It's a machine that gathers thermal energy from one source and transfers it to another. This would normally be transferred to the water for the heating circuit within the building.

The basic principle is the same as used in a standard refrigerator. The excess heat from within the cabinet is absorbed by a liquid (called the refrigerant), this heat vaporises the refrigerant which then passes through a compressor and onto a condenser where it gives up its heat - in the case of a refrigerator to the air but in this case the water within the heating circuit. At this point it reverts back to its liquid state and the cycle starts again.

With the heat pump the principle is the same except that the thermal source is air, land or water.

With an air source heat pump (ASHP) the unit stands outside the building and it draws the surrounding air through itself. This part looks similar to a car radiator. The refrigerant is heated and then passes through the compressor and then onto the condenser.

With a Ground Source Heat Pump (GSHP) or as they are sometimes called, Geothermal, the same principle applies but the thermal energy is taken either from the land or it is possible from a lake or the sea.

With this type of unit a collector is used, being in the form of a single loop of pipe either buried down a bore hole approximately 100m down or horizontally approximately 2m below the surface in either a well-spaced loop or a in the form of a coil if there's not much space.

If there is a nearby lake this same principle can be employed but the collector has to be firmly fixed to the bed of the water source.

When looking at heat pumps you will not see the word `Efficiency` used. Instead you will see three letters: COP. This stands for the Coefficient Of Performance and this describes the ratio of heat output to work input. It would appear that the heat gained is free, however power is used to drive the compressor and in the case of ASHP the fan as well, so it is the ratio between the electricity used and the heat output. In the case of an ASHP this figure would normally be around 3 to 4 but will decay as the air temperature drops. However with a GSHP this would normally be around 4-5 but in winter there should be very little difference as the ground temperature at a depth of more than 1.5m is typically 10?C all year round in the UK.

As can be seen this is not quite free heat as power is required to drive the compressor, and in the case of the ASHP the fan as well.

Advantages Disadvantages
The source for heat will be unlimited and will not vary with time of day or cloud cover Can be noisy
Relatively compact in size of appliance They have to be supplemented with another appliance to get the water up to temperature
Have multiple options of where to obtain the heat source from (ground, air & water) They have to use energy to make little heat
  Some installations you have to be a land owner in order to insall them
  Installation costs are high