JamesPa

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be capable of thinking out of the box) said this, I would not trust either of the others, who are either grant chasers, outdated in their thinking, or boiler fitters who haven't bothered to understand heat pumps properly, or a combination of the above.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 Based on what you say about the house Im doubtful heat loss is really as much as 7kW, whats the age and construction? 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, 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.

I suspect there are a lot of cases of ufh by one organisation, ashp by another so they are not designed together. Ufh after all tends to be for new builds and by it's very nature is essentially part of the fabric. Thus it's almost natural to build a ufh system and later tack a heat generator on the side. So ufh comes with lots of unnecessary controls so it could operate stand alone, which essentially needs to be bypassed if you are running it with a weather compensated heat pump or boiler. Based on cases I've seen here and elsewhere it feels like this is a significant part of the 'problem' (amplified of course by our obsession with micro zoning that the control manufacturers have ruthlessly nurtured over several decades).

Mitsi firmware clamp output so its 11.2kW more or less irrespective of FT and OAT. Well thats true at least for the databooks from Mitsi (which BTW are very good) that I have looked at.

Or the 11.2 Mitsubishi. Roughly same min output from what I recall.

Well you don't want the 15kW grant then unless it's sticker output is a joke. If you can refine the figure by any means it might help choose between one if the many 8kW ish machines and one of the 10-12 kW machines. I had the same dilemma but managed to convince myself that my loss was no more than 8kW and probably only 7 so ended up with the 7kW sticker output (8.5 actual according to the spec) Vaillant. Yours sounds like it may swing the other way

Should be heat capacity according to my degree in physics. Personally I don't have an objection to thermal mass as a colloquial term. Mass, after all, is responsible for inertia, and the heat capacity results in thermal inertia. One can be too precious over these things although, like steamy tea, I get fed up of unit abuse mostly because it tends to indicate that people don't understand the underlying concept.

I didn't, I just used the default. It doesn't make much difference to the plot other than at the high oat end which doesn't matter much. I did once try a best fit to my data exercise on degree days.net and I think the best fit was if the base was 16 or 20. The latter is obviously silly and 15.5 is close enough to 16 which gave me confidence to use the default. As you see from the plots I came at the answer in more than one way further increasing confidence. I had also clamped boiler output at 8.5kW (house remained warm) further increasing confidence. The time average plots seemed to me to confirm that anything above 7kW was rare and transitory, so house would just 'ride it out' I did have a bit of a concern that the loss might be as high as 8kW, but concluded that if it was (a) the heat pump may well quote as the data says it does and (b) for the couple of days per year where it's this cold all day and night, I could use supplementary heating. The need to use supplementary heating hasn't arisen yet.

Same isn't it. 2900 is between 2000 and 3000. 290 is because 1l of oil is worth about 10kW Fair enough, but I said 2000 and separately by 3000 and the loss lies between the two, which easily encompasses these uplifts.

Unfortunately if the heat pump is grossly oversized, thats impossible. Heat pumps have a practical modulation range of about 3:1, sometimes less. This means that about the best you can expect is that they will run steadily up to about 11-12C then cycle. if its a factor of 2 over it will cycle at all temperatures, either because of defrost or because of oversizing. The sole exception I know of is the Mitsubishi 8kW R290 which has 2 compressors (2kW fixed, 6kW variable. There may be others but 3:1 or thereabouts is very common. I have a 1930s house with partial fabric upgrades, some modest modern extensions and originally solid walls. The surveyors (2 lots of them) ignored any fabric upgrades they couldn't see (mostly IWI but they also well overestimated the double glazing loss), double counted room to room losses and assumed ACH=2-3 depending on room. Correcting for fabric and room to room losses brought the calculation down to ~10.5kW. 7kW, the measured loss, requires one to assume an ACH of 0.5-0.75. I have heard several installers say that this is a more reasonable assumption anyway and one who says that he has never measured more than 1 ACH on the occasions when a measured result has been obtained. If you have your annual gas consumption in kWh a very rough estimate can be got by dividing by 2000 and separately dividing by 3000. The loss is very likely somewhere in between unless you run a Chinese restaurant so have a wok burner going all day. even in summer (or something similar) I did the attached using 2 years worth of half hourly metering data from my gas boiler. I had been running the boiler 24*7 so somewhat like a heat pump although its a matter of some debate how much difference it makes. basically this was used to size the heat pump and the installer 'adjusted' assumptions to fit. The one I chose was not the only one that was prepared to do this, but many were not. One way or another I would strongly recommend you sense check. If you need to get an air tightness test its probably worthwhile - £300 or thereabouts I am told. How big is your house and what is the construction? That might also give a sense check and @JohnMo s set of questions also. . Measured consumption - Copy.pdf

15.5kW is a lot, are you certain? Surveyors got the loss for my retrofit wrong by a factor of 2 (16kW 'surveyed' 7kW actual), just saying! Vaillant do a 12kW and I believe now a 16kW and actual output, according to the datasheets, tends to be more than sticker value (although there is some evidence that output during defrost may be much closer to or even slightly below the sticker value. They likely will be cheaper than Nibe, Viessmann and personally I like the way the Vaillant controls work, its a much more intelligent interface to weather compensation than many because changing the set temperature, whether programmatically or manually, shifts the WC curve so you aren't reliant on a thermostat, with the inevitable performance penalty, to have setbacks etc. Also adjusting the WC is just a single dial, not 4 parameters common in many others. I should state that I dont know about Nibe, Viessmann controls and, as they are European and manufactured boilers for countries where WC was compulsory, they may have a similarly well though or even better interface to WC Ditto Grant although their R32 models were the usual, not terrible good, interface.

The important thing with ashp system design is KISS. You just need a correctly sized (not grossly oversized) ashp connected directly to the emitters with a diverter valve to the UVC, maybe a volumiser, (almost) never a buffer. No external controls, not even a thermostat. No glycol, use anti freeze valves. Use the ashp controller to set the weather comp curve as low as possible and leave ticking over 24*7 (unless you plan to batch charge your slab to use it like a storage heater which is a whole different design). Choice of ashp itself not so important, but some have better controls, quieter, less ugly etc than others. Many of the ones with poor (personally I would say inadequate) controlled can use homely, an external physical controller, or havenwise, a virtual controller service) as an overlay. Under no circumstances fit any smart thermostats etc other than these (or adia) which are designed specifically for heat pumps. Reject any installer who is going to install a buffer, phe or llh between ashp and emitters, or external controls other than those mentioned above. Be suspicious of any installer who specified a 'pre plumbed cylinder'. This is often an indication that they are employing rookie plumbers and some pre plumbed cylinders come with built in llh. Post back for explanations of the above if needed.

... Which, if I follow through on the logic above, is apparently entirely fair, because they wouldn't have charged Roger as much for the infrastructure upgrade needed to support his load had there been an additional house on the pole.

I agree there is little point in continuing the debate because you appear to think that a utility that we all pay for should provide infrastructure to support what you want free of charge to you, so the rest of us have to pay, even though your requirement is out of the ordinary. Like it or not its your load that is abnormal and it's entirely irrelevant to the argument that there are some hypothetical circumstances in which the dno could accommodate you cheaply. Those circumstances don't exist. What does exist is your abnormal load and the rest of us shouldn't have to pay for it. Hopefully one of the suggestions others are making will work.