Ground source heat pumps use pipes buried in the garden to extract heat from the ground. This is usually used to heat water for radiators, underfloor heating systems and hot water. Beneath the surface, the ground stays at a constant temperature, so a ground source heat pump can be used throughout the year - even in the middle of winter.

A ground source heat pump circulates a mixture of water and antifreeze around a ground loop buried in the garden. Heat from the ground is absorbed into this fluid and is pumped through a heat exchanger in the heat pump. Low grade heat is then extracted by the refrigeration system and, after passing through the the pump compressor, is concentrated into a higher temperature (up to 55^oC) producing useful heat capable of heating water for the heating and hot water circuits of the house. The ground loop fluid, now cooler, passes back into the ground where it absorbs further energy from the ground in a continuous process while heating is required.

The efficiency of a ground source heat pump is measured by a coefficient of performance (CoP) - the amount of heat it produces compared to the amount of electricity needed to run it. A typical CoP for a ground source heat pump is around 3.2 if used with under floor heating (it can be reduced if used with radiators). This means for every unit of electricity used to power the pump, you could get 3.2 units of heat.

On average a ground source heat pump could save around 540kg of carbon dioxide every year when replacing an oil boiler. Emissions can be reduced further if the heat pump is partly powered by another renewable technology, such as solar electricity (PV).