The refrigeration cycle is utilised so that the heat rejected at the condenser is used to heat a building as shown below. This is the opposite function of the condenser when the ‘fridge cycle is used for air conditioning, in which case the heat rejected is lost to the atmosphere.
The compressed gas from the compressor is passed to the condenser where heat is removed for use, and in the evaporator the refrigerant absorbs heat at a relatively low temperature from the heat source.
A typical example would be an air to air heat pump, which extracts heat from outside air even at very low winter temperatures by using the vapour compression refrigeration cycle in reverse mode. This can produce warm air at the condenser to heat a building.
Heat pumps work best when a building needs to be heated during winter and cooled in summer.
One of the disadvantages of a heat pump is the amount of electrical energy used to drive the compressor. This can be overcome in large installations by running the compressor from a gas driven engine. Another difficulty is noise and several heat pumps can be linked to one remote compressor to reduce local noise. This has the added advantage that compressor maintenance is centralised.