JamesPa

In a split its quite possible that the indoor unit is adjacent to (or integrated with) the DHW tank and point where the primaries to the heating system split off, and thus also possible that little or no 28mm is needed (the connections between the ODU and IDU in this case are refrigerant not water) and you would get away with a short length of 22mm at 9kW. However he should know the difference!

Have you hit the max output of the heat pump?

Just to add to what @johnmo says, these variations typically work in addition to weather compensation not instead of. Some more middle-market-upwards heat pumps feature a mode called something like 'adaptive' (or similar terminology), which automatically 'tweaks' the basic WC curve based on what the heat pump learns about the house over a period of time. This is not the same as responding directly to room temperature (like a thermostat would), it had a much longer time constant for the response. Various third party controllers, of which the best known is Homely, do something similar (and other smarts in addition) for heat pumps that don't feature this kind of supplementary control. Note though

Probably best not to do that. Many heat pumps (it is said) want to be exactly horizontal to balance the fan bearings. I certainly wouldn't tilt the pump without asking the manufacturer if its OK.

probably not, just use the immersion or fit a phe & pump as suggested by @JohnMo

One thing to add to the above. If your heating load at OAT 10 is only 750W, then your house loss at -2C is nowhere near the 8kW+ that the Vaillant 7 is capable of at -2/45C. Probably more like 3-4kW. You could very likely have got away with the next size down. This might have given a bit better COP at mild OAT. However with an electricity consumption of only 250W the other factors would still be significant, so perhaps not!

That's an interesting observation. Looking at OPs figures eg 12Oct total energy consumption 6kWh, which is only just over 250W. 7th Oct was half that. At this level water pumps, control electronics, trace heaters etc all start to matter. Looked at this way its not surprising that high loss houses get better COP, at least when it's mild. I guess the supplementary question is, how much does it matter. If your house is warm for 250W input that's a good result in many ways, perhaps there is a point where chasing COP becomes a bit academic. The better approach might be to look elsewhere to reduce the electricity baseload (which is typically ~250-400W). This will push up heat pump load but at a cop of 3+ rather than 1 (I'm assuming that all or most electricity consumed in the house eventually appears as heat, which I would think is highly likely).

Its good to see somebody is now offering this as a kit, which sort of brings the option a bit more mainstream.

By 'wiring box' I mean one of those typical central heating 'wiring centres' that many use to do all the interconnections between thermostat, valves, call for heat for the boiler etc. Not necessary so far as I can see for the Vaillant, or indeed most ashps, as almost all wires go back to the ashp control, unless you are doing something silly with external controls (which I'm not). Yes the ac isolator is fitted and wired already. Actually a 32 A one with a 6mm feed (but a 16A MCB) just in case it needs to be upgraded in future. I put the cable in, sparky fitted the isolator and connected to the cu. They are. The current boiler is run off the ring main. I'd ideally prefer to have it separate as we do get the occasional nuisance trip, but not worth the effort until I replace the cu for some other reason.

BTW I recall you too were trying to avoid replacing the HW cylinder. Should be easier for you with a smaller HP. How has that worked out? I could have avoided replacing the DHW cylinder (one installer, still in the frame, was willing to work with existing), but I eventually decided to replace. TBH I'm fed up of the two pumps I have to boost the DHW pressure (particularly galling since my mains pressure is about 9bar) and the mess of pipework in the airing cupboard, plus I finally found a do-able way for the D2 vent (which is the key issue). So it will go UVC. Still TBD whether its a 'Heat pump' UVC or a 'direct' UVC with a PHE on the side - that depends who wins the job which I'm (hopefully) sorting out with final questions at present. I'm pretty sure plumbers connect these up if they choose to, and I'm pretty sure there is no regulation that says it has to be an electrician, its just connecting to an existing circuit via the 'wiring box'. But then again that's why I still think an electrician may turn up and disagree with the signed-off installation. However if the installer has in fact budgeted for a day of electrician time, its not going to take more than a couple of hours to splice a small CU into the current circuit that my electrician was perfectly happy with, so its hardly a disaster if that does happen. No (because I didn't ask him to) and No. There is a switched fused spur and typical heating wiring where all the electronics will be, and the one installer that specified what he wants didn't specify any sockets, because he presumably reckoned he could connect to what was there. That said, thanks for the tip-off, I might just put a double socket on the end of the fused spur before anyone turns up😀 its a wonder anyone ever gets a heat pump installed given how difficult the industry manages to make it.

Interesting The two candidate installers have both said that so long as mine has been signed off by an electrician they are happy. However I can easily see that changing when they get on site if their electrician (assuming they bring one - I dont think there is now anything to do for which an electrician is mandatory, circuits are all in place its just a case of making a connection) looks into it in that much detail.

Thanks for posting this and I'm pleased to hear it mostly went well. Do you have any control at all over dhw. Is that regulation or technical. My electrician (not connected to the hp installers, a separate bit of prep) has just provided an MCB and no RCD because he couldn't find an RCD to fit my cu (which is an older model where some of the circuits are just on mcbs not RCDs) and didn't consider it worth upgrading or necessary to do so. My suspicion is that he may strictly be right but perhaps I would have preferred an RCD in a separate cu for maximum safety.

I can't decide whether I violently agree or violently disagree. It's clear that, as we electrify, there are real opportunities (and a need) for grid demand management which probably are best managed centrally not locally, and equally clear that the new infrastructure costs, which ultimately we all have to bear, will be enormous if we don't manage demand. On the other hand, like you, I value independence. My considered view is that we need infrastructure build out and demand management that acts solely in the public interest, not commercial interest. But we also need the option to opt out of demand management. However if you do opt out of demand management then you should bear the full cost including making a significant personal contribution to the infrastructure in addition to any contribution through tax. That will translate to very high electricity charges if you don't want demand management, but so be it. With a bit of luck that should focus minds on what matters individually in a way that doesn't prejudice choices others may wish to make

Thanks for the explanation. I cant find a business case for batteries, at least for me, and the power round here only fails rarely for short periods. So if a battery doesn't help the environment then there is absolutely no reason to have one. Thats why its so important to understand if it actually helps the environment, as some people claim If I understand you correctly, your conclusion is that local storage, where the choice to store or not is based essentially on retail price, is currently (and wont be for perhaps a decade) of benefit to the environment, and in fact is probably harmful. Is it possible to envisage a chart that demonstrates this simply without going through all the arguments?

Sorry to be dense, but I'm still not seeing how to interpret that to work out if retail price and carbon intensity correlate or not