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Ah sorry, and now I look I see you did in fact mention them!

Excellent progress as usual. Hope you mend quickly! You don't really show it (maybe the start) or mention it but I presume those external MHVR ducts were heavily and carefully insulated before being boxed in?

Indeed. Depends a lot on what you have. Just to note, I'm not particularly knowledgeable about this so just repeating things I've seen written here by others. Common improvements over last 30 years: 1. Low E glass coatings etc 2. Argon filling vs plain air 3. Larger gap between panes 4. Better insulated spacer bars 5. Better frames (especially if you you look at the older metal frames but UPVC too) and probably more Suspect so, especially if they are small windows and seem well sealed.

Can you do something like this (red line):

75oC is wrong for a new system. A lot of existing systems run like that which I guess is why they use it (though when I just tried it it defaulted to 70). Either way if you are designing for efficiency, the lower the better. IIRC @marshian's system runs at something like 35. This does mean you need to specify bigger radiators. I think this sounds a little high, checking out their tool it doesn't ask how much insulation you have or how good the double glazing is for example. So they are going to assume for an average property, which as you are in the process of upgrading yours probably won't be. New double glazing is a lot better than 20 year old. A single skin wall with 50mm PIR on top better than an uninstulated cavity wall and likely better than a lot of older insulated cavity walls. I think it's worth trying out some other calculators to see how things line up. Jeremy's one mentioned above is here but it's less user friendly than others: Others here are better to ask as I've been more focussed on new build than renovation but you didn't mention anything about the floor. If you have a wooden ground floor which is ventilated then insulating that and especially putting in an air barrier to stop the air from the ventilated space exchanging with your warm air from the room is something to look at. To be clear, you don't want to block the air bricks, you want to stop the air moving from below the floorboards to above. You mention adding insulation to single skin walls, but not to other external walls, there might be scope to beef up insulation in other areas (though sounds like you might be a bit past that point now). Edit to add: if you have fireplaces/chimneys thats an area to think about. If you are maintaining the use of a fire then unless you are using a completely sealed unit you will need to maintain airflow into the room to provide air for the fire (which is a problem if you want to air seal), and if you are air sealing you also need to think about maintaining enough ventilation to stop problems with humidity/stale air.

Something to bear in mind. A lot of discussion on this forum related to heating is based around trying to build the most efficient systems an for most situations that is to run heating systems "low and slow", ie, at a low water temperature continuously, rather than the traditional UK approach of only putting the heating on for relatively short periods (morning and evening) and letting the house cool down at other times. If you have a very uninsulated house then the low and slow approach will possibly consume more energy but for any house with reasonable insulation/airtightness (nothing cutting edge) then the low and slow approach will be much more efficient and comfortable* but it feels 'wrong' to many people who think it will cost a lot more to have the heating on continuously. A benefit of designing any system for the low and slow approach is that this is the best approach for heatpumps and a swap to a heatpump at a future point should be fairly simple. For a low and slow approach, even a big house will likely be fine with a small boiler, but if you are really attached to the traditional burst heating approach where you put out a lot of heat in a short period of time more might be needed. As you are doing a refurb, if you haven't put much effort into the insulation/air-tightness yet, now is the time to do it. * It results in the structure of the building warming up which can reduce dampness/humidity issues (though ventilation required as always), avoids big temperature gradients (feet colder than head), etc.

Nick's suggestion should work. It's a very cheap thing to do (<£10) so why not try it and come back if it doesn't work? You just need to stop warm air touching cold surfaces.

My comment was based on your moving from an insulated raft to either an uninsulated one or traditional foundations. If you always were on traditional foundations with the slabs just as a floor surface then sure. Is that the case? If that is the case, then am I right in thinking that you wouldn't have had a completely flat and open space for the concrete guys to do the slab? ie, they'd have to work in room shaped areas with lots of corners? That's bound to have put them off if so.

I haven't watched it all myself but the early bit covers most of it. Maybe ~8min - 18mins in. Watch at 2x speed to save time (though that takes a bit of getting used to

Below is a video with a discussion on this with Hager talking about how they worked with Valliant (at their request) to build something specifically for their heatpumps.

That is my understanding. If the cable is otherwise protected its fine, but at least on retrofit installs doing so may be tricky. Only if to the right spec, hence need to follow the manufacturer. As an aside, either I'm missing something or it feels like the regs are calling out heatpumps, etc, but ignoring a whole load of other devices that could be as problematic or more. I suppose BIG heatpumps could be more of a risk but smaller heatpumps aren't drawing much different power from many other home devices these days that will also have issues with high frequency leakage. Induction hobs, computer power supplies (which can be 1kw+ on gaming PCs) and high power battery chargers. All of which will work at freqencies far higher than the 20khz spec that Valliant is talking about. The trend is faster and faster switching speeds, cutting edge stuff will be working at 1Mhz+.

After posting that I checked, it might not be regs that I saw updated but installation guidance: https://www.heatpumps.org.uk/wp-content/uploads/2025/03/HPA-RCD-Guidance-March-2025-Final.pdf Edit: quoting from that: BS 7671 Regulations 134.1.1 and 510.3 both require the installer to take account of the manufacturers’ instructions

If the regs say something along the lines of appropriate RCD protection needs to be installed. Then you need to choose the appropriate RCD. RCDs come in many flavours and the issue here is that Valliant heatpumps put out waveforms of a higher frequency than standard RCDs can protect against. If they installed filtering in that heatpump to eliminate the higher frequencies then such RCDs would not be required. That the manufacturer recommends something is a consequence of them helping installers meet the regs for their device as currently designed.

This doesn't really match what I've seen before (though again it's been a while). I suspect this is a case where they are thinking aircon not cooling and designing for the 'cool a hot room down rapidly' type situation with all its other complications.

I think the RCD is to protect the cable so inside the heatpump doesn't necessarily work. You do make an argument that the manufacturers should install better filtering inside the unit so the noisy signals that need complex RCDs don't leave the unit. That would impact efficiency though I'd guess.

Ah, trying to make something that wasn't designed for it do cooling. I thought we were talking about things that did support it but the manufacturer didn't recommend.

Not Valliants fault the electrical standards were clarified that these things were required.

That sounds like you are thinking of switching from an insulated slab to a non-insulated one. Thats a different decision to just changing how you finish the floor. You'd need to get the drawings all adjusted as your levels will be out and it might affect the heat loss calculations + SAP too depending what you are doing. You can just top the insulated slab with a leveller and a normal floor finish if you just can't get the powerfloating done. That would save a lot of the rework of drawings etc, though you might need some depending how much height you are adding.

That looks fun to move

Not sure I understand why they said any of this. It's been a while since I looked but when I was looking I'm pretty sure I saw plenty of examples of shallow depth (1m) coils being set up for heating and cooling without a problem (I was tending to look at other countries as not so much used here). The problem with them is sometimes you need a lot of space/length to stop them freezing in winter, not a problem to dissapate excess heat in summer. DHW cylinder to dump excess heat sounds like they've gone way off topic. Sorry, I feel I'm missing something here. If you use a GS heat pump to cool a property the heatpump is actively heating the ground during this time. The problems with ground source I thought were more with the ground freezing when you try to pull too much heat from it during winter. Didn't think adding heat to the ground during summer was a big issue comparitively. This was my understanding and it's commonly used and supported in other countries. Though I'm sure I've also seen plenty use the heatpump for greater effect. As at least in some ground conditions it helps recharge the ground for the winter heat demand.

Oh I get that. My assumption in making the point (and assumptions are always dangerous) is that the funding would be available in a way similar to heat pump grants, where you choose the contractor and agree a spec before moving forward. In that case, with Rogers knowledge it would seem possible to seek out a reasonable contractor, not the worst, and make sure the spec is reasonably close to the end goal and be able to monitor the work. If the schemes are more in the form of sign up to it and basically hand over all control and decisions to a 3rd party and the end result it whatever they decide with you having no influence then it's a completely different situation.

*deleted I'm talking about something different from this thread*

I think the issue is with the mixing valve. A normal analog thermostatic valve can't have different set points for hot and cold. I'd guess that many digital ones also require more setup work/software support to do this. Maybe their inbuilt control of digital valves is the problem? If so, wouldn't seem hard to make sure you bought valves that could handle two set points and wire them seperately from the rest of the system. Not sure off hand why they would need to be wired into the heatpump except for monitoring reasons, they should be set and forget.

Definitely sounds like it should be possible. They are relying on mixing valves and running the heatpump at the highest (heating) or lowest (cooling) temperature so it's not necessarily the most efficient but I wouldn't expect you to need the very low cooling temps outside of heatwaves.

You've complained before about how the building regs make it difficult to do smaller improvements because if you start then you feel you would be forced to spend a lot more to bring everything up to spec. (Though from what I've read here it sounds likely that this is a worst case fear rather than what is likely to happen in practice if you actually consult with building control). You've also complained about not being able to get a government funded upgrade to your electricity and this combined with the insulation improvements issue is why you don't feel you have the option of moving away from oil heating. If you can get such a large amount from the government to make improvements to your house (improvements that almost certainly would deal with the building control issue) even if the work has issues, couldn't you use it as a starting point to then make your own improvements (that you would like to) along with fixing up any issues you saw with the install? This would likely then open up the possibility of electric heating options for the future if you decided to go that way. Reckon you'd still save an awful lot even if there were some issues that for expediency you decided to fix yourself and it sounds like you have the skills to closely watch the installers and pick up any issues to have them fix the majority of them before they left site.