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Sizing a heat pump correctly is vital for good performance

Posted by Gabriel Wondrausch on 9 July 2012 at 9:17 am

Unlike a conventional fossil fuel boiler it is essential that a heat pump is sized accurately to meet the heat demand of the property. The reason for this is because over-sizing will significantly increase the capital cost for little operational savings. It will also mean that the heat pump operates at part load for more of the time and frequent cycling will reduce overall efficiency and equipment life.

If the heat pump is under-sized it will not be able to meet the heating requirements of the building. To size a heat pump system the design heat load of the building must be known. This must be calculated on a room by room basis in conjunction with the latest standards to give an accurate understanding of individual room heating requirements. It is also crucial to ensure the heat emitter (whether radiator or underfloor heating) meets the heating requirement of the room in which it’s located.

Heat pumps are designed to operate most efficiently at low flow temperatures between 35˚C - 55˚C. Most conventional heating systems have radiators sized to operate at flow temperatures between 65˚C and 80˚C. To meet these temperatures with a heat pump is possible but will significantly reduce the efficiency. Ideally to maintain high efficiency and reduce the running costs of a heat pump the best heat emitters are either underfloor heating, fan convectors or over sized standard radiators.

Heat pumps are often characterised by their COP (coefficient of performance) which is the amount of heat energy produced from every kW of electrical energy imported. The higher the coefficient of performance the more efficient the system. A typical COP for a heat pump is between 3-4 units transferred per unit of electric energy supplied. Electrical heating for example has a coefficient of performance of 1 (for every unit of energy you get one unit of heat).

The COP will commonly decrease when the flow temperature increases. This is the reason why low flow temperatures are much more efficient. A flow temperature of 60˚C+ can have a COP of 2.1 where as a flow temperature of 35˚C will have a COP of 3.6+. This emphasises the importance of a low temperature heat emitter. A standard fossil fuel boiler has an efficiency of approximately 85-95% in comparison to a heat pumps average 300% efficiency.

Although it is preferable to have a low temperature heating system at times it is not practical. 

We always advise that every measure is taken to improve the energy performance and heat loss of the property before considering a renewable heat source. In certain cases only minimal measures can be taken to improve the heat losses i.e. listed buildings with single glazed windows, walls without cavities.

In these situations a low flow temperature may not be practical therefore requiring high flow temperatures to ensure the individual room heating requirements can be met and maintained. This is still achievable with a heat pump and worth considering as the efficiency can be double the efficiency of a fossil fuel boiler.

Blog by Matthew Lauri: Photo SunGift Solar

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

If you have a question about anything in the above blog, please ask it in the comments section below.

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2 comments - read them below or add one

SunGift Energy

SunGift EnergyComment left on: 11 July 2012 at 10:45 am


Thank you for your comment. 

Indeed you are absolutely correct in making a comparison to the running costs of a gas boiler. The national average cost of gas per kW is 4.8pence whereas the average cost of electricity is 14.5p/kW.

At 300% efficiency with a heat pump the running cost will be 4.8p/kW, which matches that of a gas boiler. For this reason, the Government have not allowed on gas grid heat pump installations the entitlement to claim the RHI as there is little justification for changing their current system from a gas boiler to a heat pump.

Oil, on the other hand, is an average 6p/kW (and rapidly rising) which gives a much more valid reason to make the change. Also, not forgetting that a ground source heat pump or low temperature heat pump offer 400% efficiencies, which reduces running costs and CO2 emissions retrospectively.

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circolocoComment left on: 9 July 2012 at 11:12 pm

You say

" A standard fossil fuel boiler has an efficiency of approximately 85-95% in comparison to a heat pumps average 300% efficiency."

However please do beware that more often than not the fossil fuel in question will be 3 to 4 times cheaper than grid electricity and maybe 3 time less CO2 producing (currently) than grid electricity so indeed you would need the heat pump to be at the very least 300% efficient to even match that of lets say a gas boiler.....and rarely do heat pumps keep a constant. 

Undersize or choose the wrong fuel and you may end up causing more damage to not only your pocket but also the enviroment.

There are many companies that really do not understand these systems and are ruining what can be a very good product if understood. 

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