Heat pumps use the difference in temperature between a building and the external environment to provide heating or cooling. They can be air, ground, or water source.

Requires: Insulation.

Works well with: Photovoltaics, Wind Turbines, Hydroelectric.

Heat pumps extract energy from the environment through differences in the internal and external temperature. This energy can then be used to heat a building. They have the advantage over other technologies described of being able to provide cooling as well as heating. All heat pumps work on the same principals to extract energy from the environment as outlined below.

Heat pumps circulate a fluid around pipework in the external environment, and the environment heats the fluid up. A compressor then decreases the volume of the fluid, which heats it up further. Thereafter this fluid, (or another fluid with a heat exchanger imparting energy from the external one to the internal one,) is circulated internally around the building, heating it up. Having transferred heat energy into the building, the fluid cools, and it (or another fluid with a heat exchanger imparting energy from the internal one to the external one,) is circulated in the environment once again, restarting the cycle.

When cooling is desired the cycle can be run in reverse, like a fridge, to cool the interior. This process is only effective for systems that operate on air cooling within the building, rather than the use of wet central heating.

Heat pumps use electricity to run the compressor. They extract more heat energy than the electrical energy they need to run, so are over 100% efficient. Heat pump efficiency is measured in coefficients of performance, where a coefficient of performance of 1 means gaining as much heat energy as electrical energy is used for compression (100% efficient), 2 means gaining twice as much heat energy as electrical energy is used for compression (200% efficient), and so on.

The amount of energy heat pumps extract is dependent on the environmental temperature. More energy is extracted when the environment is warmer, because then there is less temperature difference to heat the fluid over. As a result, heat pump efficiency changes over the year. In cold spells when there is minimal thermal energy to be extracted from the environment, heat pumps may need to run constantly to compensate. Heat pumps are still cheaper to run than fossil fuel boilers or electric heating, provided the heat pump is properly installed, well maintained, in an insulated building, and working efficiently thanks to appropriate sizing and use patterns. (1)

It is possible to utilise an artificial thermal store in collaboration with cooling the building, storing the heat energy being extracted in the store for easier, cheaper access later. The system could also use excess energy from renewable generation systems to increase the temperature of a thermal store in the summer months, making it cheaper to heat the building in the colder periods when less energy is available from Photovoltaics, for example.

Heat pumps work best in conjunction with water underfloor heating (1, 2). This is because heat pumps and underfloor heating are most efficient when they use lower temperature heat for longer periods of time, keeping a building at a more consistent temperature.

Despite this, technically heat pumps can be used with radiator central heating and hot water systems but, due to the much greater water temperatures required compared to underfloor heating, coupling to these systems involves more powerful and expensive heat pumps for less benefit. It is recommended that if heat pumps are used in conjunction with radiators then radiators be fitted that are designed to operate at lower temperatures. This can be achieved by installing either wider radiators and bore piping or low flow temperature radiators (3, 4).

(1) Energy Saving Trust, Generating Renewable Energy, Air Source Heat Pumps. Available from: https://energysavingtrust.org.uk/advice/air-source-heat-pumps/ [Accessed 26th January 2021].

(2) Note that air-to-air heat pumps are a warm air not wet central heating system so are an exception.

(3) Evergreen Energy, Do I Need Low Temperature Heat Pump Radiators? Available from: https://www.evergreenenergy.co.uk/heat-pumps/do-i-need-bigger-radiators-to-have-a-heat-pump/ [Accessed 15th March 2021].

(4) Doug Johnson, Mesh Energy, Low Flow Temperature Radiators: The Next Best Thing? Mesh Energy Explains. Available from: https://www.mesh-energy.com/insights/low-flow-temperature-radiators-the-next-best-thing-mesh-energy-explains [Accessed 15th March 2021].