Posted by: @bontwoody

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The point I am making is that a slightly undersized heat pump may not just be cheaper to run because is smaller, but also because it is running at a higher COP. I am not a heat pump engineer so I am just relaying opinion from people who know more than me and you may not agree with those views.

but a somewhat oversized heat pump that is cycling in low demand, can also give a high COP, if everything else is done properly. Mine from this morning with a very low heating demand, cycles 3 times, COP is 5.2

one cannot simply make a blanket assumption that smaller heat pump gives better COP at low load.

Posted by: @bontwoody

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The DHW issue is a bit of a red herring in my opinion as I used to recharge a 300lt cylinder to 52C daily in well under 2 hours using a 5kW ASHP in my last house.

disagree about the DHW red herring: with a 16kw, I can reheat my 300L UVC in just over 30 mins. which means less downtime for the heating loop, and less "grumble time" if someone is complaining that hot water has run out.

Posted by: @bontwoody

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@squiff As I said in my first post "Its probably best to do your own research for this and go with what you think as you may not get a consensus here." LOL

agree 🙂

Posted by: @jamespa

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It's unfortunate that we currently don't have a fool proof way to measure the loss that is generally accepted, if we did then choices could be more finely tuned and better informed.

totally agree. that, in a nutshell, is the root cause. If one genuinely knew, for absolute certain, what the house needed, then you could make an informed decision what size you wanted.

but if the 10kw you've been given by the survey has already been doubled over the actual figure by the uncertainty of the heat loss methods, and then someone adds more "for safety" you could end up with a 16 when your house only needs a 5.

Posted by: @iancalderbank

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one cannot simply make a blanket assumption that smaller heat pump gives better COP at low load.

Thermodynamically it must be less efficient to cycle than not to cycle, because a higher flow temp is required to achieve the same output from the emitters.  Whether this is enough to matter is of course a different thing,  If it only happens at reasonably high OATs, almost certainly not.  If it happens throughout most of the heating season then maybe, depending on personal priorities.

Posted by: @bontwoody

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Posted by: @kev-m

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Sometimes I think we lose sight of why we spend all this time and money on expensive and complicated heating systems. Isn't it to keep warm and have lots of hot water (literally) on tap?  Why would anyone spend many thousands on something that can partly heat your house and hopefully give you enough HW?

Of course an undersized ASHP will cost less to run if it's not heating your house when it's below zero.  But then, you'll either be cold, not have enough HW, or have to spend money on logs or - and I can't believe this is even suggested - electric fan heaters. Or all three. This will happen more often than the much  quoted 5 days (or whatever) per year, because of defrosting. 

Being a little less facetious, an undersized ASHP will, under most circumstances, cost less to run than a correctly sized one just because it's smaller.  Like a Polo is cheaper than a Golf.  But even this may be offset a bit when the small ASHP has to run at full throttle where the big one would be cruising.  Actually that's the main argument against oversizing; that a big ASHP costs more to run (and buy) than a smaller one, just because it's big.  Not because of cycling btw, which makes relatively little difference, is only a factor in warmer weather and can often be 'tuned out'. 

As @iancalderbank says, it's your money and your choice. I'd rather have a bit in reserve and my car and ASHP are both a bit bigger than I really need. By about 1.25 to 1.5 times I think. 😉   

While I dont disagree with you about being warm and having enough hot water Kev, most people also have to consider cost as an important factor.

The point I am making is that a slightly undersized heat pump may not just be cheaper to run because is smaller, but also because it is running at a higher COP. I am not a heat pump engineer so I am just relaying opinion from people who know more than me and you may not agree with those views.

The DHW issue is a bit of a red herring in my opinion as I used to recharge a 300lt cylinder to 52C daily in well under 2 hours using a 5kW ASHP in my last house.

@squiff As I said in my first post "Its probably best to do your own research for this and go with what you think as you may not get a consensus here." LOL

 

@bontwoody

when I said it was cheaper to run because it was smaller I meant that it would have a better COP.  If you compare Mitsubishi Ecodans, the smaller ones tend to have a better COP for a given OAT/LWT.   

The 2 hours to heat your tank with a 5kW ASHP may be a problem if your heat loss is 5kW, it's -2 deg outside and the ASHP has just finished a defrost. 

Of course cost is a consideration but other heat sources are probably going to be more expensive than an ASHP.  I just think it's a bad idea to undersize a heating system.  As you say, others don't.

@bontwoody

There are quite a number of aspects to consider when it comes to the sizing of a heat pump.

As James has correctly pointed out, the starting point must be to try to obtain a reasonably accurate heat loss assessment, which would appear to be easier said than done. I have some ideas upon which I will expand later.

Assuming that a reasonably accurate heat loss assessment has been obtained, when you talk about slightly under-sizing what exactly do you mean? Would it be an 8kW heat pump for a heat loss of 8kW, or a 7kW heat pump?

The next most important factor is the correct sizing of the heat emitters, or rather installing the maximum heat emitter capacity in a balanced manner. It is somewhat pointless fitting a larger radiator in a particular room, if a similar sized room has a smaller radiator installed. The heat emitters need to be sized to the heat loss of each room if at all possible. This then dictates the required LWT needed to meet a certain level of heat loss.

Using the data for the various sizes of Midea heat pumps, I applied the following operating criteria. A hypothetical home with a heat loss of 8kW at an IAT of 21C and OAT of -3C.

With total heat emitter capacity of 16kW the following results would be obtained:-

A 8kW heat pump would need to supply 7kW of thermal energy when the OAT is 0C at an average heat emitter temperature of 47.5C, so probable LWT of 50C, which it can achieve at a predicted COP of 2.78.

If the OAT was to fall to -5C, the heat loss would increase to 8.67kW, with a required LWT of 54.7C, with the result that the IAT would not be maintained at 21C, and would likely fall to approximately 14.3C.

A 10kW heat pump would need to supply 7kW of thermal energy when the OAT is 0C at an average heat emitter temperature of 47.5C, so probable LWT of 50C, which it can achieve at a predicted COP of 2.75.

If the OAT was to fall to -5C, the heat loss would increase to 8.67kW, with a required LWT of 54.7C, with the result that the IAT would not be maintained at 21C, and would likely fall to approximately 17.6C.

A 12kW heat pump would need to supply 7kW of thermal energy when the OAT is 0C at an average heat emitter temperature of 47.5C, so probable LWT of 50C, which it can achieve at a predicted COP of 2.42.

If the OAT was to fall to -5C, the heat loss would increase to 8.67kW, with a required LWT of 54.7C, with the result that the IAT would be able to be maintained at 21C.

A 14kW heat pump would need to supply 7kW of thermal energy when the OAT is 0C at an average heat emitter temperature of 47.5C, so probable LWT of 50C, which it can achieve at a predicted COP of 2.47.

If the OAT was to fall to -5C, the heat loss would increase to 8.67kW, with a required LWT of 54.7C, with the result that the IAT would be able to be maintained at 21C.

So undersizing a heat pump would mean running the risk of cold feet and earache from 'she who must be obeyed'.

I feel certain there will be some who state "well I don't mind it being a little cooler" or "I'll just fire up the wood burner", and these are all valid comments. The other thing to remember is the reason that an IAT of 21/22 is used for the heat loss assessment, because most may find these to be acceptable temperatures. The installed heating system must therefore be capable of achieving these temperatures, otherwise any new owners may not be too happy when they find they cannot keep warm come the next Winter.

The above figures do not include any allowance for performing defrost cycles, which remove thermal energy from the system, which then has to be replaced. This can only be accomplished if the heat pump has sufficient capacity to do so.

The other thing to consider is DHW production, which has the effect of removing the thermal energy supply to the home for a period of time. The heat loss of course does not stop and has to be replaced when the heat pump commences central heating once more. This is a further reason to have some excess capacity available.

As others have said "at the end of the day it is your choice", but I would caution prospective heat pump purchaser's to heed the advice given by James, do your research, questions any statements or figures provided by any possible installers, do your own heat loss calculations and be suspicious if your installer is suggesting 2 x 16kW heat pumps to supply your calculated heat loss of 8kW. Could the fact that your installer just happens to have 2 x 16kW units sitting in the back of their stockroom have anything to do with their proposal? 🙄 

I am surprised that no manufacturer, as far as I am aware, has produced a heat pump unit containing two compressors. One capable of say 4kW output with the second rated at 8kW, giving a total capacity of 12kW.

The 4kW compressor being used for DHW during the Summer period and milder weather when heating is required. The 8kW compressor taking over as heating demand increases, and both compressors being employed when the occasional 'beast from the East' arrives. Simples 😎 

@Derek M

I would just like to reiterate here that I am not a heat pump engineer and I don’t have an axe to grind about whether it is better to slightly oversize or undersize a heat pump J. All I wished to do was point out that there is a school of thought that takes the latter view.

You have obviously gone to some trouble to do those calculations and thanks for taking the time to do that.

@JamesPa has already linked to the data that I was referring to and there is an interesting discussion about it here: https://forum.buildhub.org.uk/topic/34566-bre-heatpump-calculator/

“when you talk about slightly under-sizing what exactly do you mean?”

In my mind I guess I was considering 5-10% as being ‘slightly’. I would however consider 50% oversizing not to be slight. There are multiple sources that consider that  the increased cycling caused by having an significantly oversized heat pump would reduce the longevity of the system, even if as @iancalderbank ‘s graph showed the COP was better than acceptable.

John Cantor is one source of that opinion and while he doesn’t advocate undersizing a heat pump, the following statement from his website does I think give food for thought:

 “In Sweden and other countries, they have successfully used small heat pumps, but the UK’s MCS scheme (Microgeneration certification Scheme) had a requirement for the system to be large enough so as to supply all of the building’s needs on the coldest day.

In my personal opinion, a lot of heat pump units are too big, and some heat emitter (e.g. radiators) circuits too small.”

With respect to the heating of DHW, would not a successful strategy be to heat it overnight when the heating is either off or setback? I appreciate that if you completely deplete the cylinder during the day then that wouldn’t work, but my wife and I easily manage with a fully charged 130lt Mixergy cylinder for 2-3 days

To come back to the original post by @squiff , he mentions that he has two wood burners and while I am a soft Southern Midlander **** who like his living room a balmy 23C J, topping up the heat pump on particularly cold days when COP is low seems like a viable strategy, particularly if logs are cheap/plentiful as in my neighbourhood.

I like the idea of dual compressors, can you patent it quick!

If you have a smart meter and gas heating, you have in principle very accurate data on demand.  It's not entirely easy to interpret, and some additional info may be needed on heating patterns (particularly spatial patterns, ie heating only some rooms - temporal variations are evident from the data) but most of the data needed is there.

I would strongly advise to compare smart meter readings, if you have them, with any spreadsheet calculations if only as a sense check.

I'd lay odds that Octopus are working on using the data they already have more intelligently.

The combination of a smart meter plus a couple of cheap temperature/humidity sensors would absolutely nail it.  Together these would give enough data to calculate load, house heat capacity and much more if properly processed.

Posted by: @jamespa

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If you have a smart meter and gas heating, you have in principle very accurate data on demand. 

for those not aware, hildebrand glow brightapp allows you to get your own hourly gas data for free, regardless of supplier. I used it during my own analysis 2 years ago.

Posted by: @bontwoody

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DHW, would not a successful strategy be to heat it overnight when the heating is either off or setback? I appreciate that if you completely deplete the cylinder during the day then that wouldn’t work, but my wife and I easily manage with a fully charged 130lt Mixergy cylinder for 2-3 days

well yes of course  in winter I heat the DHW overnight to start with.

But: we are 4. our teenage boys sing in the shower and can take 20 mins plus. long hair wash day for mrs: 30 mins. I have next to no hair and take 3 mins :-). we have eco shower heads. But even the 300L I have isn't always enough for a day 🙁 . DHW usage varies a lot between households.

Also , the cylinder might be smaller in some houses for space reasons or perhaps it was previously boiler driven so a little smaller due to the higher temp of boiler driven was ok. so running out and needing a reheat mid-evening might be par for the course every day in that situation.  I think that being able to quickly reheat matters in many scenarios.

Posted by: @jamespa

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The combination of a smart meter plus a couple of cheap temperature/humidity sensors would absolutely nail it.  Together these would give enough data to calculate load, house heat capacity and much more if properly processed.

Looks like these guys are onto it, albeit for energy efficiency not system sizing.

https://www.current-news.co.uk/new-smart-meter-technology-to-offer-uk-households-tailored-energy-efficiency-solutions/

But: we are 4. our teenage boys sing in the shower and can take 20 mins plus. long hair wash day for mrs: 30 mins. I have next to no hair and take 3 mins :-). we have eco shower heads. But even the 300L I have isn't always enough for a day 🙁 . DHW usage varies a lot between households.

@iancalderbank lol maybe frogs have the right idea after all with respect to child care 🙂

Also , the cylinder might be smaller in some houses for space reasons or perhaps it was previously boiler driven so a little smaller due to the higher temp of boiler driven was ok. so running out and needing a reheat mid-evening might be par for the course every day in that situation.  I think that being able to quickly reheat matters in many scenarios.

Good point!

Posted by: @bontwoody

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But: we are 4. our teenage boys sing in the shower and can take 20 mins plus. long hair wash day for mrs: 30 mins. I have next to no hair and take 3 mins :-). we have eco shower heads. But even the 300L I have isn't 

Also , the cylinder might be smaller in some houses for space reasons or perhaps it was previously boiler driven so a little smaller due to the higher temp of boiler driven was ok. so running out and needing a reheat mid-evening might be par for the course every day in that situation.  I think that being able to quickly reheat matters in many scenarios.

Good point!

True, but surely its alright to go without, or just use the immersion, on the handful of days during the year when the OAT is at or around its design minimum. 

OK silly me, I watched, with disbelief, the TV coverage of the wildfires in Rhodes, with people complaining that the authorities  (who seemed to be doing a rather good job) needed to explain why the tourists were inconvenienced (er... its because you are addicted to burning fossil fuels and jetting around for holidays stupid).   Meanwhile the locals were losing their homes and livelihoods.

Thus it should be obvious to me that the system "must" work perfectly even in the most extreme circumstances.  

How hopelessly decadent we have become!  

@jamespa I sympathise James, the same arguments apply around the 'war on cars' once people get used to something, having something slightly less convenient is a very hard sell.

@bontwoody

It sounds that you are contemplating becoming 'bad consumers' like me. Please remember that the Chinese economy will collapse if you all stop buying things that you probably don't actually need. 🙄