlier post. Space heating using the rads doesn't have this loss, and so is slightly more efficient. @ProDave has done a fantastic job, hasn't he? @RandAbuild, about 80% of our electricity use is at the cheap which is currently 17.96p / kWh so our energy cost during this cold snap is around £12 / day. Do whatever you think works for you. @jack, I remember that episode, back from the days when Grand Designs was watchable. I always wonder what the actual as-built performance of these builds is.

@S2D2, the rads that I have are in practice silent. Any convection is passive. And yes they are ~100% efficient -- that old entropy: everything ends up as heat. ASHPs are at a CoP / efficiency of maybe 400% if you have enough buffer to avoid cycling and keep the O/P flow to under 40°C. But as I've said before you only fall within permitted development if you use an MCS certified installer and installation, so in our locale this would work out at perhaps £10-15K plus we'd need to find space for a buffer tank because using a slab as the buffer (as we do currently) is not in the MCS playbook. Not worth it for maybe £1K savings at the current hiked electricity prices. My view is that ASHPs are still on the early-adopter end of the engineering experience curve. They will significantly fall in price over the next few years and there will be more variants to cover different use cases. So there are advantages in waiting this one out for a few years: the real price savings will fully offset any short term increased energy costs.

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.

Yup, something like De'Longhi Dragon 4 TRD40820T Oil Filled Radiator- White where the fins are encapsulated and can thus run safely hot, rather than CYBL20-7 Freestanding 7-Fin Oil-Filled Radiator 1500W. You get what you pay for.

@Temp, yup it's really a design flaw / characteristic mismatch with the market labelling. One would expect a radiator marketed as 1500W is capable of heating at roughly that output. Reading a range of review on Screwfix and Amazon, they broadly fall into one of extremes: (The user only needs ½kW): wonderful little heater ***** (The user only needs more than ½kW): useless; doesn't heat the room * Each only cost me £30 + another £10 for the MQTT-enabled SmartPlug with power monitoring, so I will use both at opposite ends of the landing during the coldest months so I can use my HA system to top-up the upstairs heat using off-peak electricity.

As a codicil to this, you need to be deeply suspicious of any power claims for these radiators. I thought that my old rad was big and clunky so I got a couple of these Freestanding Oil-Filled Radiators at a nominal 1½ kW from Screwfix. My intent was to control them from my HA system using power monitoring smart plugs. No matter what power setting you choose, they only output roughly half a kilowatt. Yes you can toggle between the ½, 1 and 1½ heater options but the heater heats the oil and cuts out when the oil is about 50°C. The oil does a convection circulation through ducts in the fins and hence the fins only heat up to about 45°C or so. If you do the fins have a total surface area of ~1 m² at a Δt ~ 25°C. Radiative + convective emission is going to be ~20W/K/m² so the radiator can only output ~½ kW tops. At the 1 kW setting the heater switches on a 1:1 mark:space ratio; at the 1½ kW setting the heater switches on a 1:2 mark:space ratio. Hence the effective output is always ½ kW. My old big and clunky had external baffling and the (i) the internal fins could run a lot hotter without a surface scald risk, and (ii) this double structure ducts the air through the rad, thus improving specific emission. Double bonus = about 1¼ kW output. 🙄

I did a post on my calibration process. I just bought 20 generics and put 2 batches of 10 DS18B20s in a bowl of hot water wrapped in a towel and ran a small logger to take the temps every couple of mins overnight and until it cooled down. I then did a curve fit. A couple of them had a gradient that was off the average by about ½°C over the ~30°C range so I chucked those. I noted the offset from average for each and my logger offset the reading accordingly. This meant that they were all consistent to within the 0.125°C tolerance.

@S2D2, I don't want to segue into the next blogpost that I am drafting, but thanks for your comments. Our first house was a 1920s mid terrace, so we had effectively zero heat losses through the party walls. What we've got now is a 3-storey detached house. The bar or baseline is very different for different house geometries. I have also been pondering this old entropy lark: how much of our energy input doesn't end up in adding to the fabric heating> The answer is two shades of bugger all. For example, our DHW waste outflow does run out of this environment representing a partial direct heat loss, but this is relatively small as we dont use that much HW, and as our pipe runs flow through the warmslab before dropping into the waste flow under it; so 10s of ltr per day @ maybe an average of perhaps 30°C. Radiative losses are small with our windows, but quite honestly on the other hand our solar gains are bugger-all on our net heating days for our house aspect and window geometry. Pretty much everything else eventually heats the house fabric.

I've only been collecting external temperature data for the last three years, so I have been using this plus daily peak + off-peak values to examine correlations between these. OVO has recently updated its logging query API, so I have updated my data pull NodeRED routines and have repulled the data from my last 4 years of fixed-price E7 usage, daily actual meter readings (which is what is used for billing), and the reported daily and ½ hourly usage. (The old API reported usage incorrectly, basically because the dual tariff is based on 0-7 AM UTC for off-peak and a 0:00 UTC split on daily billing, but the OVO portal uses UK timezone for reporting and the current version still botches the hour to 1AM during daylight savings.) However this is only a reporting issue and not a financial one, since this bug doesn't impact meter reading actuals. Returning to the data, the strongest trend is shown by the external temp vs daily total power use scatter plot, which fits to P = 60 - 2.45T, that is each drop of 1 °C in average daily outside temperature requires an extra 2.45 kWh heating. I also checked this against my pre-build design calcs which predicted 1.92 kWh, i.e. the as-built house performs about 25-30% worse than as-designed. (See my original post for the likely reasons for this off-nominal performance). Maybe disappointing, but still factors better than a typical new build. Realistically, given that we have an electricity-only home, I can do little to change this line. As I have said, I can't make the investment case for installing an ASHP at the moment so I have to live with a CoP of 1. The main behavioural handle that I have would be to drop average house temperatures a degree or two. The other handle that I have is to control the unit price that I pay for my heating, this is by absolutely maximising off-peak use, and have a blitz on avoidable peak rate use, but I will do a separate post on my plans here.

Why not? The house was secure with a turn of a key from day 1. It was also weatherproof, so the slab and everything else could slowly dry out. It was about 9 months before we plaster-boarded and skimmed then another 4 before we decorated. Five years after moving in, and not a single settlement or other crack in our entire plasterwork. The polythene protection meant that we had no scuffs or other damage to our aluclad windows and doors. Also if you have read the blog then using Continental-style inward opening windows and hiding about 75mm of the frame behind the stone skin gave a visually pleasing thin-look frame that mirrored traditional window furniture rather than contemporary frame which look like they've as much wood framing as glass.

We fitted our windows and doors immediately following the last day of the frame erection, but we had ours standing out into the 50mm airgap with the skin lapping over the front of the windows. We stapled heavy duty polythene sheeting over the windows to protect them when the building we going on and carefully cut this out as one of the finishing off jobs once the scaffolding was down. See my blog for profiles and more details.

It surprised me too. 🤣 Jan and I are retired. My son is only working a part week ATM, so we are in the house pretty much all of the time and my son effectively has his own bed-sit on the top floor. So we have 1 freezer (plus a small supplemental one brought on when needed); three fridges; 1 DW, 1 WM; our cooking is electric / induction; SunAmps for HW; MVHR and slab recirc; 1 gamer PC and 2 laptops on; tablets and mobiles of course, and 3RPis. All lighting is LED. The fridge/freezer in the utility is old, so it has a relatively poor energy rating, but it is not economically worth replacing. It seems quite easy to sustain 600-750W baseload for this lot, and that equates to 15-18 kWh /day. 😢

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.

@ProDave, Accredited Installer route or not, we would still need to comply with the noise requirements complete with calculation sheets. We would need a BInsp to sign off on this and the danger that the assigned BInsp might just demand an accredited installer do the worksheet. Plus all of the fees of course. Also since the ASHP would only be 1.2m from our boundary fence and 7m away from the neighbour's nearest opening fenestration, we would need a quiet ASHP such as the Ecodan 5kW. This with the controller comes in at around £4½K. But another is that our house (even with the poorer as-built performance) is still factors out the operating regime and templates that most ASHP units and installers are used to. We need the equivalent of 1kW continuous heat at somewhere between 30-35 °C (max) into the slab during the coldest months (or say 8×3 kW if we want to use mostly E7 low rates). It is a huge thermal store and so will soak this up happily. My algo for this is quite simple: set the O/P flow temp to something sensible, say 32°C if can be set that low, though putting a decent PHE between the ASHP and the UFL circuits might simplify this gearing; turn on the ASHP demand; integrate up the delT out-return on the UFL to calc the heat dumped into the slab and turn off when the desired total kWh reached. Need to think more about the peak variation causes, but I'll reply separately on that.

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.

Most commonly Microsoft Certified Solutions Expert but in this case a Microgeneration Certification Scheme certified engineer. The installation must comply with MICROGENERATION INSTALLATION STANDARD: MCS 020. In essence this determines the maximum noise level at the neighbours boundary. This is normally achieved by using a certified engineer to sign off the installation. The Planning Portal states that planning permission is not required if ASHP "equipment must be installed by an installer who has been certificated through the scheme using a certificated product" -- that is self-certification is not permitted. It also lays down other limitations. All other cases must be covered though a planning application, but whether your BInsp will allow self-certification is another matter.