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Sizing of Heat Pumps

Dan Gates


The sizing of heat pumps has a critical effect on the running cost, maintenance and capital cost of your installation.

Traditionally with gas central heating the ‘combi’ boiler is massively oversized compared to your property’s actual heat load. 

Heating installers have done this to cope with the instantaneous hot water load. To heat water with a ‘combi’ boiler you may been to heat say 10-15 litres per minute of hot water for your taps and showers on demand. This take a lot of ‘power’ e.g. the kW rating of the boiler. Hence an installer would fit 20 to 30kW gas boilers to meet this requirement. These then go into central heating mode when not needed for hot water. Sizing in this scenario is easy and ‘rule of thumb’ guides can be used like this one here, suggesting a 2 Bed house needs 25kW. 

However, the average heating heat loss for a UK home is around 8-15kW in mid winter.

The modern gas boiler is typically not “fixed output” but can modulate to say 20% of it’s output. So your 24kW combi can run effectively down at 4.8kW and no one really notices the difference (apart from various issues shown here by Adam at Heat Geek such as  shorting the life of your boiler).  

Heat Pumps are much more expensive and need to be carefully sized to the actual heat load. 

“Oversizing heat pumps can mean your heat pump runs at <300% efficiency when it should be at 360% efficiency”

How to size your heat load ?

  1. Design Day- Typically heating is done for the ‘design day’ – this results in systems to cope with the peak demand on the coldest days of the year. In Glasgow lets say this is -4’C. This is the 99% chance of being coldest day according to analysed weather data. There is 1% of the year there’s a chance its much colder than that, but most of the time your building will need much less capacity as the graph below shows. 

Heating curve

We know what the peak power to size the heat pump for on this day (the kW rating) by measuring or calculating the rate of heat loss from the property. Lets say we have a new build and have minimised the rate of heat loss from the fabric (walls, roof, windows and so on) and also made the building more airtight (loss from draughts etc is reduced). In this case lets when it is -4’C outside then the heat load for a new build is 4kW. 

2. We then think maybe I do not want to get this wrong and undersize it? (everyone really dislikes being cold in midwinter). So we add some ‘safety’ factors and think 5-6kW. 

3. You approach heat pump companies and installers and they say: “how about a 8kW heat pump?” (they might not have a smaller model, or haven’t spent a lot of time looking at your design and actual heat loss and want more safety). They show you government endorsed graphs from MCS that show the ‘efficiency’ (a COP- Coefficient of Performance) of this 8kW heat pump is 380%.  Sounds ok?

4. Well maybe not- the larger heat pump needs more bigger fan units, pipes, buffer tanks and so on. This could cost you up to 20% more for your heat pump capital cost compared to the smaller model. Remember the cost difference between the small gas boiler and a larger gas boiler might be £100, but in heat pumps the difference between a small heat pump and a larger on could be hundreds of pounds (if not £1-2k). 

5. It could also be running 20% less efficient – the first graph below shows the frequency of time the 8kW heat pump would be running on our 4kW property – i.e. nearly 50% of the time is in the “under 20%” of its 8kW power zone.

6. Heat pumps use compressors to move the heat energy into your building. In the second graph below we can see how a compressor will change under different load characteristics. Lets say this manufacture has a Scroll compressor, so following the green line, you can see the efficiency will drop from 360% (COP 3.60) at 42% of capacity to 300% (COP 3.00) at 10% capacity.

7. The final graph below shows the time a 3.5kW heat pump would sit in the ‘sub 20% output zone for the compressor’- i.e much less at 165 of the year. The load of the heat pump compressor is much more matched to the building needs and hence the overall system efficiency is much improved. This could be a 20-30% saving on your fuel bill for heating. It will also mean less ‘starts’ for the system and thus less wear and tear for the heat pump and prolong it’s lifespan.

In Sweden they would typically size your heat pump at 90% of building load and allow (or even in some cases mandate) the use of say a wood stove to supplement your peak heat loss of the coldest days. In the UK installers are instructed to size to 100% of your building capacity from the heat pump with no back up. 

Not every project can afford a ‘dynamic ‘ energy analysis like the graphs above but you may save more then the design cost back in running costs in your first year alone. In any case its worth considering the hourly load profiles and the heat pump characteristic when selecting them and not just taking manufacturer data. 


Further information is also found in Nicola Terry’s blog here:

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