Heat pump
A device that uses a refrigeration cycle to move heat from a cooler source (air, ground, water) to a warmer sink, delivering 2–5 units of heat per unit of electricity.
A heat pump runs a vapour-compression cycle: a refrigerant evaporates at low pressure, absorbing heat from the source; a compressor raises its pressure and temperature; it condenses in the indoor heat exchanger, releasing heat; and an expansion valve returns it to low pressure. Reversing the cycle provides cooling.
COP = Q_heat / W_elec
Carnot limit: COP_max = T_hot / (T_hot - T_cold), temperatures in kelvin
Example: 35 °C flow, 0 °C source: 308.15 / 35 ≈ 8.8 (real units reach 40–55% of Carnot)
Practical COP is roughly 2.5–4.5 for air-source and 3.5–5 for ground-source units. Efficiency falls as the temperature lift grows, so low-temperature emitters (underfloor heating, oversized radiators at 35–45 °C) give the best results. Common refrigerants are R32, R410A and, increasingly, natural refrigerants such as R290 (propane) and CO2 (R744), driven by the EU F-gas Regulation.
The IEA estimates heat pumps could cut global CO2 emissions from building heating substantially; global sales were around 10–12 million units a year in 2022–2024. Compared with a 90% efficient gas boiler, a heat pump with seasonal COP 3 uses about 70% less final energy for the same heat.
Related terms
All terms →- COP (Coefficient of Performance)
The ratio of useful heat (or cooling) delivered by a heat pump to the electrical energy it consumes, measured at a specific operating point.
- SCOP (Seasonal COP)
A heat pump's average coefficient of performance over a standardised heating season, weighting test points by how often each outdoor temperature occurs.
- Air-source heat pump
A heat pump that extracts heat from outdoor air, supplying space heating and hot water (air-to-water) or warm air (air-to-air), and the most widely installed type.
- Ground-source heat pump
A heat pump that exchanges heat with the ground through buried loops or boreholes, benefiting from stable soil temperatures for high seasonal efficiency.
- SEER
Seasonal Energy Efficiency Ratio: total cooling output over a cooling season divided by electricity used, in BTU/Wh (US) or W/W (EU).
- District heating
A network of insulated pipes distributing hot water or steam from central heat sources to many buildings for space heating and hot water.