thumb|External heat exchanger of an [[air source heat pump|air-source heat pump for both heating and cooling]]

thumb|[[Mitsubishi Electric heat pump interior air handler wall unit]]

A heat pump is a device that uses mechanical or thermal energy to transfer heat from one space to another. The mechanical heat pump uses electric power to transfer heat by compression. Specifically, it transfers thermal energy by means of a heat pump and refrigeration cycle, cooling one space and warming the other. Heat pumps driven by thermal energy are known as absorption heat pumps.

In winter, a heat pump can move heat from the cool outdoors to warm a house; in summer, it may also be designed to move heat from the house to the warmer outdoors. As it transfers rather than generates heat, it is more energy-efficient than heating by gas boiler.

In a typical vapor-compression heat pump, a gaseous refrigerant is compressed so its pressure and temperature rise. When the pump operates as a heater in cold weather, the warmed gas flows to a heat exchanger in the indoor space, where some of its thermal energy is transferred to that space, causing the gas to condense into a liquid. The liquified refrigerant flows to a heat exchanger in the outdoor space, where the pressure falls, the liquid evaporates, and the temperature of the gas falls. Now colder than the temperature of the outdoor space being used as a heat source, it can again take up energy from the heat source, be compressed, and repeat the cycle.

Air source heat pumps are the most common models, while other types include ground source heat pumps, water source heat pumps, and exhaust air heat pumps. Large-scale heat pumps are also used in district heating systems.

Because of their high efficiency and the increasing share of fossil-free sources in electrical grids, heat pumps are playing a role in climate change mitigation. At a cost of 1 kWh of electricity, they can transfer 1 to 4.5 kWh of thermal energy into a building. The carbon footprint of heat pumps depends on how electricity is generated, but they usually reduce emissions. Heat pumps could satisfy over 80% of global space and water heating needs with a lower carbon footprint than gas-fired condensing boilers: however, in 2021 they only met 10%, 3 million European heat pumps were sold in 2023. Although sales have grown significantly, adoption remains limited. In 2025, REPowerEU provides a clear roadmap to transition to this high efficient and flexible air conditioning system.

Operation

thumb|A: indoor compartment, B: outdoor compartment, I: insulation, 1: condenser, 2: expansion valve, 3: evaporator, 4: compressor

Heat flows spontaneously from a region of higher temperature to a region of lower temperature. Heat does not flow spontaneously from lower temperature to higher, but it can be made to flow in this direction if work is performed. The work required to transfer a given amount of heat is usually much less than the amount of heat; this is the motivation for using heat pumps in applications such as the heating of water and the interior of buildings.

The heat pump works by the use of reverse cycle conditioning. Liquid refrigerant flows through coils in the cooling unit and absorbs heat, becoming a gas which is then compressed to further raise the temperature. This gaseous refrigerant is pumped into more coils in the heating unit, where a fan blows air over the coil to absorb the heat and liquefy the refrigerant. Most heat pumps are capable of transferring heat in both directions.

The amount of work required to provide an amount of heat Q to a higher-temperature reservoir such as the interior of a building, while extracting heat from a lower-temperature reservoir such as ambient air is:

<math display="block">W = \frac{ Q}{\mathrm{COP</math>

where

  • <math>W </math> is the work performed on the working fluid by the heat pump's compressor.
  • <math> Q </math> is the heat released in the higher-temperature reservoir.
  • <math>\mathrm{COP}</math> is the instantaneous coefficient of performance for the heat pump at the temperatures prevailing in the reservoirs at one instant.

The coefficient of performance of a heat pump is greater than one so the work required is less than the heat released, making a heat pump a more efficient form of heating than electrical resistance heating. As the temperature of the higher-temperature reservoir increases in response to the heat flowing into it, the coefficient of performance decreases, causing an increasing amount of work to be required for each unit of heat being transferred.

  • As the temperature of the interior of the building rises progressively to the coefficient of performance falls progressively to 10. This means each joule of work is responsible for transferring 9 joules of heat out of the low-temperature reservoir and into the building. Again, the 1 joule of work ultimately ends up as thermal energy in the interior of the building so 10 joules of heat are added to the building interior.