JamesPa

Pretty much. A few months ago I wondered if its main advantage was the absence of the need for a vent with all that comes with that malarkey. That said most (but not all) actual applications where a tank this small was the only practical option would probably be in a location where running a vent wasn't too difficult. Excepting the cases where running a vent is difficult, the question becomes - if you have only enough space for something this small (and not enough money for/dont trust a sunamp), whats the best solution - a 80l (or smaller) UVC or a mini store? Thats a very specific problem, which likely applies to a fair number of smaller houses which currently have a combi. In either case you may be forced to reheat at times of day other than cheap periods (depending on your pattern of use). I have a feeling that a very well designed small UVC that is well stratified so you can reheat whilst running water off and maintain stratification may well work out better, but Im not absolutely sure, it may depend on run-off rate and is there such a beast? The mini-store is a solution (if its a solution to anything) to the specific problem of insufficient space for a 150l+UVC. But, in the grand scheme of things, that represents a large sector of the housing stock so the question 'is it a valid solution given these constraints' (and if not whats the alternative given the constraints) is relevant. Horses for courses maybe?

That's a bit of a generalisation, and note that the mini store has only one heat exchanger between heat pump and dhw not two like a 'conventional' thermal store plus it's designed for the purpose. I'm not trying to advocate it just saying I'm not sure you can dismiss it quite so simply. I think it may actually depend on use profile, bearing in mind that the alternative (in the situations of interest) is a very small uvc

The general rule as Im sure you know is that COP goes up as flow temperature goes down. So it comes to first order down to the question - whats the flow temperature? This comparisons will depend on which variables you keep constant. Both a cylinder and the mini store have one heat exchanger (coil). In the case of a cylinder heating water circulates through the coil, in the case of a mini store its the DHW (and the mini store itself is filled with heating water). So in principle the difference between DHW temp and flow temp is about the same, so COP could be about the same. However for any given cylinder volume and stored water temperature the yield (of DHW) from the mini store is different to the yield from a cylinder. At low draw off rates it can be more (according to the Newark specs), at higher draw off rates it can be less. If you are in the latter category and as a result have to increase the stored water temperature to get the yield you require, COP will suffer. I think the only way to work this out is to understand your demand profile and then plug in the numbers to estimate the temperature of the stored water required and hence the flow temperature required. This should probably be done anyway to be certain that the mini store/small cylinder is big enough, or whether you need to consider additional measures like water saving shower heads, waste heat recovery, or just not showering for 10 mins at 20l/min (if thats what you do - Im not suggesting it is, but apparently some people find this necessary!).

From discussions on another forum different ASHPs do different things. eg Vaillant has a PWM pump which it adjusts down at installation time to match its max output @ approx DT5, then leaves constant. There is also the possibility to manually adjust in the installer menu. Samsung, if you fit a PWM pump, apparently adjusts to constant DT depending on load, but with a floor rate. Daikin does something different still. Mitsubishi is constant speed I am told, although it does support a pwm pump so maybe same as Vaillant. The question, to which I have yet to find a satisfactory answer, is whats best for efficiency and/or is that itself heat pump dependent. Pure Carnot thermodynamics would argue that low emitter DT = higher average emitter temperature for any given flow temperature = better COP, so make DT as low as possible (consistent with noise, pipe erosion etc). The folks on openenergymonitor trade this equation off against water pump consumption to get an 'optimum', which is fair enough. Some others argue that there are compressor related factors which aren't just the Caront equation, however they arent specific and until they are the argument has only limited credibility. If anyone has a 'chapter and verse' on this with the physical principles on which the argument is based id be interested!

Vaillant 7kW here, double glazed living room window just 30cm from the unit and adjacent utility room window immediately behind the unit. Most of the time its totally silent indoors and whisper quiet outdoors, inaudible from >3-4m away. When it working really hard, which it does for typically 5-10 minutes after a defrost at - temperatures and very occasionally for longer periods up to say 30mins, its just audible indoors, but still quieter than the fridge in the same room and quieter than the gas flue was in the bathroom to which the flue was adjacent. Outdoors in this relatively rare situation there is a buzz which seems to appear at compressor modulation >80%. I've turned on noise reduction mode at night out of respect for the neighbours, however I only did this about a month in and after some quite severe defrosts, and they now tell me that they didn't even know I had the heat pump! Their bedroom window is about 6m away, albeit screened by the building. No noise from pipework, which is odd because there was pipework noise with the gas boiler when it was heating DHW. Overall, for me. no more of a problem than a gas boiler.

@berkshireselfbuild If the heat loss calculation is about right then the 5kW Vaillant is a pretty much perfect fit. It claims 6.5kW at -3/35C (as you have UFH I presume you will be operating at 35 or less), but note these figures may not include defrost.. Its a nice unit with good and very capable controls - better than many). As your installer says there is no point in going for the 3kW model its just the 5kW one software limited at the top end but identical at the bottom end. Your installer sounds like they may be a goodun! As @Johnmo says keep it simple 3 port diverter valve, UFH/radiators. Minimal zones ideally one, no buffer/LLH/PHE separating heat pump from emitters (2 port volumiser OK - but with the house area you have even that probably isn't necessary - but no harm if it is)

There seem to be lots of people who still believe that weather compensation is a third class control mechanism specific to heat pumps solely to overcome their limitations, and that micro zoning in both time and space is the one true and ultimate control system.😒 Perhaps the installer is in this deluded camp?

Not sure you would want to do that, two different flow temperatures (heating or cooling) means you have to operate at the least efficient of the two (the colder for cooling, the warmer for heating) and mix down/up which sacrifices cop/eer. I don't know how effective fancoils are at 16-18 TBH, hopefully someone will. I don't get to find out until summer when I will try it on mine! I can't see why they wouldn't be pretty good, after all you are forcing air over the fins pretty quickly.

Assuming lg support the function you could use the wiser controls to set the demand for heat on the heat pump, then set an appropriate bivalent point on the heat pump controller and use that to control the third party boiler. Obviously I can't tell if that's what your installer did. Whether that is wiser (sorry couldn't resist it) than just optimising the system for the heat pump is another matter. I have a suspicion that optimising the controls for the heat pump and selecting an electric tariff will work out about the same or cheaper, assuming you have a reasonably sensible heat pump set up. If there is a difference it's likely to be small and the risk is that by being 'clever' you degrade the heat pump performance and thus end up costing more.

Doesn't that depend on the design objective eg: to run at minimum cost (if so what electricity tarrif) because the heat pump alone isnt enough even at the design temperature because the heat pump alone isnt enough at some extreme temperature sub design temperature to run at minimum carbon emissions something else or some combination I cant see how you can design a control strategy until you know what you are trying to achieve and why the system is so complex (presumably one and the same thing - the system is complex because you are trying to achieve something complex)? Also whats the objective of having wiser - is it micro zoning or just dealing with extreme events like solar gain?

Not sure what the problem is. As I understand it you want to combine (for heating) fancoils and UFH. Both will run at a low temperature so thats OK No performance crippling mixing or buffer needed, just some valves so you can balance (not that you will need a lot of balancing if the fancoils have built in thermostats which modulate the fan - they will look after themselves!). One water pump (the one in the heat pump) only unless house very large. For cooling you want to run both fancoils and possibly UFH at at 7C - the problem is you will get condensation on all exposed pipework and the fancoils. The fancoils probably have condensate drains which will need to be connected up, you may have to insulate and seal the exposed pipework. Alternatively just run above the dewpoint for light cooling - some heat pump controllers handle this natively. Still cant see why you need a buffer. What function is it that you think is needed and that only a buffer can perform (I cant see one personally). The only problem comes if you want to heat and cool at the same time. You cant.

Like it. No system separation for the heat pump, only for the boiler, max efficiency. Why cant installers think like this? (rhetorical question) is the boiler used to reduce cost or because the heat pump does not have sufficient capacity?

Ok but does it really make sense to compromise the heating performance for occasional cooling? Is it flow that's the problem or cooling capacity. If the latter it will presumably just cycle, but so what. Some inefficiency but for a handful of days per year rather than throughout the heating season. Fridges cycle, nobody gets concerned. @joth seem to have a sensible suggestion which he has tried. Adding a buffer (other than a 2 port volumiser) to your heating will almost certainly give you headaches and increase running costs, because you will almost certainly end up with mixing forcing you to increase the LWT and thus inevitably reducing COP. Best avoided IMHO. (Health warning - I'm not a heating engineer, just someone with a background in physics and engineering who has spent 2 years studying this as an amateur, in order to inform his own heat pump installation)

Why make it that complicated. Connect the heat pump to the emitters, do a bit of balancing with manifold valves if necessary, get the WC curve as low as it can go whilst just heating your house, enjoy cheap low carbon heating. If you buy a heat pump with decent controls no need for hacked control solutions either. Seriously ashp systems dont have to be complicated and buffers etc (unless well set up which is rare) both cripple performance and make fault diagnosis difficult. More to the point they are unnecessary in almost all cases. What problem is the buffer purportedly solving in your case? If you lack system volume for defrost add a 2 port volumiser.

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