JamesPa

As @johnmo says above its a good question, there are multiple answers (ie different heat pumps adopt differnt approaches), and flow rate isn't anyway that important (so long as its sufficient). In principle faster flow rate = lower deltaT across the emitters = higher average emitter temperature for any given leaving water temperature = higher COP. However it also equals higher water pump power consumption, which is certainly significant in a very low loss house (say <4-6kW) but irrelevant in a high loss house. Then there may be 'engineering' factors which we cant quantify, and there definitely is the very practical factor that the velocity of water through the pipes should be limited for reasons of noise and erosion (typically to somewhere between 1 and 2 m/s depending on material and location). By the time you put all this into the mash you come to the conclusion that the rule of thumb to design for 5C DT across the emitters is a good starting point, but if you can do less without busting the water pump do so, and if you are forced to do a bit more (up to say 8C) don't sweat too much. Unless of course you have a heat pump that insists on 'control'. There are occasionally discussions on other forums about this, for example on openenergy monitor someone has done an analysis of water pump consumption VS COP and from that come out with an 'optimum' (which depends on various factors including house loss), and on the Renewable Energy Forum there is currently an individual that is claiming (without any theoretical justification and contrary to available empirical evidence) that heat pumps which maintain a fixed DT of 5 by modulating the water pump speed are somehow superior. So unless there is a good reason to the contrary, design for around 5C, if you end up doing better then do so. Absolutely do NOT design in any system separation (3 or 4 port buffer, LLH or PHE) unless there is a very solid justification (rare in a domestic environment) and you have a well thought through control strategy to avoid mixing, because if you do you are almost certain to incur a COP penalty for ne good reason. Basically KISS.

4kW with a reasonably well insulated semi 173 sq m is definitely very plausible. My 200sq m detached in 7kw (two professional surveys, each taking 3hrs, got to 16kW - perhaps you can understand why I am sceptical about heat loss surveys) So if this is correct you are looking at a 5kW pump perhaps, directly connected to the heating system without 'hydronic separation'. UVC for DHW and a diverter valve to switch between the two. Hopefully, given you have UFH, operating at 35 or below. Is it UFH or rads upstairs. You can only really run at one flow temp for space heating so if its rads they will need to be large enough to run at the same low FT as the UFH. Should be simple provided the pipework or locating the outdoor unit doesn't cause a problem. Just be aware that quite a few 4 or 5 kW units in fact software limited higher capacity units. The problem with that is they wont modulate down as far as you ideally need in the shoulder season. By the same token there is not much point in buying a 5kW unit if it is in fact the 8kW unit software limited. Good installers have twigged this and therefore avoid certain models in any given range. The combination of physical size and refrigerant charge frequently gives the game away. I would recommend you contact several installers including one that is Heat Geek certified. Heat geeks tend to be more expensive, but purport to understand the technology better. You will want to hear/read what they say. The website I pointed you to does provide recommendations for installers (I have no connection and cant vouch for them). I would eliminate any that insist on fitting a buffer or LLH (low loss header), other than a 2 port volumiser. I agree with the sentiment, but we have to deal with the world as it is not the world as we would like it to be. In this field, as in many others, it helps to be an intelligent customer and increases the chances of getting a good job. For the past 20 years many, perhaps most, of us have been paying 10% more for our gas heating than we needed to, and enjoying lower levels of comfort, because 'the industry' shunned weather compensation (which was made mandatory in some countries) and set up our condensing boilers so that they mostly don't condense. Unfortunately the only thing to be learned from this is not to trust the heating industry! From what we hear there are other countries that are in not dissimilar situations so we appear not to be unique. I should stress once more that there are good installers out there, the trick is to avoid the bad ones and this is where a bit of understanding can come in useful. This comment probably applies to almost any aspect of building, and indeed many other trades!

I have a 1930s property in not so rural Herts/Essex borders and I have recently replaced my gas boiler with an ASHP. Like you my (originally solid) walls are insulated with 50mm PIR, and I have 300mm loft insulation. I dont have floor insulation. Heating is by radiators. I am very pleased with the heating performance (Its much more stable than I ever got with the gas boiler) and its working out about 12% cheaper like for like to run. Its also more comfortable - low temperature heating is a bit of a revelation, the reduced thermal gradients both in time and space are noticeable. However you are right that there are quite a few poor installers out there who will fit buffer tanks, low loss headers or plate heat exchangers, a heat pump double the size that you need, lots of 'smart' external controls (the worst type unless specifically designed for heat pumps, which most are not), and will then turn the flow temperature (which is the critical parameter) up high, all of which clobber efficiency. Actually these people are really just grant harvesters and wont be around once grants subside, but in the meantime they are giving the industry a bad name. I would strongly advise you to take a bit of time to understand the technology, perhaps starting here where there is a good newbies guide, get several quotes and ,if you have the time, take your time over it. If you know how much oil you use per annum you can sense check any heat loss surveys. You will need to have a heat loss survey done as the basis of the design, but be warned that they can be complete nonsense - for example there are definitely installers who will refuse to take into account your wall insulation because they cant see it, and can easily get it wrong by a factor of 2 - which can matter a lot! There are also some very good installers out there who will take care to sell you an excellent, well matched system (and please note the system design matters more than the heat pump 'brand'. If you post your floor area and annual oil consumption, plus tell us a bit about your current heating patterns and age of boiler, we can get some idea of what you might (subject to survey - and then sanity checking of the survey) need. Cool energy sell a limited range of heat pumps and, whilst they have a good reputation in many respects, it would be a mistake to limit your choice at this point. Hope that helps

OK it makes sense if that is in fact the case. I dont understand why the architect doesnt just say that, and instead feels obliged instead to spout nonsense!. But it doesn't matter if it is in fact the case.

Sorry but as you stated it (which is presumably how your architect stated it) that is complete and dangerous nonsense. Rads need to be sized correctly for the desired flow temperature because you need them to emit the correct amount of heat at that temperature, and the amount of heat they emit is dependent on the flow temperature. So (for example) a radiator that emits 1kW at 70C will emit only 370W at 45C and only 230W at 35C (the normal temp for UFH). If they are not big enough for the design flow temperature the only things you can do are (a) increase the flow temperature above the design, until it is high enough that they emit the right amount of energy or (b) replace the rads or (c) freeze If your architect actually meant this then he doesn't know what he is talking about! Seriously.

Ok that's makes more sense. The homely controller will help get the best out of the heat pump. So in this case, if price is ok, question some detail. For example the bed 2 rad could presumably be physically smaller if they made it a type 22/K2. What ft is it designed for and how does the ufh ft relate to the rad ft. Some things still don't quite stack up but may be ok in fact.

Why are they suggesting a 9.5kW heat pump for a 3kW house? Unless 'peak power demand' means electrical power, but it cant, surely mean this. There is no way a 120sq m recently built house is 9kW. My house, 200sq m, was built in the 1930s with a couple of modest 1980s extensions, only partial internal wall insulation (a minority of the walls are solid brick) and no floor insulation and its 7kW I hope they arent putting in a buffer tank/LLH further to clobber your performance (but fear they may be) Do yourself a favour and find another installer.

I cant find this requirement in the definitive MCS document which is here. The BUS rules say that the system must be capable of providing 100% of the space heating requirements, but say nothing about rooms You might reasonably argue that the guidance (but not the actual requirement) of Part L of the building regs requires separate heating of each room so you might want to take a careful look at this (note that most of the document is guidance, the actual requirements are just a couple of paragraphs.) DHW is frequently heated to 48 with an ASHP If you heat rads to 41-45 then your flow temperature will have to be be 41-45 with mixing down for UFH, so you lose the efficiency advantages of UFH. You want a design where the space heating is one flow temperature only (with weather compensation obviously). The towel rail in my bathroom is hopelessly undercapacity according to the calcs (because it wasnt upgraded from the DT50 design). The bathroom is nice and warm from the adjacent hall and, when its in use, steam. I would go with those who say find another installer/designer.

... particularly if its a long run from tank to outlet. To get hot water to flow in my kitchen I have first to drain 9l of cool water from the pipework. If Im using less than 9l of hot water at a time, which is likely, a COP of 2 at generation stage becomes a COP of less than 1 at point of use.

Is it doing a defrost?

@marshian Im assuming you mean kW not kWh in the above. If so with calculated losses of 4.7kW and measured 4.2-4.6 you have a pretty good indication what the actual loss is, I would say. You do, in the latter, have to make an assumption about gas boiler efficiency (75-110% depending on age/operating mode) but any fabric calculation also makes many assumptions. Many may suggest you go for 6kW so you have oomph to heat DHW in a sensible time (its worth doing the calculation on reheat time if you have a low loss house and use a lot of DHW!); also some manufacturers 4-5kW models are just hobbled 6kW machines so the lowest output is no better and there is little or no point in going for these. Of course the latter us manufacturer dependent

@DREG Just to amplify what (I believe) @JohnMo is saying. You want thermostats set well above the target temperature so they are constantly calling for heat. You then operate your heat pump 24*7 and reduce the weather compensation curve to the lowest value possible consistent with the house reaching the target temperature. You may need to balance radiators and/or ufh loops to get all rooms at precisely the design temp.

Some mcs installers will work with a more realistic value than the 2-3 ACH that is the default. If it matters in your case @Dillsue (which it may well not given your low loss), collect together the evidence and shop around. There is flexibility in the 'rules', some installers choose to use it, others don't. Don't be bullied!

Well they have at least fixed one flaw in many other heat pumps, including my Vaillant, namely the flat and level top. As you say they have had the good sense to put the top on a slope. Revolutionary for somewhere it rains! 64dBA isn't going to win any awards for being quiet though, this may rule them out of quite a few opportunities.