What is a heat pump?

Answer by Dale Blundell (Atkins)

A heat pump uses electricity to move energy from a heat source to a heat sink.  In heating mode, the source will usually be the air, ground or a body of water (such as a lake, river or the sea).  The sink is the building or process that requires heat, and the energy from the heat pump is delivered in the form of hot water or warm air.  More heat energy is provided to the sink (or user), than is used in the form of electricity, which is the primary driver for the use of heat pumps

A working fluid is moved around the closed loop heat pump circuit to capture, move and release heat through a heat exchanger.  The most easily considered example of a heat pump is a domestic refrigerator where heat is extracted from the low temperature food within the fridge, and delivered to the air outside the via the heat exchanger at the back of the unit.


The operation of the individual parts of the process is described below:


The Evaporator uses the thermal energy of the heat source to boil the liquid refrigerant to turn it into a gas.  Although the temperature of the heat source might be low, down to as low as -20°C for an air source heat pump, this is much higher than the boiling point of the refrigerant.

Most commonly heat pumps use the ambient air (air source: ASHP) or ground (ground source: GSHP) as the source of energy but water source heat pumps are becoming more common.


The motor driven Compressor compresses the gas to a higher pressure, also making it hotter in the process.


The heated and pressurised vapour passes through the condenser, which is a heat exchanger that transfers the heat from the gas to the surroundings.  It may transfer the heat to air or water, depending on the type of heating it is providing.  This loss of thermal energy means that the gas condenses back to a liquid.


The expansion valve turns the liquid back into a gas, also lowering its temperature, ready for flow into the evaporator.  The cycle then starts again.