JamesPa

If you had both a heat pump and E7 it would be cheaper than E7 alone, by a factor of 2-3. Night time water heating (with E7 or similar) on a heat pump is nearly free, even if the COP isn't the best. (PS I assume you mean 125l not 25l, otherwise its just a footbath)

... and thats why OP needs to work out their DHW requirement properly. 125l/person is a lot, unless they each have long showers every day which of course is perfectly plausible but not necessarily the case!. Does the existing 300l cylinder with the existing heating system suffice and if so how often does it get reheated? This is definitely an 'edge case'!

Agree with @johnmos comments, also (Im shocked he didn't say this), NO buffer, 2 port volumiser in the flow if you need it. Operate space heating open loop, do not zone radiators. Since only engineer 3 (who appears to be capable of thinking out of the box) said this, I would not trust either of the others, who are either grant chasers, outdated in their thinking, or boiler fitters who haven't bothered to understand heat pumps properly, or a combination of the above. Sadly there appear still to be loads of these people around who between them risk being responsible for the destruction of the industry @dilsue is also right in his general comment but (a) some '7kW' heat pumps will do more than 7kW and (b) based on what you say about the house Im doubtful heat loss is really as much as 7kW, whats the age and construction and (c) you don't want to go too high otherwise you will compromise efficiency. That said you do need to calculate and take into account your DHW consumption with that many people living in the house. As @johnmo says you may well need >300l unless you space usage out. its an unusual requirement so many installers wont take it into account; I would strongly recommend you do some sums on this and understand how its phased through the day, Plate loading an existing (unvented?) tank is certainly a sensible retrofit option if its large enough for your requirement. Bear in mind with an ASHP you typically heat the DHW only to 50C to maximise efficiency. That said, with a modern R290 heat pump there is nothing actually stopping you heating even to ~70C albeit COP for the DHW wont be as good (mine gets to 70 during the weekly legionella cycle which is done by the heat pump alone). If you have a low tariff period you could do it then and top up later, it would still be cheap and the payback period for fitting a new, larger tank just so you can operate at a lower temperature is probably rather long. Again this comes back to understanding your DHW requirement. Engineer 3 may just be spot on with his/her proposal, particularly of the '7kW' pump he would fit is actually a bit more than 7kW under the conditions you will be operating at and you are qilling to be a bit flexible in your thinking, but I cant stress enough that you do need to work out your DHW requirements.

Interesting, presumably to boost a high resistance leg. Was it just plumbed in series or did you employ any separation?

Point 3 on the post by d438a1. Interestingly triggered a disagreement between MCS inspector and installer apparently Surely you can just leave out the room stat as far as the LG is concerned, it will then satisfy the buffer based on return temp and its WC curve. Im not saying this is a good thing to do, just saying that the plumber may have done it if, like the one in the reference above, they prefer to do it this way.

@chris47 All above makes sense. I didn't realise you had cosy which does make the picture of how best to operate the heat pump a tad more complex (unless you also have a battery of course) Have you discounted the possibility that the ashp is simply satisfying the buffer then? There is another thread on this forum discussing alternative ways to wire controls when a buffer is present, and the installer in question is advocating (he claims for efficiency) doing it in the way that is guaranteed to cause the symptoms you observe. Given that one installer favours this it's likely that others do, so quite possible yours did.

Is it, or is it heating up the buffer? Given you are in Bristol its most unlikely to be frost protection because its (presumably) not yet cold enough. You may be able to adjust the frost protection settings to find out for sure, but its equally likely you cant, given that the freezing point of water is the same worldwide and frost protection is a safety measure to protect the equipment. Certainly my Vaillant, and several other heat pumps the manuals of which I have read, have no provision for adjusting the frost protection settings (neither did my boiler), for the simple reason that there is nothing really that it makes sense to adjust. Im still thinking its most likely the heat pump satisfying the buffer because it has no way to know that there isnt a call for heat. @Dillsue is suggesting ruling out the programmer, but I think you have already done this. He is also suggesting turning down the buffer target temperature to detect whether its the heat pump satisfying the buffer. If you can do that he is right it is a good way to confirm. Alternatively trace the route from the receiver end of the wireless thermostat. Does it only switch the secondary pump, in which case what I suspect is the cause is guaranteed to happen, or does it also switch the call for heat contact on the heat pump? Or just ask your installer! Although you are right to be concerned about the short cycling, probably more significantly long term is your mode of operation. In most houses heat pumps are most efficient run 24*7 on weather compensation and without a buffer, with all or most of the TRVs and thermostats turned up a couple of degrees above the desired set point. Depending on what your flow temperature is, the buffer alone will increase your running costs by 15%. Running at a flow temperature 5 degrees too high and bouncing off thermostats could easily be another 15%. And, depending on your house characteristics part time running another 15% (unless your heat pump is well oversized in which case you may be incurring the penalty anyway). Really your installer should come back, replumb the buffer as a 2 port volumiser in the flow, attach the Honeywell to the call for heat from the heat pump, operate it 24*7 then turn up all the TRVs/Thermostats and adjust the WC so it just heats the house. That's generally a good starting point from which to tweak. Of course there are always exceptions to the general rule, but in a well insulated house (you say yours is reasonably well insulated) with a reasonably right-sized heat pump the general rule is almost always the right starting point.

So unlikely to be frost prevention then (I presume it's not getting down to 5C in Bristol yet) This increases the probability that the heat pump is doing what it has been asked to do, and the 'problem' is due to a complete separation between the control systems either side of the buffer tank, as set up by your installer. In a sense you shouldn't worry too much about it, the heat delivered will stave off the point at which your heating is properly on. On the other hand it suggests, as @JohnMo says, a sloppy installer. You probably will benefit from some tweaks to your system which people here can advise on if you are willing to stay the course.

Frost prevention already? I guess you are quite a bit further north than me! Its a good point though, I dont think we know where OP is, if he is in the south its not frost prevention, if quite a lot further North it could be.

OK. A possible explanation is this: Installer has kept the control systems on the secondary (emitter) side of the buffer totally separate from the control system (ie the heat pump controller) on the primary side. Most of the time this works more or less as expected. However when there is no call for heat from the secondary for a long time the heat pump has no way of knowing that. So if its also cold outside, it tries to keep the buffer at whatever temperature it is required to according to the WC curve and other settings. Thus it switches on occasionally during the night, draws power for a while, switches off because the buffer is warm and no heat is being drawn from it by the secondary. It only switches on again when the buffer cools due to heat loss. Stupid way to wire a control system IMHO if that is what has been done, but then putting a buffer in was quite possiblystupid also! Was the honeywell etc already present when the installer fitted the ASHP. If so he probably was trying to avoid touching it. Feel the buffer tank (or better the pipes leading to the buffer tank) after this has happened, if its warm then thats the culprit, if its not warm then its something else.

So you have a buffer tank? The heat pump may be switching on to heat it up even though there is no call for heat from the secondary side, depending on how your installer has set up the control system. Has this started only when it got colder at night outside and is this your first winter? The fact there is no immersion is irrelevant, its the heat pump which will switch on if the system is wired in the way it might be

So you have a buffer tank? The heat pump may be switching on to heat it up even though there is no call for heat from the secondary side, depending on how your installer has set up the control system. Has this started only when it got colder at night outside and is this your first winter?

Have you got a buffer between ashp and emitters? If so then, depending on how your installer has set the controls up, it could be turning on to heat the buffer up when it gets cold outside, even if there is no call for heat from the rest of the system.

Its also a little pointless because the coil isnt adding much extra heat transfer! KISS!

@Michael_S No experience of this particular one but can you disguise it without putting anything in front. I opted for top and two sides as the front of mine is anyway acceptable in appearance and there is no side intake.