A heat pump operates most efficiently when the temperature gap between the heat source and the heat demand is minimised.
The efficiency of a heat pump is given by the Coefficient Of Performance (COP). At a COP of 3, it will give 3 units of heat energy for each unit of electricity used. The COP will vary as the source temperature changes.
However, the COP excludes factors such as any auxiliary electric heater or an immersion heater used to ‘top-up’ heating or hot water, or electricity for pumps and fans. It is more useful to look at overall system efficiency – total heat output compared to total electricity use across different weather conditions. A Seasonal Performance Factor (SPF) can be used for this, so you may see this term quoted as well.
See Are heat pumps environmentally friendly? for more about why reaching a high SPF/COP is important.
To reach an SPF or COP of about 4 you must have a very well insulated house - usually a new-build or an extensive renovation. Heat pumps will not be able to heat water to 75°C for standard radiators, so you’ll need a low temperature system. Large radiators can be run on water at about 50°C, whilst underfloor heating can be run at only 35°C. Underfloor heating can be fitted under solid or suspended timber floors, but thick carpets should not be used – they’ll stop the heat coming through.
So that a heat pump can work at maximum efficiency all year round it is usually sized to meet about 90% of the heating demand. This means some form of backup heating is likely to be needed during very cold spells – for example a wood-fired room heater. Heating domestic hot water to 60°C will further diminish efficiency, so an immersion heater is often used - which means more electricity use and higher costs. Solar water heating is a good alternative for hot water - see our solar water heating section for more advice.
High quality heating controls are needed to run the system as efficiently as possible. Weather compensation controls regulate internal heating according to the outside temperature. For example, in cool weather underfloor heating may need water at only 25°C to provide suitable comfort.