Ed Davies

I think Temp is a bit optimistic. That 140 kPa is presumably the BS EN 826 value (it is for Celotex XR4000) which is for 10% compression. If you have 200 mm of foam under the door you'd be a tad disappointed to find it moving up and down by 20 mm every time you slide it. Also, the weight is obviously important but you've also got to consider the forces as the door moves, the weight of hefty people stepping on the runners as they stagger in and out… I think you'd want it bearing on a minimum of 20 times that area (10x to reduce the compression to 1% then double it for the extra forces).

3 port, 2 way? AIUI, it only has two positions (AB→A and AB→B). I suppose a mid-position valve could be described as 3 position.

This article, which includes an interview with John Larsen, is pretty clear on the subject: https://www.greenbuildingadvisor.com/article/all-about-larsen-trusses . Similarly, this one (http://www.suncertifiedbuilders.com/en/Larson Truss) even suggests that it should strictly be external to the sheathing of the structural wall. Whatever, I think it'd be better to keep the name specifically for non-structural trusses and call, as that first article does, a structural version a wall truss.

If it's load bearing (e.g., propping up the roof) it's not a Larsen truss. A similar truss system which is load bearing might well be a good idea, though.

Heat pumps have a long history of problems/failure in social housing. Don't know what the real problems are but it seems like a mixture of poor and inappropriate installations and lack of user understanding.

The official word used for those, at least in the Scottish building regs, is “manifestations”. Makes me think you have to stencil a ghost or something, which would not be entirely inappropriate.

The point is that the earth under the floor will provide some resistance to be added to that provided by the materials you lay on top. How much is very dependent on the properties of the soil (dry sand will insulate a lot better than wet mud) and also on the geometry of the house because heat loss has to go sideways. A round house (or, more likely, square one) will lose a lot less through the soil than a long thin one. This could all be calculated using finite-element analysis or whatever but in practice is it's approximated based on the perimeter to area ratio, as @PeterW says. Sorry, don't know the details of the calculation. But, I'm not sure that chart you quote is quite right. It gives “Ext surface” R = 0.040. That's the value for an external surface to air, I don't think it applies for an interface to mud and I think the value should be approximately zero. However, it's a tiny fraction of the total resistance anyway so will not make any significant difference.

Mine's single storey (plus loft space but that's not habitable) but still requires fire retardant on timber surfaces.

That'd presumably need some coating to prevent surface spread of fire?

The couple of houses I looked at at various times were single storey or 1.5 storey with no windows above ground floor in the end gables so I'm not sure if that would have been an issue. Timber cladding crossing multiple stories is normal, isn't it, so maybe it'd just need a break in the timber studs. Not sure. Maybe a block of PIR between?

Sorry, stupid typo on my part: I meant “meter” not “relay” (corrected now) so your answer isn't too relevant. (Actually, I think you'll find you'd need a beefier relay/contactor to break 30 A at, say, 12 V DC than the same current at 230 V AC. E.g., 16 A MCBs for 230 V AC are cheap and plentiful but only a few are rated for low-voltage DC as well. That's because at 50 Hz there'll be a zero crossing along in at most 1/100th of a second to help quash the arc.)

I saw cheaper DIN-rail relays meters on eBay last night but didn't post about them partly because I needed to go to bed but also because I was a bit confused about their current specifications. E.g., from the one @JSHarrisposted about above “50Hz 5(30)A KWH Power Energy Meter” and Basic current (Ib) 5A Maximum rated current (Imax) 30 or 32A Operational current range 0.25A-30 or 32A So what's the 5A bit about? And do they really have such a large minimum current? What do they do if the current's less than that, not register? Wish my meter worked like that, as it'd save me ~= £20/a.

I looked into doing up an old house a few years ago (while the purchase of my plot was stalled over crofting issues) and quite liked the idea of a Larsen truss for that. Mineral wool is about the cheapest insulation in terms of R-value/£ (just thicker than most other forms) but with the wood, etc, yes, overall it might be a bit more expensive in materials but it'd be a lot more flexible from the point of view of doing bits as the weather permitted, for adapting to oddnesses in the existing walls and for extending the roof over it. Would have timber clad, vertical larch or whatever, if the planners allowed it with a ventilated cavity with woodfibre board behind.

Is a DIN mounted relay “similar switchgear” for the purposes of the 17th edition amendment 3 rule about metal boxes (the one mostly relevant to consumer units)? Is that why they're cheaper now, because they should only be used in non-domestic applications? (Seems a daftly worded paragraph to me. AIUI the problem is poorly made, so high resistance, connections in high current circuits causing overheating. Therefore the problem is not whether there is “switchgear“ present but whether there are connections not protected by a low-enough current limiter (fuse or breaker). Why does a consumer unit have to be metal when a connection box in the loft for a lighting circuit can be plastic? Either can have poor connections. Perhaps they should have said metal boxes for connections if the current is not limited to 32 amps, or whatever. Similar sort of thing with the special rules for low voltage lighting circuits. The problem isn't the low voltage, it's the high current. Low voltage, low current lighting (e.g., LED) doesn't need anything special but it's caught up in this badly worded mess.)

PS, if you do go for those DIN rail-mounted meters then I'd suggest putting them in the line to the relay which switches the heater. In the UniQ Heat batteries reference manual ver_20180719_v2.0 figure 6.1 between the connection 1 on the main terminal block at bottom right and connection 2 on R1, the 16A relay with the neutral taken off connection 2 on the main terminal block. This causes the meter to see just the heater's consumption but keeps it powered all the time for the backlight on the display for readability. Dunno, but I'd hope that meter keeps its reading when it's not powered but maybe it doesn't.

9 kWh. Seems like a job for a @ProDave solution, one or two cheap electricity meters. It'd be cheaper than any current clamp. Bear in mind that even with a current clamp you'll need to mess with the wiring at least a bit to separate out the line (or neutral) to put the clamp on unless there are already big enough loops accessible inside your control boxes. You can't put it on the whole cable (line, neutral and earth) as the line and neutral would cancel out. If you've got room in the boxes then adding something like one these in each might make sense: https://www.bellflowsystems.co.uk/smartrail-x45db-mid.html?category_id=606

Indeed, some protection. AIUI, it just stops calling for heat if the temperature reaches a certain point. Assuming that's wired to a zone valve which in turn controls the pump (as an S-plan system) that won't help if the problem in the first place is the zone valve sticking in the open position. Something more multi-level is needed to tolerate plausible single-point failures.

Any ideas how much less (if anything) the ones without the electric element are? Ideally I'd prefer to know what the temperature sensors are and not bother with the controller, too.

If not, may well be a good plan. Being off-grid I haven't had to deal with them apart from the roads people needing me to check with them there wouldn't be any of their pipes across my entrance. Trying to get something out of the official office was a huge run around but my landlady's husband who'd done some building work around here said just pop into the local office. When I found it (official website was 5 years out of date) they couldn't have been more helpful: chap took me to his desk, called up their maps on his computer to confirm their pipes ran a different route then had a useful discussion of possibilities if I ever did want water or how anybody else could get it for the other plot on my site.

Maybe it is in SAP but in the real world, I'm not so sure as back-of-envelope calculations with actual grid intensities show they're much more evenly balanced. I've read that the SAP numbers (gCO₂e/kWh) are being revised down to more closely match actual grid numbers over the last few years. Of course, there are complications: 1) Space heating (as opposed to DHW) needs to be evaluated against the grid carbon intensity over the heating season, not the whole year. 2) In the short term any increase in electricity demand will likely be met mostly by gas-fired generation so the marginal intensity will be higher than the average intensity but 3) in the longer term new generation capacity will likely mostly come from renewables and, perhaps, other low-carbon generation (i.e, nuclear) so in that sense adding new electricity demand should eventually reduce the average intensity. Financially gas is probably cheaper but if your demand is low the standing charge can't be ignored completely.

Indeed, at £0.15/kWh. And the saving vs just the Willis would be twice that (£1356). So compared with a sensible heat pump price of around, say, £3000 (including a day for a plumber and a day for an electrician for the installation) you're talking about a break-even in just over two years which is also very reasonable. But it's not freezing all year and any use of cheap-rate electricity and/or PV will extend that significantly. The RHI tax seems to be giving heat pumps a bad name, though. One neighbour was shaking his head about another neighbour's new £11'000 heat pump - “ordinary people can't afford that”. Will have a longer conversation with him at the next opportunity!

You never copy other people's designs. You always design your own taking, at most, inspiration from there's. ?

Absolutely. If you're just about at Passivhaus levels (15 kWh/m²·a) and use the typically assumed 3 kWh/peep·day for DHW for three people in a 120 m² house then it's just over 5000 kWh/a. Even with full-rate electricity at £0.16/kWh that's “only“ £800/year. Add E7 or E10 or PV (remembering that the bulk of the use is for DHW which happens at least as much in the summer) and the ROI on anything more complicated begins to look a bit weak. Even the standing charge on mains gas will bite into any further savings to be made. I think an ASHP makes sense but it's got to be as cheap and simple as possible.

The thing with CarbonLite (now known as Carbon Dynamic) is that the location for most of the project is already determined: Invergordon. When I spoke to them years ago they'd just done a house on Lewis which was 36 hours from the house leaving their factory (then in Alness) to watertight on site including lorry journey across Scotland and ferry ride. So, yes, transport costs are an issue but it's not like they have workers away in B&Bs for a month or two.

I think they just build a house paying and reclaiming VAT like any normal business then sell it either without VAT or zero VAT (not sure which, technically different but same effect for the purchaser). Assuming the purchaser isn't VAT registered.