A heat pump is a device able of upgrading low value surrounding heat (from ground, well water, solar, air, internal building heat, waste heat), in a very efficient manner, to a useful temperature level for the required application (domestic and service water, pool, and process heating).

In a physical sense, all refrigeration equipment, including air conditioners and chillers with regfrigeration cycles - so also the refrigerator in household application, are heat pumps. In engineering however, the term ‘heat pump’ is reserved for equipment that heats for beneficial purposes, rather than that which removes heat for cooling only.

The source of which heat is withdrawn from has to have sufficient energy content, so that continuous drop in temperature and depletion is prevented. Besides the suitability of the heat source is also determined by its temperature. Therefore, for example, outside air is not a useful heat source, because at the time of the largest heat requirement (in winter) the source temperature is lowest. The ground on the other hand is an extremely suitable heat source for a heat pump.