TerryE

We have a ~10 year old fridge freezer and a small top-up freezer in the utility, and a large larder fridge in the kitchen. We only commission the small freezer as and when we run out of capacity in the freeze half of the fridge freezer. The fridge freezer has the largest consumption of the three, but that's because it is old. We can't justify replacing it because it is cheaper to accept the slight inefficiency, than pay for a replacement. By way of comparison, our base load is around 150W, and this is for: 4×Asus mesh routers, 3 × RPi, 1 × Laptop; all at near idle. the 3 fridge / freezers when always closed the MVHR the slab circulation pump on a 10% duty cycle. So this is a few factors lower than your numbers. I am not sure why. This is an aggregate as measured by my Smart Meter and as billed by Ovo, rather than using a smart plug to estimate individual device use. My son's computer and TV kit when on adds around 70W on idle and 200W in use (most of the day 🤣). Opening and closing the chiller appliances, kettles and induction hob can easily add another 150 - 200W average during waking hours. Kettles and microwave are high load, short duration. Using the oven for cooking and baking causes a noticeable peak on measured use. We don't have a TV or media centre as we do all of our viewing on a tablet or Chromebook. All of our space and DHW heating, and machine washing are scheduled during the off-peak window.

The Great War poets are my favourite, though I have read a Larkin anthology decades ago. That's not the High Windows that I remember though the lines are familiar Larkin, so I looked it up: This Be The Verse. And nope, Direct Grant grammar school, then Cambridge, then the Royal Engineers before entering the real world. 🙂 My father similarly had an anathema to the use of face cloths that bordered on a phobia. We just wash ours regularly, say about once a week, and because we have multiple ones, that's probably after ½ doz uses. Yes they do harbour microbiota, but so I don't recommend sucking on one, but the moot issue is whether the stale oil and microbiota burden on the skin is reduced or increased by their use. PS. @revelation sorry for this wandering digression, but this sort of friendly interchange between the regular posters is what makes this a community and keeps us helping everyone. 😉

Our total energy actual use for this closing year is 10.9 MWh. All electric, green tariff. That's for a reasonably large detached house with 3 bedrooms (2 with en-suite) and a bathroom on the first floor and my son's bedsit on the 2nd floor. Total internal floorspace around 170m2, IIRC. We are retired so live in the house pretty much 24×7. Apart from heating, and normal whitegoods / cooking, the biggest chunk of our base-load is my son's gamer PC and his two 60+in screens. 🤣 As I said in my OP, adding an ASHP could drop this by up to 3mWh, but I can't make the RoI case when I crank the numbers. Food, booze, rates and electricity costs are about our only major outgoings, with no other maintenance or depreciation to speak of, because we designed the house with zero-maintenance in mind. Unlike the Willis which is a long life resistive heater, an ASHP has a typical life of perhaps 10 years, and well as needing annual maintenance.

Hot showers are far more economical (unless you do them @ToughButterCup style🤣). I am a little anal in my technique: I use about 5-10 ltr for a typical shower: a quick wet down; turn off shower and full body lather and rub down; leave lather on for at least 60 sec whilst flannelling; another wet down; flannel off any water and suds; a final slightly longer wet down, flannel off and towel down. Soap + 60sec contact + mechanical rubbing is better than an immersive soak (or just standing under a deluge shower) for getting rid of detritus, and surface bacteria and fungal spores. That being said, Jan took a tumble a couple of weeks back and she managed to crack a rib and tear some muscle and ligaments. Time is the only remedy for this type of fall. The pre-bedtime bath is better for getting a night's sleep during recovery. We've got a water softener and there's very little crap sticking the the bath. I find a quick rub down with a cleaning flanel gets it off.

I don't have a networking cupboard; just a 20cm deep shelf above my consumer unit and a flush mounted patch panel in the wall with a wall mounted ethernet switch below it. (This is in my utility room so having it all on view isn't really an issue.) I have a couple of Argon ONE M2 RPis (running Home assistant and a general Docker host) as well as my ASUS master router and BT 180Mb modem on the shelf. I also have 1 × Gb eNet to each room (a couple have 2 and in retrospect I should really have doubled these. I also have 3×ASUS mesh slaves (one in my study on the 1st floor, one in my son's bed-sit on the 2nd, and one in a shed covering the garden); these also act as local 4-port Gb hubs. I also have a PoE switch in the shed from which I run my external cameras.

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.