Although we may not know it heat pumps are very familiar to us - fridges and air conditioners are two examples. Ground source heat pumps (GSHP) transfer heat from the ground into a building to provide space heating and, in some cases, to pre-heat domestic hot water.

For every unit of electricity used to pump the heat, 3-4 units of heat are produced. As well as ground source heat pumps, air source and water source heat pumps are also available.

There are three important elements to a GSHP:

1. The ground loop. This is comprised of lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground.
2. A heat pump. This has three main parts:
   
   • the evaporator - (e.g. the squiggly thing in the cold part of your fridge) takes the heat from the water in the ground loop;
   • the compressor - (this is what makes the noise in a fridge) moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit;
   • the condenser - (the hot part at the back of your fridge) gives up heat to a hot water tank which feeds the distribution system.
3. Heat distribution system. Consisting of under floor heating or radiators for space heating and in some cases water storage for hot water supply.

The ground loop can be:

• borehole;
• straight horizontal - trench costs less than a borehole, but needs more land area;
• spiral horizontal (or 'slinky coil') - needs a trench of about 10m length to provide about 1kW of heating load.