TerryE

My friendly Gemini AI gives me 17 kWh/K so (1) is the right ballpark. However, I can't recall Mike giving details on his slab dimensions and profile. You need some assumptions about the profile, so mine has ring beams cross beams etc with a 10cm fill-in for the slab itself. If you simplify this to a cuboid slab with area 11×7½ then the average depth is 35 cm which is too high. I am at a loss as to where (2) comes from. If the slab is sitting on 20cm PIR then the downward loss would be ~ 120W for a slab at 25 °C sitting on a subbase at 10 °C, plus the edge losses which should be smaller. The amount going usefully into the house is ~ 7×A×ΔT which more in the 2-3 kW pallpark if is ~ 5°C

No, that's me. 🤣

Mike, one thing that you might try is get an idea of the time constant for the house at some point. Heat the house to some set-point and keep it at that for a couple of days, then tiurn off the heating for 24 hrs, say, and track how the house cools. We have low overall U-value loss, but high internal fabric total specific heat ("thermal mass"), so we lose maybe 1°C / per day with the heating off. This means that we can shift the heating windows to pretty much any time that the electricity costs are cheap and make maximum advantage of a ToD tariff. You might find the same, but some build techniques use a smaller slab volume inside the thermal envelope, plus low SH insulation so the time constant is shorter and the house loses 2-3 °C / per day. This impacts your best heating strategy, so it's a characteristic to understand. 🙂

Mike, it looks very much as I'd expect, that is no surprises. I did warn you about the consequences of the 185s before you poured concrete but that conv obviously went into the information overload bin, but this falls into the 'it is what it is' category rather than something that needs fixing. The main consequence is that the flow speed is double what it would have been, with the increased circulation noise, and hit on your pump life. Your return temp is still 5°C more than room temp, so the avg slab surface temp is prob 7+°C warmer than the room env, and so you are dumping ~100W/m2 from the slab into the house, which seems to be heating the house by maybe 5°C per day. That's a hell of a lot for a passive class house, and you'll need nothing like that once the internal fabric is at target temp. You might want to experiment setting the house set-point to say 18°C and let the system reach equilibrium, leave a day or two; then step up the set-point another °C; rinse and repeat. That way you aren't going to be stressing anything too much.

I've had ME/CFS hovering around in my life since my first collapse in my early thirties. Luckily I spiralled out into reasonably good health over the next year and for the next 25 years, albeit with the odd post viral wobble. However, I then had a second collapse in my mid 50s and was bedridden for more than a year and had to retire early. Again 8 years of slow recovery getting to a point in my 60s where we could take on the self build. This really was a sweet spot for us: we had the time, energy and enough capital to take it on. But TBH the build was gruelling and by the time we finished, I was "running on empty" so another ME collapse, and another year+ pretty much bedridden. So doing the build extracted a toll on my health and as I stretch into my 70s I am still dogged by fatigue problems. However we've downsized from an old stone farmhouse full of character (but also high-maintenance / expensive to heat and maintain) to a cosy and cheap to maintain modern passive-class home. At the same time we freed up some capital to help our kids on the housing ladder. So yes, it was all worth it, but it did have a life cost. I couldn't do this again.

+1 to Thanks for the update. We're all growing older. Jan and I are into our 70s now. We moved into our house a couple of months after you did. Overall we are delighted with our house, its environment and performance. We have made one change: we dumped the SunAmps for a decent UVC, and Jan is now keen to do a partial refresh of the kitchen, because some aspects of the design really grate. I hope all goes well with you both, all things considered. It would be good to get any further updates; A few of us are genuinely interested. 🙂

You can get a range of contactors typically with 240VAC, 12VDC or 24VDC coil voltage. They all draw ballpark 1W to keep the contactor closed to the load on the control relay is ~40mA in the case of 24DVC, doubled for 12VDC, and a tenth of that for mains. I prefer using DC simply because all of the control stuff is low voltage which makes it easier to get signed off.

My excuse is that the idiom is a US one. We would probably say "Do the sums". 🤣 I had a shower in my 20°C unheated en-suite earlier. I survived.

None? We have 3 story passiveClass house in NN7. Lived in it for 8 years now. We have no heating on the top two floors. The master bedroom en-suite has a 60W towel rail on a Zigbee-enabled fused spur which automatically turns on overnight for about 6 hours in the heating months. We have an electric oil-filled rad in the hall well that outputs about 1kW and the CH system turns on as necessary in the cheapest (typically) 4-6 hrs on the Agile tariff in the winter months. This tops up the upper floors. Our master bedroom is about the coldest in the house and its at ~ 20°C ATM. It the house as constructed is truly energy efficient then you won't get a 10-15 payback on the installation costs of fancy solutions that you'll never use. E.g. that hall rad costs us maybe £100 p.a. and does the job. Do the math.

I use ½ blanks between my contactors, but a couple of W in a steel CU isn't really an issue. I had probs with my DIN mounted SSRs especially as my sparky had used an abs box. Luckily it was in a large closed cupboard so I could leave the cover off. I prefer my current setup. The RH unit was my sparky's work. The LH contains: 24VDC power, <gap>, contactor 1, <½gap>, contactor 2, <flyback diodes>, contactor 3, <½gap>, contactor 4 The 24V Tasmota 4×relay module is below. The red /black switch loom is all 24V.

@Mike, The power loss of SSRs is appalling IMO. The main issue is that the DIN rail form-factor ones get hot and this can cause oxidation on the power leads from the resistive heaters. Even with crimp covers the oxidation can cause probs. I haven't had any issues with the contactors. No you don't need snubbers on the resistive loads and I don't bother on the AC pump either as this only toggles ½doz times a day. No this issue is with your Shelly Pro 4PM which outputs 4 × wet 240 VAC signals for contactor coil switching. The inductance of the contactor coils is quite high so you can get field collapse arcing when they switch. I don't know if Shelley has internal snubbers, but this is something you need to check IMO

I moved away from using SSRs just under 2 years ago to an ESP32 module using 24 VDC to drive contactors. See p3 and 4 on this topic. My issue with the SSRs was that they had about a ½-1% heat loss in the SSR when on -- that's 15-30 W per relay. This caused real overheating issues with the DIN mounted SSRs. The contactor coil losses are a few W, so absolutely no overheating issues. I prefer using 24 VDC for relay control as this minimised safety issues in the microcontrolled modules. You just need to include flyback diodes to prevent the 24V relay contact arcing / wear. You really need to add snubbers for the 240VAC switching relays for the same reason.

Thanks for poking me on this BTW @Nickfromwales, and I hope that you and the family have a good Christmas. Mike, Do a deep search on my posts on how I heat my slab etc., as I've covered this in depth elsewhere. A few comments / adjustments: As I described in my topic Micro-controller based power switching revisited in 2024, this showed my thinking and eventual rework of my UFH control and switching revised design. This was very much a discussion leading to final solution over the course of this long topic, so read to the end. But the key points were: I replaced the SSDs with DIN mounted CU-compatible 20A 240VAC contactors for switching all power circuits, e.g. Willis, pump, DHW×2. These have 24VDC contactor coils. (The parasitic heat losses of the Contactors are a few W which is a lot more enclosure friendly than 30+W of the power SSDs.) I use a generic COTS relay module powered by 24VDC and running Tasmota firmware; this allows me to use MQTT to drive all controls. These 24VDC outputs have dry contact outputs, which I prefer safety-wise so you should have flyback diode protection across all contactor coils, again see the topic. I use ESPhome firmware ESP modules with remote DS18B20 temperature probes on my Willis, loop out and returns, DHW top and bottom and central hall to aid my sequencing and control. I have my own DIY NodeRED app doing all of my CH sequencing and control, and I also use Home Assistant (HA) as my GUI (linked via MQTT), although the CH app is independent of HA and can keep the control regime running for days without HA input. The system implements various safety interlocks (e.g. the DHW Immersions are powered off once the UVC reaches target temp, the Willis heater is powered off if it gets above 40°C.) The Willis does UFH only, as the UVC is heated by Immersion, so I have no buffer tanks or low loss headers. The heating regimes are optimised for use with Octopus Agile tariff. I have a low-energy, high "thermal mass" house, so in general I (or at least my CH control app) pre-calculates the day-ahead heating requirements and then time-shifts UFH and DHW heating to the cheapest Agile time-slots in the day to buy the electricity at lowest prices, so last night for example we put about 22 kWh into our slab and UVC at about 3p / kWh so under £1 (and another £2 to charge our EV). As I describe in my blog ,the Willis OP gets dumped into our ~17 ton of insulated slab so this heats up slowly over the, say, 6 hr heating period by 5-10°C and the return goes directly into the Willis, so it never goes above 30°C in normal operation. Yes I do have a fairly complex NodeRED app controlling everything, but the electronics and hardware set up is all stack items and cheap as chips.

@Gus Potter I think that you should optimise any design within sensible engineering tolerances (say a 20% margin in the case of the thermal calcs for a self build, if you have proper review in the design process and QA in be construction). In our case, we split our quite large plot which ran parallel the the street and hence we had enough land to create a separate house plot with its own street access. This adjacency, plus being retired, and that we had previously completely refurbished our old farmhouse on the other half ourselves meant that we had enough knowledge and time to keep a close QA on the build process and to prevent any material off-spec deviations from the as-designed build. The as-lived-in performance of our new house as a system is about 15% lossier than the design and that was down to a single design thermal bridging mistake, which we didn't pick up during review, but did during construction and could therefore largely mitigate. As I said we peak at about 2 kW average heat loss mid winter (when the temperatures outside are below 0). I just can't see the point in having an UFH pipe layout that is capable of delivering 5× that or more. Our 3 loop ground floor UFH keeps our 3 storey house nice and warm, so we have no fixed upstairs CH at all. At some point we might consider an ASHP: I already have the pipework and dedicated 40A spur out to the target pad in place, but just not connected at the house end. We aren't eligible for the BUS and we would only net maybe £400 pa. in running cost savings, so we just can't make an investment case for installing one. A home battery is probably the next item on the wish list ATM.

@John Carroll thanks