TerryE

In my topic Modelling the "Chunk" Heating of a Passive Slab, I discussed how I used a heat flow model to predict how my MBC WarmSlab heated by UFH + Willis heater would perform. What I wanted to do in this post is to provide a “6 years on” retrospective of how the house and slab have performed as built based on actual data that I’ve logged during this period, and to provide some general conclusions. In this, I assumed 15 mm UFH pipework, but we actually used 16mm PEX-Al-PEX pipework with an internal diameter of ~13mm. At a nominal flow rate of 1 m/s, say, my three pipe loops in parallel have an aggregate flow rate of 0.4l/s or 1.4 m³/hr. At this flow, a 3kW (2.88 kW measured) heater will raise this stream temperature by 1.7 °C. However, when I commissioned the system, I found setting the Gunfoss manifold pump at a high setting (roughly equivalent to this flow rate) gave a very noticeable circulation noise in the adjacent toilet, so I tried the pump on its lower settings and found that the flow was almost inaudible on lowest one with in to return delta at the manifold still only about 5°C, so I stayed with this. The actual as measured delta for two loops of 4.9°C and the third slightly shorter loop of 4.1°C (close enough not to bother balancing the flows out). This corresponds to an actual flow nearer to 0.4 m/s or 0.56 m³/hr by volume. When scaled to adjust for this lower flow rate, the actual measured temperature profiles are pretty close to those modelled. I measured the actual Willis heater’s heat input as 2.88kW. In analysing the actual slab heating rates, I found that this raises the overall slab temperature by some 0.45 °C / hour after the initial start up. Plugging typical specific heat and density figures for the concrete, this is empirically equivalent to heating 25 tonne of concrete (Cmass = Ewillis/ΔT/SIconcrete = 2.88*3600/0.45/0.9 kg), or 10.6 m³ concrete by volume (23000/2400 m³). In the case where the Willis provides heating for the full 7 hour off-peak window (just over 20 kWh), at the end of this heating period the flow input to the slab is +9 °C above the initial slab temperature and the flow return is +4.4 °C. The temperature of the concrete immediately in contact with the pipe will follow this same gradient, with this temperature excess decaying radially away from the pipe centres. By the end of this heating window at the slab surface, there is barely a noticeable difference in the measured temperature of the floor above the out and return UFH pipe runs (perhaps 1°C). These temperatures and gradients are also comfortably within the reinforced concrete’s design parameters. As soon as the Willis is turned off, the internal temperature gradients start to flatten and any unevenness redistributed across the slab; the rebar reinforcing has a thermal conductivity 60 × that of concrete and this accelerates this, so that within an hour or so of the heating turning off, the overall slab is left about 3.1 °C warmer than at the heating start time (actually about 10% less than this, as the slab has already started to dump heat into airspace). In my original modelling topic, I mentioned that my passive slab has ~73m² of concrete 0.1m thick (~ 17½ tonne of concrete with another ~10 tonne of perimeter beams, cross bracing and steel rebar, with the UFH runs laid in 3 × ~100m long standard “doubled back” spirals (common to most UFH designs) on ~150mm centres and roughly 50 mm below the slab surface. (Actually only 75% of the slab is covered by the UFH runs, because of the need to avoid proximity to ring beams, partition walls, areas under fitted cupboard areas, etc..) Nonetheless, this empirical 25 tonne figure is still consistent with the total volumetric 27½ total estimate if we assume that the rebar is effective at spreading heat through the wider slab over this multiple hour timescale. In conclusion, based on this modelling and observation: First recall our context: our house is near passive in class with a lot of internal specific heat capacity. We only need about 1kW overall heater input in the coldest winter months to maintain overall heat balance, e.g. either by a resistive heater such as a Willis or an ASHP. IMO, there are two extreme approaches to house heating: (i) “agile” tracking of occupancy patterns so the living spaces are only heated when and where occupied; (ii) a 24×7 constant comfortable temperature everywhere within the living space. Our warm slab design is very much optimised for this second case, and our slab supplier did a good job in designing an UFH layout to match the slab characteristics to this The slab is covered in “doubled back” spirals with each loop using up a full 100m roll spaced on roughly 150 - 200 centres (and avoiding partition walls and cupboarded areas) so that each heats roughly 15 - 20 m² slab. In our case three loops were enough, and there was no advantage in trying to squeeze in a fourth. Our 3 loops will happily take up 3 kW heat input. Circulation speeds between ⅓ - 1 m/s seem to work well, with the only real difference being the slower the flow speed, the higher the delta between in and return temperatures. The slab does just as its trade name suggests: it can be treated as a huge low temperature thermal store, but because of its extremely high thermal inertia, one that is not rapidly responsive to heat input. In our case, a heat input of 3 kW input will only raise the slab temperature by 1°C over a couple of hours, and radiating 1kW will drop the slab by only 1°C over roughly six hours. In a true passive class house, one key to heating economy is the high level of thermal insulation coupled with a substantial internal heat capacity. Trying to drive such a house in an agile manner is a fruitless exercise, so forget the traditional having room-specific thermostat control; forget having traditional time-of-day heat profiles. It is far easier to treat all ground-floor rooms as a single thermal zone to be kept at a roughly constant temperature. In my view, using a resistive heating approach (such as a Wills heater) as well as an ASHP can both work well. In this second case something like the 5kW Panasonic Aquarea ASHP would be a good fit as it uses a modulated inverter compressor so it can heat the slab directly without needing a buffer tank. The choice is a trade-off between running costs vs. installation costs. In our case, switching from a Willis to this type of ASHP would save me about £600 p.a, in electricity cost, so I would really need to do the install for a net £ 3-4K to make the investment case feasible. However I would like to defer this discussion to a separate thread because there are other issues that such an approach would need to address.

We pretty much need two threads here covering the use cases of: The likes of me, @MikeSharp01, etc. who have (near) passive-class houses with only limited heating requirements of say 20-40 kWh heat input / day during winter months. Here a 5-8 kW range ASHP would be more than adequate. Here a typical warm slab where the heating loops are embedded in a min 75mm concrete + other concrete ribbing and structural beams can easily soak up 5kW heat, say, at a ~10 °C delta between average slab and circulating water. Also remember we only need an average of 1-2 kW input over the day. In this specific usecase the buffer tank is functionally redundant, and really only required for insulation templates optimised for typical house installations. My RPi-based control system which computes daily heating requirements based on external weather forecast and actual average house temperature, and schedules heating blocks during the 24hr period to input this into the slab works well here. More legacy installations such as @oranjeboom's, which IIRC is a reasonably efficient extension on a more traditional house. His heating calcs in this case are even more complicated by the (lack of) EWI and there are all sorts of wrinkles here that need addressing. I suggest that we keep this topic to orangeboom's issues and discuss these warmslab / passive templates on a separate thread.

We are in a similar position, in that I am now probably spending about £700-£800 p.a. excess by using the Willis heater at a CoP of 1 rather than an ASHP which should give a CoP of ~4 at 30 °C. In our case we need about ~30 kWh heating during winter months and I can spread this over the day so I could probably get away with a 5 or 6 kW heat output unit, though these are quite difficult to source. We also pre-laid our in/out heating pipe to the pad where we would place our ASHP. Like you, I see no point in having a water buffer tank as I don't have the groundfloor space to house it, and we already have a 70 tonne buffer warm slab. I would rather directly circulate through the slab or poss via a PHE. I would also prefer to directly drive the ASHP with my own on demand rather than have my control system fight some fancy predictive ASHP CS that is optimised for a conventional house. You need to be aware of planning issues here as you only get a planning consent waiver if the system is installed by an MSCE registered installer who would probably be unwilling to install a config that you want. Lots of complexities and gotchas here. And picking up @joth's point 👍. In my case I would use my existing calibrated slab model the measured out to the slab vs return flow temps to calculate the actual heat input from the ASHP and do a decent block during E7 overnight plus a couple of extra say 2hr heating chunks during the day (if needed) at a ~30°C output set point. It doesn't really matter if I am ±20% on any day-by-day basis as this only causes maybe an extra ¼°C or so ripple on the overall house temperature. My job over winter is to research all of this and make a call on the way I am going here.

Do the cut and fill calcs. If you are digging out 10s of m³ of sub soil then it has to go somewhere. You need to calculate the final levels to make sure the profiles make sense. Also remember that you want to keep topsoil on top In your garden areas, so you might need to strip off and heap topsoil before spreading the subsoil. If you have a large plot and existing gradients then you might be able to lose the excess on site by terracing. But also remembered that your site might not be able to carry the excess. Check your planning submission site levels, street scenes etc. You might need to run the gauntlet of PP changes / risking running foul of P Enforcement, if they decide you are raising levels unacceptably. It might just be cheaper in the end to pay for off site removal. We didn't have a basement, but had to drop a large part of our site by about 0.6m to achieve roof lines, etc. That was about 20 × large truck loads. Luckily a farmer in our village was doing culvert backfill and took it all, so a 10min round trip and no dumping costs.

We have an MBC twinwall with 300mm blown cellulosic filler between panelvent on outside and coated OSB3 racking on inside, with battened out service cavity inside that. We use 2 × 110 foulwater piping for through-slab main ducts, but we added then added our other penetration ducts after the racking out was done, and most after filler was blown. We use a standard process which my wife and I did ourselves (not one to delegate): We decided on the placement and o.d. of each duct depending on the requirement e.g. external lights, satellite cable, ... these varied from 20-40mm. I then used a 60mm × 15mm masonry bit to drill a pilot hole in-to-out and then used the appropriate hole cutter inside and out to right-size the opening by pulling the masonry bit back to on the opposite side to give clearance for the cutter but other than that, returned the bit as a through guice to avoid losing the hole. I then cut a piece of the correct o.d. abs pipe to about 100mm longer than the hole, and notched on end with a multi-tool to form a simple cutter. This pipe could then be slid over the bit, then hand twisted / pushed to core the hole through the insulation using the 60mm bit as a centre guide. Once through, each pipe was taped to the panelling and siliconed to seal, then multitooled to leave a ~10mm flange. Once the cable / service was in place, the pipe void was foamed and siliconed to make air and moisture tight. The whole process once practiced took about 30 mins per service opening.

@embra, sorry for the delayed response. If you look at the top pic, you can see how we handled the door thresholds. We cut a 50 mm slot at the top of the EPC insulation / formwork and then added an extender to the planned depth of the span. The guys added rebar in this and tied it to the ring-beam rebar so that the slab pour also flowed into these cut-outs, leaving a cast 50mm deep reinforced "ledge" to carry the door. Well, "carry" is really the wrong word as the Internorm frames were also supported on the sides and top in the same way as the windows, so the door base carries minimal load. You will also see that these door cills were also slated externally to finish off; under the slate, we had a 50mm upright EPC at the edge of the cill to act as a thermal break between the warm slab and the external skin. OK, there is a small thermal path between the cill and the slate, but even in the depths of winter we don't notice any significant temperature drop inside the doors. I did do a spot-thermostat survey the winter after we moved in, and the floor immediately by the doors got down to around 15°C -- enough to notice on bare feet, but well above any condensation threshold.

Not much to be done about that apart from shopping around online. We got a far better price than my builder got with his full discount from TP.

The socks worked well for us, but we had a TF inner. I'll defer to @nod on this one since he's the one with brickie expertise. Even so it all depends on how particular your brickies are. Some are very clean and hardly drop any snots into the gap, and some couldn't give a toss. In this last case, snot build up above the DPC can be a real problem and cause a dampness bridge if not properly mitigated, IMO. In our case we lived adjacent to the build site so we could clear out the socks and reposition them each night after the workmen left for the day. This meant it was zero hassle for them. You just need to agree a workable approach with the tradesmen onsite and check that they are following it. Are you filling the 200 void with insulation? Maybe using some runs of ABS or metal gutter resting on the tie rows might work just drill a hole in one or both ends and tie a cord to it so you can pull it out if it does fall.

IMO, this whole issue of where to place Air Tightness Layer (ATL) and Vapour Barriers (VB) is not settled in terms of proper scientific investigation informing building advice and guidelines, so at the moment there is no right answer. Note ATLs and VBs are not quite the same in that a VB is breathable but generally moisture resistant. As I said in an earlier post, our MBC house adopts the approach of having a minimally breached ATL as a racking plasticised OSB layer on the inside, allowing to frame and insulation to breath outwards. @ggc's seems to flip this. As I said , this isn't settled so we can't say one is right and the other wrong. As I see it, the MBC approach has the advantage that the temperature gradient from inside to out goes uniformly from house to ambient temperature to the relative humidity gradient goes from low to high helping to keep moisture out of the frame itself. But I might be wrong here. However I can make some observations. If you want an airtight (+MVHR) build then you need to have a robust and simple strategy / design for ensuring airtightness from design through to build completion. Tradesmen will not understand this and can easily compromise it. So you need a simple rule to enforce. In our case there is a 45mm service void in front of the ATL, so all wiring, cabling, plumbing, etc. was run in this void; flush to plasterboard pattresses sat in the void, and we personally fitted any through viod ducting. Tradesmen were not allowed to break it. If you don't do something like this, then your as-built house will end up leaking like a sieve. Any twin wall void needs to be breathable on one side. Double ATLs are an absolute no-no, because if there is any water ingress then it has nowhere to go; it can't dry out and rot will set in. Someone on the build must fully understand this strategy and police it. This could be you, a PM or an architect, but this needs to be done preferably on a daily basis, because even with the best intentions some tradesman will make mistakes, and these need to be picked up and remedied before the mistake is hidden and buried. All of the above also applies to the thermal insulation design, thermal bridge prevention, etc. Same arguments apply: Maybe trust, but always verify.

The alternative is to counter-batten the 50×50s. Somewhat like this Swedish guy does here on YouTube, but this does mean that you need to lay your PB in landscape mode. At least this way the studs are supported on 600mm centres. I like this continental idea of running all cabling in ducting, though note that they use multicore, rather than the UK practice of single core.

IMO, one the best things that we did was to provide regular Tea and biscuits, plus morning bacon sandwiches. IMO, tradesmen are good people in general, but with natural biases: they will tend to try to do a better job if they like you and feel that you respect them and their work. And yes, Ian has a point. Many aren't too tidy, but keeping the site ship-shape and making sure that nothing that you are directly or indirectly responsible for impedes their work can help.

I broadly agree with Mark except with one caveat: for some things, you really do need to do what you are doing because lack of knowledge and experience can endanger your home and it's occupants. For example, I wouldn't plumb an Unvented Cylinder because doing this is yourself is against BRegs, and because getting this wrong can be dangerous. I did rewire my last house, but that was before current regulatory requirements and the IEE wiring regs were at Rev 15 and it's now at Rev 18! You really need a qualified electrician to certify all mains wiring and register the certificate for your BInsp to sign off the work. Some electrician might be willing to allow you so do some first fix, but we just subcontracted a decent electrician to do ours. To be honest doing all woodworking, the plumbing, MVHR, all bathroom and kitchen fitting, project management, procurement, house and CH control, etc. was enough for us.

Direct ammonia fuel cells are still very much a developing technology, but even with these the end to end efficiency v.v. BEVs means that the price / kWh is almost double so IMO they will remain a niche use, e.g. for shipping and aviation.

One last codicil. It's worth covering the soil under the deck with a decent build fabric and a thin layer of gravel to keep it down, once the posts are in place; otherwise you might get problems with weeds, etc.

We built our own. There's probably some pics on my blog. We use some spare roof joists for the stringers I just cut a load of rhombus shaped bits of ?15 mm? OSB that acted at tread spacers: put in the next tread, then left and right spacers directly sitting on top with a couple of screws to fix them and repeat until we reach the top. I used 22mm chipboard flooring for the treads but each had a 40 × 60 (IIRC) bracer glued and screwed to the underside. This bracer was 2×15 shorter than the tread so it would butt against the spacer. The treads were directly fixed, but instead I put 2 × 100mm self tapping fixing screwed in from the outside through the stringer and into the center of the bracer. Two staircases because we have 3 stories, with the ground floor one using a platform to do a 90° turn halfway up. They were solid as rock, and did the job well. I precut all of the bits on my table saw so these were pre-toleranced to better than ½ mm. When I say "I" above, I really mean "we" as we swapped roles for the top set and Jan did the erection / assembly whilst I did the fetching and carrying.

The top soil / made up ground will have inconsistent ground bearing pressures across the footprint of the deck. Hence if you use pads that aren't footed on the virgin clay, then you should expect some degree of differential settlement across the pads, and some buckling of the deck in consequence. Pavers are a bad idea for pads unless you correctly load spread and the posts will tend to break them up and punch through them. The method shown in that Wickes video is about the simplest. The holes really just need to go down to the virgin clay, which might only be 30cm or so, but you won't know until you've dug a few out.

We've got an MBC TF with natural stone outer leaf. We didn't use SureCav as discussed in the following topic, but in our case we had a twinwall TF inner that was structural with the insulation internal to it. With your block inner and insulation in the void, this might not work for you. Even so I think that the suggested EPC option is a lot simpler, though I would be loath to drop the ties. The idea of bulging walls in a decade or so is horrifying. You can get long ties to span the gap and these wouldn't be a problem for the EPC fill.

The nice thing about the rivet-like anchors is that the piston gives you enough purchase that the outer flange gets buried slightly so it ends up pretty flush to the plaster. Very discrete. After touching up with paint, even with the hanging removed you can still only see a small hole.

@Temp that one is more understandable as this is big data integration internal to UncleG. All I can assume is that G monitors any pages that reference YouTube, such as @Dreadnaught's above link and it has somehow previously managed to associate my TerryE ID here with my gmail ID. Oh Big Brother where art thou?

I've got to wonder how Uncle Google is listening in. I've just gone to YouTube and for the first time in months I've had something like: YouTube -- Fixing Big Holes from Drywall Anchors! recommended to me. 🤣

The issue with quite a lot of fixings is that they don't service the fixing being temporarily removed because the back bit drops down down the cavity. A total PITA IMO. I find those squeeze-grip fixings very effective but you need to work out how to remove them with the minimum amount of damage if you need to. In the case of these fixings, I find that the easiect way is to unscrew then part rescrew in the bolt then give it a bang with a hammer. This punches the whole fixing through into the void -- at least enough to leave a small indent or even a small whole which can be filled, sanded down and painted. What ever you do, don't try to pull them out as this will tear out a big whole in the board.

A door is ~ 2m2 in area and you only have a few of them, so a U-value of 0.65 vs 1.0 or whatever is small beer in terms of the contribution to total heat loss. What is more important IMO is how airtight it is. If it doesn't seal properly then you will lose far more heat through draft cold air exchange, especially if you are using MVHR.

@Petrochemicals Any member's reputation gets a bump when another member gives them an upvote like or thank-you for a contribution, and this enhances their reputation. It takes a lot of effort / contribution helping other members to get a reputation over 1K, let alone one of almost 4½K as in the case of Nick . The reason that "Anyone would think I'd touched a nerve" is very simple: you have done so by the tone and content of your replies. Perhaps you shouldn't disrespect people when you are seeking free advice from them. I suggest that the wise thing for you to do here is a reboot: try seeking help on another forum. This is my last comment on any of your topics.

I browsed the topic and on reading @Nickfromwales first post, my honest reaction was that they were reasonable scoping Qs aimed at focusing the discussion into something valuable for you. You have now twice replied by trolling in response to reasonable points. Not a good start for a new member.

What is the domestic gas boiler penetration? I would guess of the order of 60%. ASHPs about 1%. If the government is serious about its energy targets then this ratio needs to collapse.