Beyond a metre deep, the temperature in the ground is fairly constant - it varies by only a few degrees from Summer to Winter.

A Ground Loop is a means of extracting heat from the ground. Long coils of pipe are laid in an out-and-back loop at the bottom of a sand-lined trench. The pipes are then covered with sand and gravel, back-filled, then connected to a Heat Pump and filled with a water / anti-freeze mixture.

The fluid output from a Heat Pump is very cold. Pumping it around the Ground Loop warms it back up - in Winter, to about 6C. The heat gained is then extracted by the Heat Pump for use in the home; the cycle then begins again.

In summer, the ground around the pipes has been cooled by the heat pump after heating the house through the winter. By running the fluid in the ground pipes through a simple cooling coil built into the inlet duct of the heat recovery system (see comfort cooling & air conditioning); the whole house can be cooled for the cost of running a circulation pump. This cool fluid can also be sent into the underfloor heating pipes to give additional cooling if required, and/or one room can be cooled - like a gym, a larder, or a wine cellar.