External heat exchanger of an air source heat pump
External heat exchanger of an air source heat pump

A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy from a cooler space to a warmer space using the refrigeration cycle, moving heat in the opposite direction in which heat transfer would take place without the application of external power. When used to cool a building, a heat pump works like an air conditioner by transferring heat from inside the building to the outdoors. When used to heat a building, the heat pump operates in reverse: heat is transferred into the building from the outdoors. Common heat pump types are air source heat pumps, ground source heat pumps, water source heat pumps and exhaust air heat pumps. Heat pumps are also often used in district heating systems.

The efficiency of a heat pump is expressed as a coefficient of performance (COP), or seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is and the less energy it consumes. When used for space heating these devices are typically much more energy efficient than simple electrical resistance heaters. Heat pumps have a smaller carbon footprint than heating systems burning fossil fuels such as natural gas,[1] but those powered by hydrogen are also low-carbon and may become competitors.[2]

Principle of operation

According to the second law of thermodynamics, heat will flow spontaneously from a region of higher temperature to a region of lower temperature. Heat will 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 heating of water and the interior of buildings.[3]

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


The coefficient of performance of a heat pump is greater than unity so the work required is less than the heat transferred, 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.[3]

The coefficient of performance, and the work required, by a heat pump can be calculated easily by considering an ideal heat pump operating on the reversed Carnot cycle:


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Outdoor unit of air source heat pump operating in freezing conditions
Outdoor unit of air source heat pump operating in freezing conditions

Air source heat pump

Main article: Air source heat pump

Air source heat pumps are used to move heat between two heat exchangers, one outside the building which is fitted with fins through which air is forced using a fan and the other which either directly heats the air inside the building or heats water which is then circulated around the building through heat emitters which release the heat to the building. These devices can also operate in a cooling mode where they extract heat via the internal heat exchanger and eject it into the ambient air using the external heat exchanger. They are normally also used to heat water for washing which is stored in a domestic hot water tank.[citation needed]

Air source heat pumps are relatively easy and inexpensive to install and have therefore historically been the most widely used heat pump type. In mild weather, COP may be around 4.0, while at temperatures below around 0 °C (32 °F) an air-source heat pump may still achieve a COP of 2.5. The average COP over seasonal variation is typically 2.5–2.8, with exceptional models able to exceed this in mild climates.[citation needed]

Geothermal (ground-source) heat pump

Main article: Geothermal heat pump

A geothermal heat pump (North American English) or ground-source heat pump (British English) draws heat from the soil or from groundwater which remains at a relatively constant temperature all year round below a depth of about 30 feet (9.1 m).[10] A well maintained geothermal heat pump will typically have a COP of 4.0 at the beginning of the heating season and a seasonal COP of around 3.0 as heat is drawn from the ground.[11] Geothermal heat pumps are more expensive to install due to the need for the drilling of boreholes for vertical placement of heat exchanger piping or the digging of trenches for horizontal placement of the piping that carries the heat exchange fluid (water with a little antifreeze).

A geothermal heat pump can also be used to cool buildings during hot days, thereby transferring heat from the dwelling back into the soil via the ground loop. Solar thermal collectors or piping placed within the tarmac of a parking lot can also be used to replenish the heat underground.[citation needed]

Exhaust air heat pump

Main article: Exhaust air heat pump

Exhaust air heat pumps extract heat from the exhaust air of a building and require mechanical ventilation. There are two classes of exhaust air heat pumps.

Solar-assisted heat pump

See also: Solar-assisted heat pump

A solar-assisted heat pump integrates a heat pump and thermal solar panels in a single system. Typically these two technologies are used separately (or are operated in parallel) to produce hot water.[12] In this system the solar thermal panel is the low-temperature heat source, and the heat produced feeds the heat pump's evaporator.[13] The goal of this system is to get high COP and then produce energy in a more efficient and less expensive way.[citation needed]

Water source heat pump

Water-source heat-exchanger being installed
Water-source heat-exchanger being installed

A water-source heat pump works in a similar manner to a ground-source heat pump, except that it takes heat from a body of water rather than the ground. The body of water does, however, need to be large enough to be able to withstand the cooling effect of the unit without freezing or creating an adverse effect for wildlife.[citation needed]

Hybrid heat pump

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Hybrid (or twin source) heat pumps draw heat from different sources depending on the outside air temperature. When outdoor air is above 4 to 8 degrees Celsius (40–50 degrees Fahrenheit, depending on ground water temperature) they use air; at colder temperatures they use the ground source. These twin source systems can also store summer heat by running ground source water through the air exchanger or through the building heater-exchanger, even when the heat pump itself is not running. This has two advantages: it functions as a low-cost system for interior air cooling, and (if ground water is relatively stagnant) it increases the temperature of the ground source, which improves the energy efficiency of the heat pump system by roughly 4% for each degree in temperature rise of the ground source.


The International Energy Agency estimated that, as of 2011, there were 800 million heat pumps installed on Earth.[14]: 16  They are used in climates with moderate heating, ventilation, and air conditioning (HVAC) needs and may also provide domestic hot water and tumble clothes drying functions.[15] The purchase costs are supported in various countries by consumer rebates.[16]

Heating and cooling of buildings and vehicles

In HVAC applications, a heat pump is typically a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow (thermal energy movement) may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. In cooler climates, the default setting of the reversing valve is heating.

The default setting in warmer climates is cooling. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. Therefore, the SEER rating, which is the Seasonal Energy Efficiency Rating, of a reversible heat pump is typically slightly less than two separately optimized machines. For equipment to receive the Energy Star rating, it must have a rating of at least 14.5 SEER.[citation needed]

Water heating

In water heating applications, a heat pump may be used to heat or preheat water for swimming pools or heating potable water for use by homes and industry. Usually heat is extracted from outdoor air and transferred to an indoor water tank, another variety extracts heat from indoor air to assist in cooling the space.[citation needed]

District heating

Main article: District heating

Heat pumps can also be used as heat supplier for district heating. Possible sources of heat for such applications are sewage water, ambient water (e.g. sea, lake and river water), industrial waste heat, geothermal energy, flue gas, waste heat from district cooling and heat from solar seasonal thermal energy storage. In Europe, more than 1500 MW of large-scale heat pumps were installed since the 1980s, of which about 1000 MW were in use in Sweden in 2017.[17] Large scale heat pumps for district heating combined with thermal energy storage offer high flexibility for the integration of variable renewable energy. Therefore, they are regarded as a key technology for smart energy systems with high shares of renewable energy up to 100%, and advanced 4th generation district heating systems.[17][18][19] They are also a crucial element of cold district heating systems.[20]

Industrial heating

There is great potential to reduce the energy consumption and related greenhouse gas emissions in industry by application of industrial heat pumps. An international collaboration project completed in 2015 collected totally 39 examples of R&D-projects and 115 case studies worldwide.[21] The study shows that short payback periods of less than 2 years are possible, while achieving a high reduction of CO2 emissions (in some cases more than 50%).[22][23] High Temperature Heat Pump innovations are emerging to further increase Industrial Heat Pump thermal application range and especially waste heat to energy recovery.[24]


Main article: Coefficient of performance

When comparing the performance of heat pumps the term 'performance' is preferred to 'efficiency', with coefficient of performance (COP) being used to describe the ratio of useful heat movement per work input. An electrical resistance heater has a COP of 1.0, which is considerably lower than a well-designed heat pump which will typically be between COP of 3 to 5 with an external temperature of 10 °C and an internal temperature of 20 °C. A ground-source heat pump will typically have a higher performance than an air-source heat pump.

The "Seasonal Coefficient of Performance" (SCOP) is a measure of the aggregate energy efficiency measure over a period of one year which it is very dependent on region climate. One framework for this calculation is given by the Commission Regulation (EU) No 813/2013:[25]

A heat pump's operating performance in cooling mode is characterized in the US by either its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), both of which have units of BTU/(h·W) (note that 1 BTU/(h·W) = 0.293 W/W) and larger values indicate better performance. Actual performance varies, and it depends on many factors such as installation details, temperature differences, site elevation, and maintenance.

COP variation with output temperature
Pump type and source Typical use 35 °C
(e.g. heated screed floor)
45 °C
(e.g. heated screed floor)
55 °C
(e.g. heated timber floor)
65 °C
(e.g. radiator or DHW)
75 °C
(e.g. radiator and DHW)
85 °C
(e.g. radiator and DHW)
High-efficiency air source heat pump (ASHP), air at −20 °C[26] 2.2 2.0
Two-stage ASHP, air at −20 °C[27] Low source temperature 2.4 2.2 1.9
High efficiency ASHP, air at 0 °C[26] Low output temperature 3.8 2.8 2.2 2.0
Prototype transcritical CO
(R744) heat pump with tripartite gas cooler, source at 0 °C[28]
High output temperature 3.3 4.2 3.0
Ground source heat pump (GSHP), water at 0 °C[26] 5.0 3.7 2.9 2.4
GSHP, ground at 10 °C[26] Low output temperature 7.2 5.0 3.7 2.9 2.4
Theoretical Carnot cycle limit, source −20 °C 5.6 4.9 4.4 4.0 3.7 3.4
Theoretical Carnot cycle limit, source 0 °C 8.8 7.1 6.0 5.2 4.6 4.2
Theoretical Lorentzen cycle limit (CO
pump), return fluid 25 °C, source 0 °C[28]
10.1 8.8 7.9 7.1 6.5 6.1
Theoretical Carnot cycle limit, source 10 °C 12.3 9.1 7.3 6.1 5.4 4.8


See also: Vapor-compression refrigeration

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Figure 2: Temperature–entropy diagram of the vapor-compression cycle.
Figure 2: Temperature–entropy diagram of the vapor-compression cycle.
An internal view of the outdoor unit of an Ecodan air source heat pump
An internal view of the outdoor unit of an Ecodan air source heat pump

Vapor-compression uses a circulating liquid refrigerant as the medium which absorbs heat from one space, compresses it thereby increasing its temperature before releasing it in another space. The system normally has 8 main components: a compressor, a reservoir, a reversing valve which selects between heating and cooling mode, two thermal expansion valves (one used when in heating mode and the other when used in cooling mode) and two heat exchangers, one associated with the external heat source/sink and the other with the interior. In heating mode the external heat exchanger is the evaporator and the internal one being the condenser; in cooling mode the roles are reversed.

Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor[29] and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air flowing across the coil or tubes. In heating mode this heat is used to heat the building using the internal heat exchanger, and in cooling mode this heat is rejected via the external heat exchanger.

The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.

The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser.

To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor.

Over time, the evaporator may collect ice or water from ambient humidity. The ice is melted through defrosting cycle. In internal heat exchanger is either used to heat/cool the interior air directly or to heat water that is then circulated through radiators or underfloor heating circuit to either heat of cool the buildings.

Refrigerant choice

Main article: refrigerant

Until the 1990s heat pumps, along with fridges and other related products used chlorofluorocarbons (CFCs) as refrigerants that caused major damage to the ozone layer when released into the atmosphere. Use of these chemicals was banned or severely restricted by the Montreal Protocol of August 1987.[30] Replacements, including R-134a and R-410A, are hydrofluorocarbon with similar thermodynamic properties with insignificant ozone depletion potential but had problematic global warming potential.[31] HFC is a powerful greenhouse gas which contributes to climate change.[32][33] More recent refrigerators include difluoromethane (R32) and isobutane (R600A) which do not deplete the ozone and are also far less harmful to the environment.[34] Dimethyl ether (DME) has also gained in popularity as a refrigerant.[35]

Government incentives

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In 2022, the Canada Greener Homes Grant [36] provides up to $5000 for upgrades (including certain heat pumps), and $600 for energy efficiency evaluations.

United Kingdom

As of  2021: heat pumps are taxed at the reduced rate of 5% instead of the usual level of VAT of 20% for most other products.[37]

United States

Alternative Energy Credits in Massachusetts

The Alternative Energy Portfolio Standard (APS) was developed in 2008 to require a certain percentage of the Massachusetts electricity supply to be sourced from specific alternative energy sources.[38] In October 2017, the Massachusetts Department of Energy (DOER) drafted regulations, pursuant to Chapter 251 of the Acts of 2014 and Chapter 188 of the Acts of 2016, that added renewable thermal, fuel cells, and waste-to-energy thermal to the APS.[38]

Alternative Energy Credits (AECs) are issued as an incentive to the owners of eligible renewable thermal energy facilities, at a rate of one credit per every megawatt-hour equivalent (MWhe) of thermal energy generated. Retail electricity suppliers may purchase these credits to meet APS compliance standards. The APS expands the current renewable mandates to a broader spectrum of participants, as the state continues to expand its portfolio of alternative energy sources.[citation needed]


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