Ground source heat pumps use pipes which are buried in the ground to extract heat from the ground. This heat can then be used to heat radiators, underfloor or warm air heating systems and heat hot water in your home.
A ground source heat pump circulates a mixture of water and antifreeze around a loop of pipes – called a ground loop – which is buried in your garden. Heat from the ground is absorbed into the fluid and then passes through a heat exchanger into the heat pump. The ground stays at a fairly constant temperature under the surface, so the heat pump can be used throughout the year – even in the middle of winter.
The length of the ground loop depends on the size of your home and the amount of heat you need. Longer loops can draw more heat from the ground, but need more space to be buried in. If space is limited, a vertical borehole can be drilled instead.
Reference: The Energy Saving Trust