Posted by: @squiff

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Well into my journey of moving to an ASPH, and want to get the sizing right - it seems to be a confusing topic!

I had a heatloss survey done, heat loss at -1.6c is 10.3kw.  To complicate things I'm planning a future loft extension, so want enough headroom for that, surveyor said to allow additional 1.5kw. 

Thanks for this forum I'm very aware I don't want to oversize the install, but it also needs to get MCS approval.  The heatloss survey assumes temperatures that are frankly silly (22c for bathrooms! and 21c for living spaces.  I never do either), and I have two log burners for when it's really cold, so I'm much more relaxed than MCS assume.

For various reasons I'm looking at the Daikin Altherma 3, which has a 'class' of 8/10/12/14/16/18.  Naively you think 'well I want about 11.8kw to cover lowest temps, so a 12kw will be fine esp with wood burners to top up'.  But my installer pointed me to the 'nominal rating', for instance the Daikin '16 class' is rated to only 11.54kw at -2c, the 14 to 10.17kw.  A 11kw Ecodan rates at 6kw for -7c!  It seems that all the ASPH have a much lower real rating when the temp drops, which is just when you need it!

I want to avoid having a largely oversized unit for the vast majority of the year when you don't need that power, but conscious of not undersizing for when you actually want that output.

So my question is how do you size a unit??  Do you size it based on the -2c 'nominal' kw rating?  If so it seems the name plate rating is misleading at best?

 

Where did you get the data for the Ecodan? According to the manufacturers data at an OAT of -7C, the 11.2kW Ecodan has a capacity of 10.6kW at a LWT of 50C and 10kW at a LWT of 55C.

Dependent upon the manufacturer and model, I would usually suggest sizing a heat pump at 1.25 to 1.5 times the heat loss calculation. Of equal importance is the sizing of the heat emitters, since the larger the heating capacity, the lower the required LWT, and hence the heat pump operates in its more efficient operating range and is capable of producing more thermal energy.

My mistake with the Ecodan spec, I googled 11.2kw but the link opened a 6kw spec sheet!  

Thanks re the 1.25>1.5x sizing rule of thumb, sounds like a 16 class Daikin will be right for my 11.8kw loss property.

@squiff

I have read in several places that it is better to slightly undersize a heat pump than oversize it as it improves efficiency. If you have log burners in rooms where you want the most heat (ie living room) then it would seem perfectly sensible to me to err on the small size. The rest of the house could be heated to a lower temperature. Its probably best to do your own research for this and go with what you think as you may not get a consensus here.

@bontwoody @squiff

Also you must check the manufacturers capacity tables at the design OAT and FT.  Some manufacturers 'name' their heat pumps very conservatively (eg the Vaillant '7kW' is more like 8.5kW at a -2C/45C design scenario which is not uncommon), others go well the other way.    Then there are manufacturers that, at least according to their capacity tables, cap the output at the nominal figure in the firmware over a wide range of OAT/FT.  So don't believe the nominals, check the capacity tables.  Good luck with getting capacity tables from Daikin BTW, they refused to provide these when I requested them and insisted instead that I ask an installer (obviously, in Daikin's view, Joe Public is not to be trusted with facts and is only fit to receive marketing hype

This set of data, published by BRE, purports to represent the actual efficiency of a range of heat pumps as a function of house load, and  is one of the pieces of 'evidence' quoted in support of the view that, at least in some cases, modest under sizing yields better efficiency than oversizing.  The plots are based on manufacturer data, I believe.  I have no idea if they have been verified (it would be almost impossible to do so) but BRE is well thought of.  Having said that there seem to be some strange figures so I do wonder if they have 'pushed' the model too far in some cases.

Posted by: @squiff

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So my question is how do you size a unit??  Do you size it based on the -2c 'nominal' kw rating?  If so it seems the name plate rating is misleading at best?

I just re-read your post and noticed this which I missed first time round.  The answer is a definite 'no'.  The approach to 'name plate ratings' varies a lot between manufacturers as stated in my previous reply.  Also it is true that ASHPs will tend to have lower ratings at lower temperatures (its a function of the physics, sadly). Check the capacity tables before reaching a conclusion! 

Also be aware that heat loss surveys vary in their reliability.  My property has been 'professionally' surveyed (by MCS contractors) at anything between a shade over 16kW and a shade under 8kW, depending on assumptions made about air change rate and fabric.  The 16kW surveys (two of them) totally ignored fabric upgrades despite the fact I made a point of telling the surveyor about them several times, double counted room to room losses and used the maximum air change rate in the recommendations.  The 8kW survey assumed much lower air change rates, accounted for fabric upgrades and didn't double count room to room losses.  For what its worth the measured (gas) consumption at a constant -2 is 7.5kW (we had 45 days last winter when it was -2 pretty much day and night).  If I were to go with 16kW and add 25%, the heat pump would be cycling unnecessarily during most of the heating season!

so you definitely don't trust the nameplate number. you must read the spec sheets for the actual outputs.

under vs over is more nuanced though. I have a somewhat oversized system with 16kw, likely worst case steady state heating load is 10-12 kw.

take a look at my stats (link in my sig) . These  now contain the first few heating runs of the year , with WT=36 which is the low point of my WC curve. ignore the ones where WT getting to 55+, those are for HW. There is some cycling evident as the heating load is low. but efficiency is still perfectly reasonable. 

having an slightly oversized system means it will  have the capacity to recover the HW cylinder faster (if that is important to you - it is to me - on HW runs I can and do clock 16kw) , recover the house from too-cool more quickly, and cope with defrost cycles better. slightly under means likely better efficiency at low-moderate loads. YMMV and/or "you pays your money and takes your choice", as they say.

@iancalderbank Didnt know you were on http://heatpumpmonitor.org/ Ian, good idea putting it in your signature so I've 'borrowed' it 🙂

Imho the biggest problem is to determine 'right size'.  The spreadsheet models, even if used correctly, make several more or less unverifiable assumptions about the fabric, particularly for houses that have a rich history.  That's not a criticism of the spreadsheets, it's real life uncertainty.

If you can get a measurement then do so.  It's another datapoint to supplement the (potentially Gigo) spreadsheets.

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. 😉   

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

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?

Generally I agree wholeheartedly with your sentiment.

However excessive oversizing can lead to other problems.  One of the MCS contractors who did a full survey of my house came up with 16.5kW, and thus a recommendation that I install 2 '16kW' mideas.  That would require planning consent which there would be almost no chance of getting, and is anyway impossible to site, and that's just the beginning of the issues.

The risk to my mind is adding 25-50% on top of a survey that is already over cooked (or has built in margin), either because the surveyor did a shoddy job or due to fabric uncertainty.  This can lead to a system which is double the required size or more with potentially significant knock on effects beyond the unit itself.  If this is a conscious decision with full facts that's fine, but if the decision is without knowledge of the facts, less so.

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.