When is a kW not a kW?

As an industry we don’t do very much to convince people that we are not a bunch of liars. If a homeowner wants a heat pump the first hurdle is a heat pump surveyor has to come to your house and measure it up. We do this so a calculation can be done to work out what the maximum heating requirement for the house is. It is now accepted that this is what you have to do.

It puzzles me that we are still doing this. Every year literally hundreds of thousands of these calculations are completed. If anyone had bothered to put a database together by now you could easily and accurately estimate the heat loss using the data. But no one bothered to do it. So we do the maths and the tape measure work again and again and again and again. 

The only reason we do these calculations in such detail is that they are required by MCS, the gatekeepers to the heat pump grant. We all do what we are told because we love a bit of free money. Once this incredibly accurate heat loss is complete we have to select a heat pump, and this is where the drama starts.

For example, if my house needs 10.5kW to heat it to 21C when it’s -1.8C outside what heat pump should I choose? Simple. I look in the catalogue of my friendly heat pump manufacturer and choose a unit (see below).

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What would you choose for my 10.5kW requirement? Clearly the 8kW is not going to be big enough, the 12kW looks ideal and the 16kW is going to be a bit big. If that’s what you thought you are wrong.

You see, in heat pump land the manufacturers write a number on the side of the unit that bears absolutely no relevance to the number of kWs it actually produces.

Let me explain. When a manufacturer wants to sell a heat pump they have to test it at 35C water temperature when it’s 7C outside ignoring defrost.

Weirdly, the manufacturers use this figure as the model number of the unit. It’s called EN14511. Whoever made this standard up needs taking outside and giving the kicking they so richly deserve. No one ever sizes a heat pump to run at 7C. What everyone wants to know is will the bloody thing work in cold weather when I need it.

If I was at the helm of the heat pump universe, all manufacturers would give the actual output of the heat pump at 50C water temperature when it was -2C outside. Every heat pump loses capacity as you increase the water temperature and as it gets colder outside. It is a fact of physics that cannot be changed.

So if you test a unit and it gives 16kW at 35C water temperature and 7C outdoor temperature, it’s very likely it will give considerably less than that when it’s cold outside and you want hotter water in your radiators. If you can get 12kW out of the same unit in this condition you are doing very well indeed.

This is called the drop off and manufacturers are very clever at making sure you can’t get hold of this information. Go on, I dare you to post your capacity tables in the comments below. Samsung for all their faults are actually very good at publishing this data very clearly.

Here is a typical output curve for one of my favourite heat pumps rated at 16kW running at 50C flow.

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Note how the capacity is more than 16kWs when it’s really warm outside and how it falls as the ambient falls. This graph has a big dip from 5C but starts to recover below zero.

This is proper data because it is including the capacity loss due to defrost. If you have to spend 10% of your time in defrost the total kWs output falls by 10% if measured over a long period. Note also how the capacity rises when it’s really cold. When its below 0C outside we spend less time in defrost because there is less water in the air so they don’t freeze up as much.

Some manufacturers hide the defrost figures and hope no one notices so their curves look like this. 

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This is the same unit capacity curve at 50C flow, but with defrost ignored. Whoop, whoop the unit just got bigger, and it’s pure garbage.

Now I can already hear people saying, “Our unit doesn’t loose any capacity”. Really? No one breaks the laws of thermodynamics. Sorry, that’s simply not true. These manufacturers de-rate the units when it’s warm by basically taking a 20kW unit, call it a 14kW, but above 3C cap the speed of the unit is slowed down so the capacity is flat, like this.

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It’s all a horrible mess. Every time a unit is specced up, the homeowner quite rightly thinks installers are oversizing and selling bigger heat pumps than they need to, and installers and suppliers have to then explain all of this stuff to them. It makes us all look like amateurs.

Here is how the conversation goes: “Sorry Mrs. Miggins, but we need to put a 16kW unit in your house to cover your 10.5kW load. I know it sounds like bullshit, but the manufacturers lie in the data, but I can assure you it’s a good unit.” 

I think the MCS, the Heat Pump Association and the Heat Pump Federation should kick the manufacturers’ arses and make them give real figures, so that homeowners can buy heat pumps with a greater level of transparency and confidence.

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How many heat pumps are NOT eligible for the BUS?

Graham Hendra
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Kev M
5561 kWhs
2 years ago

Graham, thanks, very interesting. A couple of questions.

(1) Can you explain how you got from Mrs Miggins’s 10.5kW heat requirement to the 16kW HP? I can see that defrost might account for 10% but I’m not sure how you got beyond 12kW.

(2) The manuals for my ASHP (Ecodan) list capacities at minimum, medium, nominal and maximum compressor frequencies and it can achieve its badged capacity across nearly all conditions by varying this. When you talk about the capacity used in ASHP calculations, which is it; the maximum, nominal or what? Surely you don’t want the ASHP running flat out much of the time?

I don’t think the curves in the Ecodan manual includes defrost but in the tables, the figures for 2 deg C do.


943 kWhs
2 years ago

Very interesting article. I can’t see the images referred to though?

John hunter
82 kWhs
1 year ago

Refreshing article – since a heat pump is usually replacing a gas boiler why can’t we estimate the requirements simply by the amount of gas used used historically at peak times? My ancient 14kW gas boiler went on/off 10 times a day but averaged 2kWh over February – so with a COP4 do I only need a 500W ASHP running continuously?

5 months ago

hello, we have these figures from our heat loss calculations :

Heat loss if known kW: 15.5
Heat loss calculator figure kW: 6.4
Heating Flow Temperature °C: 50
Total area 145 sq m

What HP will you install?

Graham Hendra
Reply to  David
5 months ago

Put your deets into http://www.genous.earth and I will sort this for you GH

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