TerryE

I've just done the query. The daily averages (rounded to nearest kWh) for Sep..May are 2, 16, 17, 23, 25, 22, 19, 9, 5 kWh or 4,148 kWh total. I also run a small oil-filled electric heater on an overnight timer in my first floor study Nov-Feb with the hours per night depending roughly on the average external temperature. I estimate that this adds another 480 kWh so the total heating load is around 4,700 kWh with about 90% at E7 tariff. I just had a look at my "JSH style" initial estimate. The main error that I made in this was due to my assumption that all other electricity use ends up as waste heat which for heating days also warms the house: our new house electrics are more energy efficient so the base load is less than I estimated, and therefore we need more top-up. I also used a temperature set-point of 21°C rather than our currently preferred average which is around 22.8°C. Adjusting for both of these, the new estimate (to my amazement) was also 4,700 kWh . On reflection, it looks like my 10kWh mentioned above was just a measure of the thermal banking in the main ring foundations within the warm slab. It looks like the mitigation did its job. TL;DR: our as-built thermal performance is a good ballpark of the as-designed. I am currently on an OVO 2 year fixed tariff at 9.19p and 15.81p per kWh + VAT, so totals roughly £490 for my annual heating cost. DHW, cooking and all other use is on top. Given that I use little or no heating for the 4 summer months, I can cross check my total annual bill against the annualised cost for these summer months and this is consistent with this figure. Incidentally I estimate that switching to use an ASHP would only save perhaps 50-60% of this (at an average CoP of 3 say, but more peak rate tariff), say £290 p.a. That's why I find it difficult to justify the cost of installation of an ASHP. I am going to switch to an Octopus ToU tariff but the process is that you have to switch to Octopus on an interim single rate tariff until they switch your smart meter, so I am waiting until April because I want to take advantage of the OVO off-peak tariff in the meantime. I also have no components in the system requiring annual professional maintenance so I have no annual maintenance bills.

@Stones Jason, I'll need to run a SQL query to give my annualised actuals. I'll post back later. My target setpoint is 22.3 °C, but the way my algo work this is more of a typical lower bound and the house cycles between about 23.3 °C at peak which is around 11:00 to a minimum at 01:00. Of course we could go lower but Jan like to wander around barefoot in sleeveless tops. However on the actuals, one thing that we do know is that a thermal bridge in out slab (I discussed its mitigation in this blog post) adds maybe 10 kWh to our daily slab losses for the ~6 months of net heating -- say £150 p.a. on our energy bills. We can estimate that by switching form UFH to space heating for a few days and letting the slab slowly chill down maybe a degree or so during this window. Annoying but we can live with it.

IMO, never use an external vendors web service to control any internal infrastructure devices.

Ah, the tragedy of the commons. It seems to me that there are two broad classes of self-builder (i) those you are using their personal efforts to maximise the house they can get for a cash constrained investment; (ii) those who are self-building primarily because they don't like the pre-built choices on offer and want to have design control over the house they want to live in. Whereas the first category might want to repeat (and improve) the experience, the second will tend to want to make the most of their "dream house" and so will have different payback trade-offs to make. In our case, we wanted a warm, cheap to run, pretty much zero maintenance house that would last us through our "3rd age" (as the French call it). BTW, in our experience designing for energy efficiency didn't add materially to the build cost. The big premium was as a result of LPA imposed aspects like using locally quarried hand dressed stone for the skin and a slate roof because they wanted the house to fit well in the local context in the village (mostly 1700s and 1800s stone houses). However the fact that our plot was a "freebie" calved from our previous large garden more than compensated for this and the result was a fine looking house that is a pleasure to live in.

The price would be an issue for me as there would be no way to generate a realistic cost-benefit payback period at our level of use. In our case, it is easier and cheaper to take the hit for the resistive heating costs which we can optimise on a green ToU tariff.

@Bitpipe @daiking thanks for the links; the more suggestions, the better for anyone readings this. The main benefits of these pre-tapped systems is that wiring them up is simple and if you have flexibility in layout (e.g. laying them out in a bed) then you can space them out or lose the excess cable by snaking and can thus working within the fixed spacing. The disadvantage can also be this fixed spacing. In our case, I am fixing thee spots to fence posts at 2.4m centres so I have a fixed layout and I don't want take-up-slack loops visible. I also plan to drill a hole through each post and run the main backbone straight along the Arras rail line. Hence for me, the wiring system which uses a bare SPT1 backbone with clamp-on taps is preferable, with the less visible joints the better. I am using a Zigbee controlled socket as I can control this directly from my Home Assistant system without and dependency on an external cloud-based service. I have a couple of Zigbee motion sensors, so my HA system can set a dusk to midnight + motion triggered on schedule.

Just a quick update on this. Jan and I have been bouncing this one around for the last few months. What we really want is a reasonable cover of "full moon" light levels or thereabouts along the boarder of our driveway and in our side passage -- enough to see the way to the bins or down the side of the house without tripping over obstacles en route. We live in a village and even through we live on the main street through the village, the general street lighting is poor and so there is almost no indirect light into our garden -- effectively zero if you are entering the garden with eyes adapted to indoor light levels. I've decided on 6×12V LED wall lights, 4 on posts along our front and a couple in the side passageway. 3W lights should give this full moon or better visibility and avoid dazzling our neighbours. It seems to me that many of these 12V systems are overpriced and over-engineered. For example one system that I looked at had fixed T taps at 2m spacing and each lighting spur required 2 more connectors and a min 1m cable to the device -- not much use when are fence posts are spaced at 2.4m and I want to run the cable across the fence under the top Arras rail with the light about 20cm from the cable. We've decided on a 12V garden system from Luxform. What I like is that this uses easily replaceable GR3.5 LED bulbs and simple close-on connector system driven by a simple 240V-12V transformer- based PSU. I'll use a Home Automation-controlled switch to control time-of-day and motion switching. I will post back an update after the system is installed and operationing.

Re UFH, you need to distinguish loops vs zones. Keep your individual loops as close to the same length as possible and < 100m. So we have 3 × ~100m loops (IIRC the shortest is 92m) all drawn off a 300m coil. 4 × 75m could have worked as well. Don't have any joins in your UF pipework below slab. The reason for the suggested 100m max is the head needed to get the necessary flow. I used the delta T across each zone to trim the flow rates and balance them, but once balanced, like @joe90 and @Pete, I run my 3 loops as a single zone. There's little point in doing anything else in a passive-class house, IMO. Ditto no point is trying to dick around with time-of-day temperature strategies. In terms of JSH's spreadsheet, the obvious Google search gave it as the 1st hit: Fabric and Ventilation Heat Loss Calculator, though IMO, you could dump most of the complexity about the under-slab soil temperature as it is going to be a pretty constant 12-14 °C for most UK insulated slabs that are in direct contact with the soil. Ventilated are another issue. Yup KISS - Keep it Simple Stupid.

We much +1 to @Bitpipe's comments. MVHR gives you lots of fresh air in the colder months without having to throw heat out of open windows or trickle vents. Brilliant -- but that doesn't stop us opening windows when it is warm enough. IMO, the only good argument for not doing this is if one or more of the occupants is allergic to pollens etc. You really need to think of the house as a system and understand where your relative thermal gains and losses are. IMO, the PH and SAP spreadsheets are just too complex to do these sorts of trade-offs. The sort of simple spreadsheet that Jeremy Harris or I developed is plenty detailed enough. The whole build process is one of trust(-ish) but validate and verify at every step. You need to understand potential thermal bridges and make sure to avoid them when practical. You need to be sympathetic to the fact that UK building trades in general have little or no understanding of the importance of the detailing needed to get good thermal performance, but this doesn't excuse sloppy workmanship. You either (i) need to pay the premium for a subcontractor that does have this understanding; (ii) validate and specify at every step, or (iii) accept that you will be disappointed with the as-built performance of your house.

As @Iceverge says, I have a reasonably large stone clad "cottage style" build to passive standards. As I said recently to @shuff27 when showing him the build, I can't think of any things that I'd tweak if we were doing this over let alone fundamentally change. Have a look at my blog.

Or a Jacuzzi, or more specifically not for DHW or conventional CH. It also has a flow inverter so it can cool as well as heat. Given that I only need <30°C for my slab, and I can't make the payback case for a premium ASHP, I must admit that I am seriously considering one of the rebadged Chinese imports for my UFH application.

That's quite a subjective view really. The hall way can act as the focal point unifying the house. We've got a fairly tight footprint and space usage was quite an important design factor, but we still have an open hall way going up all three storeys in our house. My wife and I think that this is one of its most effective design features. It pleases us and surely that's what matters.

@shuff27 has ridge height limits and this has resulted in the imposition of dormer-style windows. However I don't think this would preclude small Velux-style windows in the bathrooms. The LPA also has a think about large picture-style windows on the principle elevation, and overlooking neighbours, so increasing window areas might prove problematic.

Re the issue of cycling boilers, let's say you need 2 kW and you have a 12 kW boiler, this the boiler will be on:off on a roughly 1:5 cycle. Boilers need to be on for a decent block at a go, say 20 mins, this means that you will need to be off for 1h40 between each on and you will need a buffer tank capable of storing ~ 5kWh heat.

The issue here is gaps in the mortar. Let's say behind one board you have a gap near the bottom and another at the top, so you get cold air coming in at the bottom, being nicely heated to say 20°C by that nice board radiator and cycling back out at the top crack. The net result is that the void behind the board is cycling heat into the void between the brick and blockwork courses. I've seen horror story FIR walk-arounds where external walls are 10° colder than they should be because of this. You can't tell it is happening until the house is built and fully heated and by that stage the cost of remediation is prohibitive. I've also seen cases where it is clear from the FIR camera that whole areas of cavity insulation have been omitted. If this happens, by this stage what realistic remediation or recompense do you have from your builder? As @joe90 says, one option is to parge the wall with a cement slurry to seal the blockwork properly. Another is to go over it with a "fine tooth comb" before it is plaster boarded out, but this level of inspection is skilled and tedious.

Consider a double wall between the office and the master bedroom with the void split say 70:30 as built-in wardrobe for the MBR and shelving for the office. Such built-in features are part of the fix of the house and are VAT zero-rated. They also work much better than free standing equivalents, IMO.

+1 on the main issue with dot & dab being air-tightness. Builders tend to botch this by using the plasterboard as the main air-tightness membrane, and if you've got any cracking in or voids in the blockwork mortar lines (which you nearly always do) then you get convection cycling behind the plasterboard which just pumps heat out of the house. There are some good FIR camera walk-arounds of which show this. Far better to directly plaster or alternatively seal the block work and then batten out to carry the board. We keep our house at ~ 23 °C and I accept that we may be paying a 15-20% premium for this but that's 15% of a small bill anyway. The problem of 0.18 vs 0.12 is a more subtle one of tipping points. We loose minimal heat through our walls so we don't have or need any installed form of CH on the upper floors: no radiators, boilers or CH plumbing and this saved a lot of installation complexity and cost. You should use a Jeremy-style heating spreadsheet and have a play with the parameters and sensitivities and see what the correct mix / trade-offs is for you. Having a truly energy efficient house (if done properly) doesn't add materially the overall costs because of the consequential savings that you can make. Architects will design crappy performance houses, builders will build sloppily because that is what they are used to, and building to an energy efficient standard is just an inconvenience for them: this is the culture of the UK construction industry. But you will need to live with the consequences every day after you move in.

Turning this on it's head, why has your architect spec'ed such a crappy profile? Another layer of Dritherm would drop the U-value to nearer 0.11 W/m2K for minimal extra build cost albeit with a 470mm wall profile. It seems that architect's are just so f***ing conservative to be unbelievable. See @tonyshouse blog. Even so, you can't make any as-built thermal performance predictions until you have an air-tightness and MVHR design.

We've got some bad creaking spots in our 1st floor. Nothing to do with the anchoring of the flooring to the EcoJoists, and everything to do with the cross-joist brace only being nailed across the joists and not properly screwed in. Missed this until after we moved in and by then post second fit, so screwing the cross-brace to the EcoJoists would have been a total PITA. Workaround: avoid stepping on the small ~ 1m2 area which creeks . 20-20 hindsight, and I wish that I'd fixed that cross joist properly ?.

You need to do the thermal calcs. On a new build, you can get the wall / roof / slab U-values 0.15 or better quite easily IMO, and for minimal cost. The downside is wall depth needed, but if you plan this in, then there isn't a material cost -- except that it make makes it a little more inconvenient for your building crew -- hence you need to focus on quality assurance and avoid construction flaws (like sloppy or omitted insulation installation) which cause thermal bridges and will compromise the build. Next up if you have a thermal shell of this spec, are the air-related losses which will dominate your heat budgets, so you also need a level of air-tightness and MVHR. My personal experience of building a house of this class is that the internal heat flows are 10× greater than interior to exterior losses. The bottom line if you achieve this is that you will need to top up heat -- e.g. through a in-slab UFH installation -- but that you don't need any other CH installation or rads as room to room temperatures will only vary by a degree or so. (In the coldest 3 months, we also boost the heat for the top 2 floors by using a small oil-filled electric heater on a timer switch in my office on the 1st floor for a few hours overnight.) Overall, the whole house maintains a 22½-23°C temperature 24×7. We use a Willis for my slab heating, and I can't make the cost-benefit case to install an ASHP as I won't recover the installation costs over a 10-year payback. However, that is because we have a high-spec passive-class house. IMO, an initial install of an ASHP is a safe option for most reasonably energy-efficient new builds.

I agree that fitting two is good idea, but if you have a properly insulated slab then it will quickly get far too hot if pump 6kW into it.

Air tightness and MVHR? Thermal bridges? You have the sums using a basic heat balance spreadsheet as per Jeremy's linked above. What is the average outside temp for your winter quarter? Ditto daily heat loss? Plug in the numbers and per kWh electricity costs. Can you live with this run rate?

Something like this: 12000 BTU AC. IMO, using something like a Willis is part of a package: Wall, roof, floor U-values 0.15 or better. No major thermal bridges. Triple glazed windows and doors. Airtightness 1.0 ACHP or better MVHR with ~90% heat recovery. If you use a Willis solution, there are two separate Qs Will it provide enough heat to do the job? Will you be able to afford the monthly bills?

@bobh there is always a trade-off between upfront costs and run-rate ones. The Willis approach works for us because: My house is perhaps 10× more energy efficient than our previous one, so the total energy input we need to keep the house at a comfortable temperature is small. (e.g. the Willis ran for about 2½ hrs last night at E7 rate). I use an RPi to control how much heat and when I inject it into the slab, and so I can make most use of cheaper tariff rates. I am about to switch to an Octopus agile tariff to drop these costs further. However the weakness of this approach is that a Willis works at a CoP of 1, so you have to pay the full electricity unit price for every kWh that you use. If you can't afford an ASHP and you only have ~100m², then have you considered an air-to-air wall mounted unit? You can get these for around £500 and up for a 12000 BTU (~3 kW) unit that will run at a CoP of around 3.0. @Jeremy Harris did a topic on the one he installed. A bit noisy, and one room only but it is a cheap way to pump heat into a house.

@bobh, the Achilles heel of this approach is that at is core it is using resistive electric heating at say 9p / kWh (off peak) or 15p+ / kWh peak / single tariff to pump those kWh into the environment just like those 1970s storage heaters. Resistive = a CoP of 1.0 unlike an ASHP which might be 3-4x as efficient or gas which runs at maybe a third to a quarter of the unit price per kWh. The reason it makes sense in my case is that my house is maybe 10× more efficient in terms of heat conservation than a typical build. Yes, is is very cheap to install, but the run costs can be prohibitive is you don't have an energy-efficient build. Do the sums before you commit to this path.