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System that transfers heat from one space to another

This article is about devices used to heat and potentially also cool buildings or water using the refrigeration cycle. For other uses, see Heat pump (disambiguation).

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A heat pump is a device that uses electricity to transfer heat from a colder place to a warmer place. Specifically, the heat pump transfers thermal energy using a heat pump and refrigeration cycle, cooling the cool space and warming the warm space.^[1] In winter a heat pump can move heat from the cool outdoors to warm a house; the pump may also be designed to move heat from the house to the warmer outdoors in summer. As they transfer heat rather than generating heat, they are more energy-efficient than heating by gas boiler.^[2]

A gaseous refrigerant is compressed so its pressure and temperature rise. When operating 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 indoor 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. It is 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.^[3] Large-scale heat pumps are also used in district heating systems.^[4]

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.^[5][6] Consuming 1 kWh of electricity, they can transfer 1^[7] 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.^[8] 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%.^[4]

Principle of operation

[edit]

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.^[9]

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: $$\displaystyle W=\frac Q\mathrm COP$$ where

• $\displaystyle W$ is the work performed on the working fluid by the heat pump's compressor.
• $\displaystyle Q$ is the heat transferred from the lower-temperature reservoir to the higher-temperature reservoir.
• $\displaystyle \mathrm COP$ 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 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.^[9]

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:

• If the low-temperature reservoir is at a temperature of 270 K (−3 °C) and the interior of the building is at 280 K (7 °C) the maximum theoretical coefficient of performance is 28. This means 1 joule of work delivers 28 joules of heat to the interior. The one joule of work ultimately ends up as thermal energy in the interior of the building and 27 joules of heat are moved from the low-temperature reservoir.^{[note 1]}
• As the temperature of the interior of the building rises progressively to 300 K (27 °C) 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.^{[note 2]}

This is the theoretical amount of heat pumped but in practice it will be less for various reasons, for example if the outside unit has been installed where there is not enough airflow. More data sharing with owners and academics—perhaps from heat meters—could improve efficiency in the long run.^[11]

Milestones:

1748

William Cullen demonstrates artificial refrigeration.^[12]

1834

Jacob Perkins patents a design for a practical refrigerator using dimethyl ether.^[13]

1852

Lord Kelvin describes the theory underlying heat pumps.^[14]

1855–1857

Peter von Rittinger develops and builds the first heat pump.^[15]

1877

In the period before 1875, heat pumps were for the time being pursued for vapour compression evaporation (open heat pump process) in salt works with their obvious advantages for saving wood and coal. In 1857, Peter von Rittinger was the first to try to implement the idea of vapor compression in a small pilot plant. Presumably inspired by Rittinger's experiments in Ebensee, Antoine-Paul Piccard from the University of Lausanne and the engineer J. H. Weibel from the Weibel–Briquet company in Geneva built the world's first really functioning vapor compression system with a two-stage piston compressor. In 1877 this first heat pump in Switzerland was installed in the Bex salt works.^[14][16]

1928

Aurel Stodola constructs a closed-loop heat pump (water source from Lake Geneva) which provides heating for the Geneva city hall to this day.^{[17][unreliable source?]}

1937–1945

During the First World War, fuel prices were very high in Switzerland but it had plenty of hydropower.^{[14]: 18â€ÅÂ}  In the period before and especially during the Second World War, when neutral Switzerland was completely surrounded by fascist-ruled countries, the coal shortage became alarming again. Thanks to their leading position in energy technology, the Swiss companies Sulzer, Escher Wyss and Brown Boveri built and put in operation around 35 heat pumps between 1937 and 1945. The main heat sources were lake water, river water, groundwater, and waste heat. Particularly noteworthy are the six historic heat pumps from the city of Zurich with heat outputs from 100 kW to 6 MW. An international milestone is the heat pump built by Escher Wyss in 1937/38 to replace the wood stoves in the City Hall of Zurich. To avoid noise and vibrations, a recently developed rotary piston compressor was used. This historic heat pump heated the town hall for 63 years until 2001. Only then was it replaced by a new, more efficient heat pump.^[14]

1945

John Sumner, City Electrical Engineer for Norwich, installs an experimental water-source heat pump fed central heating system, using a nearby river to heat new Council administrative buildings. It had a seasonal efficiency ratio of 3.42, average thermal delivery of 147 kW, and peak output of 234 kW.^[18]

1948

Robert C. Webber is credited as developing and building the first ground-source heat pump.^[19]

1951

First large scale installation—the Royal Festival Hall in London is opened with a town gas-powered reversible water-source heat pump, fed by the Thames, for both winter heating and summer cooling needs.^[18]

2019

The Kigali Amendment to phase out harmful refrigerants takes effect.

An air source heat pump (ASHP) is a heat pump that can absorb heat from air outside a building and release it inside; it uses the same vapor-compression refrigeration process and much the same equipment as an air conditioner, but in the opposite direction. ASHPs are the most common type of heat pump and, usually being smaller, tend to be used to heat individual houses or flats rather than blocks, districts or industrial processes.^[20]

Air-to-air heat pumps provide hot or cold air directly to rooms, but do not usually provide hot water. Air-to-water heat pumps use radiators or underfloor heating to heat a whole house and are often also used to provide domestic hot water.

An ASHP can typically gain 4 kWh thermal energy from 1 kWh electric energy. They are optimized for flow temperatures between 30 and 40 °C (86 and 104 °F), suitable for buildings with heat emitters sized for low flow temperatures. With losses in efficiency, an ASHP can even provide full central heating with a flow temperature up to 80 °C (176 °F).^[21]

As of 2023^[update] about 10% of building heating worldwide is from ASHPs. They are the main way to phase out gas boilers (also known as "furnaces") from houses, to avoid their greenhouse gas emissions.^[22]

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 radiators or underfloor heating which releases 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. Some can be used to heat water for washing which is stored in a domestic hot water tank.^[23]

Air-source heat pumps are relatively easy and inexpensive to install, so are the most widely used type. In mild weather, coefficient of performance (COP) may be between 2 and 5, while at temperatures below around −8 °C (18 °F) an air-source heat pump may still achieve a COP of 1 to 4.^[24]

While older air-source heat pumps performed relatively poorly at low temperatures and were better suited for warm climates, newer models with variable-speed compressors remain highly efficient in freezing conditions allowing for wide adoption and cost savings in places like Minnesota and Maine in the United States.^[25]

A ground source heat pump (also geothermal heat pump) is a heating/cooling system for buildings that use a type of heat pump to transfer heat to or from the ground, taking advantage of the relative constancy of temperatures of the earth through the seasons. Ground-source heat pumps (GSHPs)—or geothermal heat pumps (GHP), as they are commonly termed in North America—are among the most energy-efficient technologies for providing HVAC and water heating, using less energy than can be achieved by use of resistive electric heaters.

Efficiency is given as a coefficient of performance (CoP) which is typically in the range 3-6, meaning that the devices provide 3-6 units of heat for each unit of electricity used. Setup costs are higher than for other heating systems, due to the requirement of installing ground loops over large areas or of drilling bore holes, hence ground source is often installed when new blocks of flats are built.^[26] Air-source heat pumps have lower set-up costs but have a lower CoP in very cold or hot weather.

Exhaust air heat pumps extract heat from the exhaust air of a building and require mechanical ventilation. Two classes exist:

• Exhaust air-air heat pumps transfer heat to intake air.
• Exhaust air-water heat pumps transfer heat to a heating circuit that includes a tank of domestic hot water.

A solar-assisted heat pump (SAHP) is a system that combines a heat pump and thermal solar panels and/or PV solar panels in a single integrated system.^[27] Heat pumps require a low temperature heat source which can be provided by solar energy. Typically, these two technologies are used separately (or only placing them in parallel) to produce warm air or hot water.^[28] In this system the solar thermal panel performs the function of the low temperature heat source and the heat produced is used to feed the heat pump's evaporator.^[29] The goal of this system is to get high coefficient of performance (COP) and then produce energy in a more efficient and less expensive way. Air source heat pumps which are preheated by solar air collectors have an additional benefit of lower maintenance as the outside fan unit can be protected from the harsh winter environment.

Solar PV energy can power the heat pump electrically to enable electrification of heating buildings^[30] and greenhouses.^[31] These systems enable electrification^[32] of heating/cooling and are normally driven by economics^[33] and decarbonization goals.^[34] Such systems have been shown to be economic in the Middle East,^[35] North America,^[36] Asia^[37] and Europe.^[38]

It is possible to use any type of solar thermal system with air or liquid collectors, (sheet and tubes, roll-bond, heat pipe, thermal plates) or hybrid (mono/polycrystalline, thin film) in combination with the heat pump. The use of a hybrid panel is preferable because it allows covering a part of the electricity demand of the heat pump and reduce the power consumption and consequently the variable costs of the system.

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.^[39] The largest water-source heat pump was installed in the Danish town of Esbjerg in 2023.^[40][41]

A thermoacoustic heat pump operates as a thermoacoustic heat engine without refrigerant but instead uses a standing wave in a sealed chamber driven by a loudspeaker to achieve a temperature difference across the chamber.^[42]

Electrocaloric heat pumps are solid state.^[43]

The International Energy Agency estimated that, as of 2021, heat pumps installed in buildings have a combined capacity of more than 1000 GW.^[4] They are used for heating, ventilation, and air conditioning (HVAC) and may also provide domestic hot water and tumble clothes drying.^[44] The purchase costs are supported in various countries by consumer rebates.^[45]

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.

Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. Therefore, the Seasonal Energy Efficiency Rating (SEER in the US) or European seasonal energy efficiency ratio of a reversible heat pump is typically slightly less than those of two separately optimized machines. For equipment to receive the US Energy Star rating, it must have a rating of at least 14 SEER. Pumps with ratings of 18 SEER or above are considered highly efficient. The highest efficiency heat pumps manufactured are up to 24 SEER.^[46]

Heating seasonal performance factor (in the US) or Seasonal Performance Factor (in Europe) are ratings of heating performance. The SPF is Total heat output per annum / Total electricity consumed per annum in other words the average heating COP over the year.^[47]

Window mounted heat pumps run on standard 120v AC outlets and provide heating, cooling, and humidity control. They are more efficient with lower noise levels, condensation management, and a smaller footprint than window mounted air conditioners that just do cooling.^[48]

In water heating applications, heat pumps may be used to heat or preheat water for swimming pools, homes or industry. Usually heat is extracted from outdoor air and transferred to an indoor water tank.^[49][50]

Large (megawatt-scale) heat pumps are used for district heating.^[51] However as of 2022^[update] about 90% of district heat is from fossil fuels.^[52] In Europe, heat pumps account for a mere 1% of heat supply in district heating networks but several countries have targets to decarbonise their networks between 2030 and 2040.^[4] 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.^[53] 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 limiting climate change by phasing out fossil fuels.^[53][54] They are also a crucial element of systems which can both heat and cool districts.^[55]

There is great potential to reduce the energy consumption and related greenhouse gas emissions in industry by application of industrial heat pumps, for example for process heat.^[56][57] Short payback periods of less than 2 years are possible, while achieving a high reduction of CO₂ emissions (in some cases more than 50%).^[58][59] Industrial heat pumps can heat up to 200 °C, and can meet the heating demands of many light industries.^[60][61] In Europe alone, 15 GW of heat pumps could be installed in 3,000 facilities in the paper, food and chemicals industries.^[4]

The performance of a heat pump is determined by the ability of the pump to extract heat from a low temperature environment (the source) and deliver it to a higher temperature environment (the sink).^[62] Performance varies, depending on installation details, temperature differences, site elevation, location on site, pipe runs, flow rates, and maintenance.

In general, heat pumps work most efficiently (that is, the heat output produced for a given energy input) when the difference between the heat source and the heat sink is small. When using a heat pump for space or water heating, therefore, the heat pump will be most efficient in mild conditions, and decline in efficiency on very cold days. Performance metrics supplied to consumers attempt to take this variation into account.

Common performance metrics are the SEER (in cooling mode) and seasonal coefficient of performance (SCOP) (commonly used just for heating), although SCOP can be used for both modes of operation.^[62] Larger values of either metric indicate better performance.^[62] 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.^[62] An electrical resistance heater has a COP of 1.0, which is considerably lower than a well-designed heat pump which will typically have a COP of 3 to 5 with an external temperature of 10 °C and an internal temperature of 20 °C. Because the ground is a constant temperature source, a ground-source heat pump is not subjected to large temperature fluctuations, and therefore is the most energy-efficient type of heat pump.^[62]

The "seasonal coefficient of performance" (SCOP) is a measure of the aggregate energy efficiency measure over a period of one year which is dependent on regional climate.^[62] One framework for this calculation is given by the Commission Regulation (EU) No. 813/2013.^[63]

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.

The carbon footprint of heat pumps depends on their individual efficiency and how electricity is produced. An increasing share of low-carbon energy sources such as wind and solar will lower the impact on the climate.

In most settings, heat pumps will reduce CO₂ emissions compared to heating systems powered by fossil fuels.^[70] In regions accounting for 70% of world energy consumption, the emissions savings of heat pumps compared with a high-efficiency gas boiler are on average above 45% and reach 80% in countries with cleaner electricity mixes.^[4] These values can be improved by 10 percentage points, respectively, with alternative refrigerants. In the United States, 70% of houses could reduce emissions by installing a heat pump.^[71][4] The rising share of renewable electricity generation in many countries is set to increase the emissions savings from heat pumps over time.^[4]

Heating systems powered by green hydrogen are also low-carbon and may become competitors, but are much less efficient due to the energy loss associated with hydrogen conversion, transport and use. In addition, not enough green hydrogen is expected to be available before the 2030s or 2040s.^[72][73]

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Large heat pump setup for a commercial building

Wiring and connections to a central air unit inside

Vapor-compression uses a circulating 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 eight 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^[74] 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. An 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 or cool the buildings.

Heat input can be improved if the refrigerant enters the evaporator with a lower vapor content. This can be achieved by cooling the liquid refrigerant after condensation. The gaseous refrigerant condenses on the heat exchange surface of the condenser. To achieve a heat flow from the gaseous flow center to the wall of the condenser, the temperature of the liquid refrigerant must be lower than the condensation temperature.

Additional subcooling can be achieved by heat exchange between relatively warm liquid refrigerant leaving the condenser and the cooler refrigerant vapor emerging from the evaporator. The enthalpy difference required for the subcooling leads to the superheating of the vapor drawn into the compressor. When the increase in cooling achieved by subcooling is greater that the compressor drive input required to overcome the additional pressure losses, such a heat exchange improves the coefficient of performance.^[75]

One disadvantage of the subcooling of liquids is that the difference between the condensing temperature and the heat-sink temperature must be larger. This leads to a moderately high pressure difference between condensing and evaporating pressure, whereby the compressor energy increases.^{[citation needed]}

Pure refrigerants can be divided into organic substances (hydrocarbons (HCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), hydrofluoroolefins (HFOs), and HCFOs), and inorganic substances (ammonia (NH
₃), carbon dioxide (CO
₂), and water (H
₂O)^[76]).^[77] Their boiling points are usually below −25 °C.^[78]

In the past 200 years, the standards and requirements for new refrigerants have changed. Nowadays low global warming potential (GWP) is required, in addition to all the previous requirements for safety, practicality, material compatibility, appropriate atmospheric life,^{[clarification needed]} and compatibility with high-efficiency products. By 2022, devices using refrigerants with a very low GWP still have a small market share but are expected to play an increasing role due to enforced regulations,^[79] as most countries have now ratified the Kigali Amendment to ban HFCs.^[80] Isobutane (R600A) and propane (R290) are far less harmful to the environment than conventional hydrofluorocarbons (HFC) and are already being used in air-source heat pumps.^[81] Propane may be the most suitable for high temperature heat pumps.^[82] Ammonia (R717) and carbon dioxide (R-744) also have a low GWP. As of 2023^[update] smaller CO
₂ heat pumps are not widely available and research and development of them continues.^[83] A 2024 report said that refrigerants with GWP are vulnerable to further international restrictions.^[84]

Until the 1990s, heat pumps, along with fridges and other related products used chlorofluorocarbons (CFCs) as refrigerants, which 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.^[85]

Replacements, including R-134a and R-410A, are hydrofluorocarbons (HFC) with similar thermodynamic properties with insignificant ozone depletion potential (ODP) but had problematic GWP.^[86] HFCs are powerful greenhouse gases which contribute to climate change.^[87][88] Dimethyl ether (DME) also gained in popularity as a refrigerant in combination with R404a.^[89] More recent refrigerants include difluoromethane (R32) with a lower GWP, but still over 600.

Devices with R-290 refrigerant (propane) are expected to play a key role in the future.^[82][93] The 100-year GWP of propane, at 0.02, is extremely low and is approximately 7000 times less than R-32. However, the flammability of propane requires additional safety measures: the maximum safe charges have been set significantly lower than for lower flammability refrigerants (only allowing approximately 13.5 times less refrigerant in the system than R-32).^[94][95][96] This means that R-290 is not suitable for all situations or locations. Nonetheless, by 2022, an increasing number of devices with R-290 were offered for domestic use, especially in Europe.^{[citation needed]}

At the same time,^[when?] HFC refrigerants still dominate the market. Recent government mandates have seen the phase-out of R-22 refrigerant. Replacements such as R-32 and R-410A are being promoted as environmentally friendly but still have a high GWP.^[97] A heat pump typically uses 3 kg of refrigerant. With R-32 this amount still has a 20-year impact equivalent to 7 tons of CO₂, which corresponds to two years of natural gas heating in an average household. Refrigerants with a high ODP have already been phased out.^{[citation needed]}

Financial incentives aim to protect consumers from high fossil gas costs and to reduce greenhouse gas emissions,^[98] and are currently available in more than 30 countries around the world, covering more than 70% of global heating demand in 2021.^[4]

Food processors, brewers, petfood producers and other industrial energy users are exploring whether it is feasible to use renewable energy to produce industrial-grade heat. Process heating accounts for the largest share of onsite energy use in Australian manufacturing, with lower-temperature operations like food production particularly well-suited to transition to renewables.

To help producers understand how they could benefit from making the switch, the Australian Renewable Energy Agency (ARENA) provided funding to the Australian Alliance for Energy Productivity (A2EP) to undertake pre-feasibility studies at a range of sites around Australia, with the most promising locations advancing to full feasibility studies.^[99]

In an effort to incentivize energy efficiency and reduce environmental impact, the Australian states of Victoria, New South Wales, and Queensland have implemented rebate programs targeting the upgrade of existing hot water systems. These programs specifically encourage the transition from traditional gas or electric systems to heat pump based systems.^{[100][101][102][103][104]}

In 2022, the Canada Greener Homes Grant^[105] provides up to $5000 for upgrades (including certain heat pumps), and $600 for energy efficiency evaluations.

Purchase subsidies in rural areas in the 2010s reduced burning coal for heating, which had been causing ill health.^[106]

In the 2024 report by the International Energy Agency (IEA) titled "The Future of Heat Pumps in China," it is highlighted that China, as the world's largest market for heat pumps in buildings, plays a critical role in the global industry. The country accounts for over one-quarter of global sales, with a 12% increase in 2023 alone, despite a global sales dip of 3% the same year.^[107]

Heat pumps are now used in approximately 8% of all heating equipment sales for buildings in China as of 2022, and they are increasingly becoming the norm in central and southern regions for both heating and cooling. Despite their higher upfront costs and relatively low awareness, heat pumps are favored for their energy efficiency, consuming three to five times less energy than electric heaters or fossil fuel-based solutions. Currently, decentralized heat pumps installed in Chinese buildings represent a quarter of the global installed capacity, with a total capacity exceeding 250 GW, which covers around 4% of the heating needs in buildings.^[107]

Under the Announced Pledges Scenario (APS), which aligns with China's carbon neutrality goals, the capacity is expected to reach 1,400 GW by 2050, meeting 25% of heating needs. This scenario would require an installation of about 100 GW of heat pumps annually until 2050. Furthermore, the heat pump sector in China employs over 300,000 people, with employment numbers expected to double by 2050, underscoring the importance of vocational training for industry growth. This robust development in the heat pump market is set to play a significant role in reducing direct emissions in buildings by 30% and cutting PM2.5 emissions from residential heating by nearly 80% by 2030.^[107][108]

To speed up the deployment rate of heat pumps, the European Commission launched the Heat Pump Accelerator Platform in November 2024.^[109] It will encourage industry experts, policymakers, and stakeholders to collaborate, share best practices and ideas, and jointly discuss measures that promote sustainable heating solutions.^[110]

Until 2027 fixed heat pumps have no Value Added Tax (VAT).^[111] As of 2022^[update] the installation cost of a heat pump is more than a gas boiler, but with the "Boiler Upgrade Scheme"^[112] government grant and assuming electricity/gas costs remain similar their lifetime costs would be similar on average.^[113] However lifetime cost relative to a gas boiler varies considerably depending on several factors, such as the quality of the heat pump installation and the tariff used.^[114] In 2024 England was criticised for still allowing new homes to be built with gas boilers, unlike some other counties where this is banned.^[115]

This section needs to be updated. The reason given is: talks about 2023 in future tense. Please help update this article to reflect recent events or newly available information. (January 2025)

The High-efficiency Electric Home Rebate Program was created in 2022 to award grants to State energy offices and Indian Tribes in order to establish state-wide high-efficiency electric-home rebates. Effective immediately, American households are eligible for a tax credit to cover the costs of buying and installing a heat pump, up to $2,000. Starting in 2023, low- and moderate-level income households will be eligible for a heat-pump rebate of up to $8,000.^[116]

In 2022, more heat pumps were sold in the United States than natural gas furnaces.^[117]

In November 2023 Biden's administration allocated 169 million dollars from the Inflation Reduction Act to speed up production of heat pumps. It used the Defense Production Act to do so, in a stated bid to advance national security.^[118]

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• IPCC (2018). Masson-Delmotte, V.; Zhai, P.; Pörtner, H.-O.; Roberts, D.; et al. (eds.). Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty (PDF). Intergovernmental Panel on Climate Change. https://www.ipcc.ch/sr15/.
  • Rogelj, J.; Shindell, D.; Jiang, K.; Fifta, S.; et al. (2018). "Chapter 2: Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development" (PDF). IPCC SR15 2018. pp. 93–174.
• IPCC (2022). Shula, P. R.; Skea, J.; Slade, R.; Al Khourdajie, A.; et al. (eds.). Climate Change 2022: Mitigation of Climate Change (PDF). Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, New York, USA: Cambridge University Press (In Press). Archived from the original (PDF) on 4 April 2022. Retrieved 10 May 2022.
  • IPCC (2022). "Industry" (PDF). IPCC AR6 WG3 2022.

• Quaschning, Volker. "Specific Carbon Dioxide Emissions of Various Fuels". Retrieved 22 February 2022.

• Media related to Heat pumps at Wikimedia Commons

Most common type of heat pump

This article is about details of the most common type of heat pump. For more general information, see heat pump.

 

An air source heat pump (ASHP) is a heat pump that can absorb heat from air outside a building and release it inside; it uses the same vapor-compression refrigeration process and much the same equipment as an air conditioner, but in the opposite direction. ASHPs are the most common type of heat pump and, usually being smaller, tend to be used to heat individual houses or flats rather than blocks, districts or industrial processes.^[1]

Air-to-air heat pumps provide hot or cold air directly to rooms, but do not usually provide hot water. Air-to-water heat pumps use radiators or underfloor heating to heat a whole house and are often also used to provide domestic hot water.

An ASHP can typically gain 4 kWh thermal energy from 1 kWh electric energy. They are optimized for flow temperatures between 30 and 40 °C (86 and 104 °F), suitable for buildings with heat emitters sized for low flow temperatures. With losses in efficiency, an ASHP can even provide full central heating with a flow temperature up to 80 °C (176 °F).^[2]

As of 2023^[update] about 10% of building heating worldwide is from ASHPs. They are the main way to phase out gas boilers (also known as "furnaces") from houses, to avoid their greenhouse gas emissions.^[3]

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 radiators or underfloor heating which releases 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. Some can be used to heat water for washing which is stored in a domestic hot water tank.^[4]

Air-source heat pumps are relatively easy and inexpensive to install, so are the most widely used type. In mild weather, coefficient of performance (COP) may be between 2 and 5, while at temperatures below around −8 °C (18 °F) an air-source heat pump may still achieve a COP of 1 to 4.^[5]

While older air-source heat pumps performed relatively poorly at low temperatures and were better suited for warm climates, newer models with variable-speed compressors remain highly efficient in freezing conditions allowing for wide adoption and cost savings in places like Minnesota and Maine in the United States.^[6]

Technology

[edit]

Air at any natural temperature contains some heat. An air source heat pump transfers some of this from one place to another, for example between the outside and inside of a building.

An air-to air system can be designed to transfer heat in either direction, to heat or cool the interior of the building in winter and summer respectively. Internal ducting may be used to distribute the air.^[7] An air-to-water system only pumps heat inwards, and can provide space heating and hot water.^[8] For simplicity, the description below focuses on use for interior heating.

The technology is similar to a refrigerator or freezer or air conditioning unit: the different effect is due to the location of the different system components. Just as the pipes on the back of a refrigerator become warm as the interior cools, so an ASHP warms the inside of a building whilst cooling the outside air.

The main components of a split-system (called split as there are both inside and outside coils) air source heat pump are:

• An outdoor evaporator heat exchanger coil, which extracts heat from ambient air
• One or more^[9] indoor condenser heat exchanger coils. They transfer the heat into the indoor air, or an indoor heating system such as water-filled radiators or underfloor circuits and a domestic hot water tank.

Less commonly a packaged ASHP has everything outside, with hot (or cold) air sent inside through a duct.^[10] These are also called monobloc and are useful for keeping flammable propane outside the house.^[3]

An ASHP can provide three or four times as much heat as an electric resistance heater using the same amount of electricity.^[11] Burning gas or oil will emit carbon dioxide and also NOx, which can be harmful to health.^[12] An air source heat pump issues no carbon dioxide, nitrogen oxide or any other kind of gas. It uses a small amount of electricity to transfer a large amount of heat.

Most ASHPs are reversible and are able to either warm or cool buildings^[13] and in some cases also provide domestic hot water. The use of an air-to-water heat pump for house cooling has been criticised.^[14]

Heating and cooling is accomplished by pumping a refrigerant through the heat pump's indoor and outdoor coils. Like in a refrigerator, a compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant between colder liquid and hotter gas states.

When the liquid refrigerant at a low temperature and low pressure passes through the outdoor heat exchanger coils, ambient heat causes the liquid to boil (change to gas or vapor). Heat energy from the outside air has been absorbed and stored in the refrigerant as latent heat. The gas is then compressed using an electric pump; the compression increases the temperature of the gas.

Inside the building, the gas passes through a pressure valve into heat exchanger coils. There, the hot refrigerant gas condenses back to a liquid and transfers the stored latent heat to the indoor air, water heating or hot water system. The indoor air or heating water is pumped across the heat exchanger by an electric pump or fan.

The cool liquid refrigerant then re-enters the outdoor heat exchanger coils to begin a new cycle. Each cycle usually takes a few minutes.^[11]

Most heat pumps can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.

As of 2024 tech other than vapour compression is insignificant in the market.^[15]

Usage

[edit]

ASHPs are the most common type of heat pump and, usually being smaller, are generally more suitable to heat individual houses rather than blocks of flats, compact urban districts or industrial processes.^[1] In dense city centres heat networks may be better than ASHP.^[1] Air source heat pumps are used to provide interior space heating and cooling even in colder climates, and can be used efficiently for water heating in milder climates. A major advantage of some ASHPs is that the same system may be used for heating in winter and cooling in summer. Though the cost of installation is generally high, it is less than the cost of a ground source heat pump, because a ground source heat pump requires excavation to install its ground loop. The advantage of a ground source heat pump is that it has access to the thermal storage capacity of the ground which allows it to produce more heat for less electricity in cold conditions.

Home batteries can mitigate the risk of power cuts and like ASHPs are becoming more popular.^[16] Some ASHPs can be coupled to solar panels as primary energy source, with a conventional electric grid as backup source.^{[citation needed]}

Thermal storage solutions incorporating resistance heating can be used in conjunction with ASHPs. Storage may be more cost-effective if time of use electricity rates are available. Heat is stored in high density ceramic bricks contained within a thermally-insulated enclosure;^[17] storage heaters are an example. ASHPs may also be paired with passive solar heating. Thermal mass (such as concrete or rocks) heated by passive solar heat can help stabilize indoor temperatures, absorbing heat during the day and releasing heat at night, when outdoor temperatures are colder and heat pump efficiency is lower.

Replacing gas heating in existing houses

[edit]

Good home insulation is important.^[18] As of 2023^[update] ASHPs are bigger than gas boilers and need more space outside, so the process is more complex and can be more expensive than if it was possible to just remove a gas boiler and install an ASHP in its place.^[3][19] If running costs are important choosing the right size is important because an ASHP which is too large will be more expensive to run.^[20]

It can be more complicated to retrofit conventional heating systems that use radiators/radiant panels, hot water baseboard heaters, or even smaller diameter ducting, with ASHP-sourced heat. The lower heat pump output temperatures means radiators (and possibly pipes) may have to be replaced with larger sizes, or a low temperature underfloor heating system installed instead.^[21]

Alternatively, a high temperature heat pump can be installed and existing heat emitters can be retained, however as of 2023^[update] these heat pumps are more expensive to buy and run so may only be suitable for buildings which are hard to alter or insulate, such as some large historic houses.^[22]

ASHP are claimed to be healthier than fossil-fuelled heating such as gas heaters by maintaining a more even temperature and avoiding harmful fumes risk.^[18] By filtering the air and reducing humidity in hot humid summer climates, they are also said to reduce dust, allergens, and mold, which poses a health risk.^[23]

In cold climates

[edit]

Operation of normal ASHPs is generally not recommended below −10 °C.^[24] However, ASHPs designed specifically for very cold climates (in the US, these are certified under Energy Star^[25]) can extract useful heat from ambient air as cold as −30 °C (−22 °F) but electric resistance heating may be more efficient below −25 °C.^[24] This is made possible by the use of variable-speed compressors, powered by inverters.^[25] Although air source heat pumps are less efficient than well-installed ground source heat pumps (GSHPs) in cold conditions, air source heat pumps have lower initial costs and may be the most economic or practical choice.^[26] A hybrid system, with both a heat pump and an alternative source of heat such as a fossil fuel boiler, may be suitable if it is impractical to properly insulate a large house.^[27] Alternatively multiple heat pumps or a high temperature heat pump may be considered.^[27]

In some weather conditions condensation will form and then freeze onto the coils of the heat exchanger of the outdoor unit, reducing air flow through the coils. To clear this condensation, the unit operates a defrost cycle, switching to cooling mode for a few minutes and heating the coils until the ice melts. Air-to-water heat pumps use heat from the circulating water for this purpose, which results in a small and probably undetectable drop in water temperature;^[28] for air-to-air systems, heat is either taken from the air in the building or using an electrical heater.^[29] Some air-to-air systems simply stop the operation of the fans of both units and switch to cooling mode so that the outdoor unit returns to being the condenser such that it heats up and defrosts.

As discussed above, typical air-source heat pumps (ASHPs) struggle to perform efficiently at low temperatures. Ground-source heat pumps (GSHPs), which transfer heat to or from the ground using fluid-filled underground pipes (ground heat exchangers or GHEs),^[30] offer higher efficiency but are expensive to install due to labor and material costs.^[31] A ground source air heat pump (GSAHP)—or water-to-refrigerant type GSHPs ^[32]—presents a viable alternative, integrating elements of ASHPs and water-to-water GSHPs. A GSAHP has three components: a GHE (vertical or horizontal), a heat pump, and a fan coil unit (FCU).

The heat pump unit contains an evaporator, compressor, condenser, and expansion valve.^[33] Thermal energy is extracted from the ground through an antifreeze solution in the GHE, transferred to the refrigerant in the heat pump, and compressed before being delivered to a refrigerant-to-air heat exchanger. A fan then circulates the heated air indoors.

Unlike conventional GSHPs, GSAHPs eliminate the need for hydronic systems (e.g., underfloor heating systems or wall-mounted radiators), relying instead on fans to distribute heat directly into indoor air. This reduces installation costs and complexity while retaining the efficiency benefits of GSHPs in cold climates. By extracting heat from stable ground temperatures, GSAHPs outperform ASHPs in low temperatures, achieving higher efficiency and reduced greenhouse gas emissions. Installation costs for GSAHPs are intermediate between ASHP and GSHP systems; while they eliminate the need for indoor pipework, they still require drilling or digging for the GHE.

Electricity consumption drives the climate impact of heat pump systems. GSAHPs demonstrate a coefficient of performance (COP) approximately 35% higher than ASHPs under certain conditions,^[32] due to the stable ground temperatures they leverage. Additionally, the operation phase accounts for 84% of its climate impacts over a heat pump's life cycle,^[34] highlighting the importance of efficiency (i.e., higher COPs) in reducing emissions. The global warming potential (GWP) of GSAHPs is nearly 40% lower than ASHPs,^[31] further demonstrating their environmental advantages in cold climates. This efficiency advantage is especially pronounced during winter when ASHP efficiency typically declines. GSAHPs consume less electricity for heating, resulting in lower greenhouse gas emissions, particularly in regions with high heating demands and carbon-intensive electricity grids.

Noise

[edit]

An air source heat pump requires an outdoor unit containing moving mechanical components including fans which produce noise. Modern devices offer schedules for silent mode operation with reduced fan speed. This will reduce the maximum heating power but can be applied at mild outdoor temperatures without efficiency loss. Acoustic enclosures are another approach to reduce the noise in a sensitive neighbourhood. In insulated buildings, operation can be paused at night without significant temperature loss. Only at low temperatures, frost protection forces operation after a few hours. Proper siting is also important.^[35]

In the United States, the allowed night-time noise level is 45 A-weighted decibels (dBA).^[36] In the UK the limit is set at 42 dB measured from the nearest neighbour^[37] according to the MCS 020 standard^[38] or equivalent.^[39] In Germany the limit in residential areas is 35, which is usually measured by European Standard EN 12102.^[40]

Another feature of air source heat pumps (ASHPs) external heat exchangers is their need to stop the fan from time to time for a period of several minutes in order to get rid of frost that accumulates in the outdoor unit in the heating mode. After that, the heat pump starts to work again. This part of the work cycle results in two sudden changes of the noise made by the fan. The acoustic effect of such disruption is especially powerful in quiet environments where background night-time noise may be as low as 0 to 10dBA. This is included in legislation in France. According to the French concept of noise nuisance, "noise emergence" is the difference between ambient noise including the disturbing noise, and ambient noise without the disturbing noise.^[41][42] By contrast a ground source heat pump has no need for an outdoor unit with moving mechanical components.

Efficiency ratings

[edit]

The efficiency of air source heat pumps is measured by the coefficient of performance (COP). A COP of 4 means the heat pump produces 4 units of heat energy for every 1 unit of electricity it consumes. Within temperature ranges of −3 °C (27 °F) to 10 °C (50 °F), the COP for many machines is fairly stable. Approximately TheoreticalMaxCOP = (desiredIndoorTempC + 273) ÷ (desiredIndoorTempC - outsideTempC).^{[citation needed][43][better source needed]}

In mild weather with an outside temperature of 10 °C (50 °F), the COP of efficient air source heat pumps ranges from 4 to 6.^[44] However, on a cold winter day, it takes more work to move the same amount of heat indoors than on a mild day.^[45] The heat pump's performance is limited by the Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach −18 °C (0 °F).^{[citation needed]}Heat pump construction that enables carbon dioxide as a refrigerant may have a COP of greater than 2 even down to −20 °C, pushing the break-even figure downward to −30 °C (−22 °F).^{[citation needed]} A ground source heat pump has comparatively less of a change in COP as outdoor temperatures change, because the ground from which they extract heat has a more constant temperature than outdoor air.

The design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange can attain greater COP and an extended life cycle. The principal changes are in the scale and type of compressor and evaporator.

Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio (SEER) respectively. In the US the legal minimum efficiency is 14 or 15 SEER and 8.8 HSPF.^[25]

Variable speed compressors are more efficient because they can often run more slowly and because the air passes through more slowly giving its water more time to condense, thus more efficient as drier air is easier to cool. However, they are more expensive and more likely to need maintenance or replacement.^[23] Maintenance such as changing filters can improve performance by 10% to 25%.^[46]

Refrigerant types

[edit]

Heat pumps are key to decarbonizing home energy use by phasing out gas boilers.^[19][11] As of 2024 the IEA says that 500 million tonnes of CO₂ emissions could be cut by 2030.^[69]

As wind farms are increasingly used to supply electricity to some grids, such as Canada's Yukon Territory, the increased winter load matches well with the increased winter generation from wind turbines, and calmer days result in decreased heating load for most houses even if the air temperature is low.^[70]

Heat pumps could help stabilize grids through demand response.^[71] As heat pump penetration increases some countries, such as the UK, may need to encourage households to use thermal energy storage, such as very well insulated water tanks.^[72] In some countries, such as Australia, integration of this thermal storage with rooftop solar would also help.^[73]

Although higher cost heat pumps can be more efficient a 2024 study concluded that for the UK "from an energy system perspective, it is overall cost-optimal to design heat pumps with nominal COP in the range of 2.8–3.2, which typically has a specific cost lower than 650 £/kWth, and simultaneously to invest in increased capacities of renewable energy generation technologies and batteries, in the first instance, followed by OCGT and CCGT with CCS."^[74]

As of 2023^[update] buying and installing an ASHP in an existing house is expensive if there is no government subsidy, but the lifetime cost will likely be less than or similar to a gas boiler and air conditioner.^[75][76] This is generally also true if cooling is not required, as the ASHP will likely last longer if only heating.^[77] The lifetime cost of an air source heat pump will be affected by the price of electricity compared to gas (where available), and may take two to ten years to break even.^[75] The IEA recommends governments subsidize the purchase price of residential heat pumps, and some countries do so.^[78]

In Norway,^[79] Australia and New Zealand most heating is from heat pumps. In 2022 heat pumps outsold fossil fuel based heating in the US and France.^[78] In the UK, annual heat pump sales have steadily grown in recent years with 26,725 heat pumps sold in 2018, a figure which has increased to 60,244 heat pumps sales in 2023.^[80] ASHPs can be helped to compete by increasing the price of fossil gas compared to that of electricity and using suitable flexible electricity pricing.^[19] In the US air-to-air is the most common type.^[81] As of 2023^[update] over 80% of heat pumps are air source.^[11] In 2023 the IEA appealed for better data - especially on air-to-air.^[78]

Many of the maintenance needs for air source heat pumps reflect that of conventional air conditioning and furnace installations, such as regular air filter replacements and cleaning of both the indoor evaporator and outdoor condenser coils. However, there are additional maintenance measures unique to the operation of air source heat pumps that concern the physical means by which a heat pump extracts heat from the outdoor air.^[82][83][84] Since a heat pump running in cooling mode operates essentially the same as a conventional air conditioning system, these measures primarily concern the performance of ASHPs during the winter, especially in colder climates.^[85][86]

In colder climates, where the compressor works harder to extract heat from the outside air, it is critical to prevent the buildup of ice and frost on the outdoor coil to maintain ASHP performance. This buildup acts as an insulation layer and decreases the rate of heat exchange by blocking the continuous flow of air over the outdoor coil.^[87] To prevent this issue, it is necessary to keep the outdoor coil clean of any dirt or grime, as this can trap moisture from the air, which freezes over the coil.^[88] In addition, it is necessary to keep the fins surrounding the condenser coil and air intake grill of the outdoor unit free of any debris, such as leaves, that could further block airflow and impede heat exchange.^[89][90] This upkeep helps minimize the need for frequent defrost cycles that put the heat pump into cooling mode and send heated refrigerant to the condenser coil to melt accumulated ice.^[91] These defrost cycles can cause pressure fluctuations in the refrigerant lines that lead to refrigerant leaks and diminish performance.^[92][93]

When heating performance drops, an ASHP can remain reliable through its auxiliary heating strip that provides an additional source of heat through electrical resistance to compensate for any heat losses, although this process is significantly less efficient.^[94][95]

It is thought that ASHP need less maintenance than fossil fuelled heating, and some say that ASHPs are easier to maintain than ground source heat pumps due to the difficulty of finding and fixing underground leaks. Installing too small an ASHP could shorten its lifetime (but one which is too large will be less efficient).^[96] However others say that boilers require less maintenance than ASHPs.^[97] A Consumer Reports survey found that "on average, around half of heat pumps are likely to experience problems by the end of the eighth year of ownership".^[98]

Modern chemical refrigeration techniques developed after the proposal of the Carnot cycle in 1824. Jacob Perkins invented an ice-making machine that used ether in 1843, and Edmond Carré built a refrigerator that used water and sulfuric acid in 1850. In Japan, Fusanosuke Kuhara, founder of Hitachi, Ltd., made an air conditioner for his own home use using compressed CO₂ as a refrigerant in 1917.^[99]

In 1930 Thomas Midgley Jr. discovered dichlorodifluoromethane, a chlorinated fluorocarbon (CFC) known as freon. CFCs rapidly replaced traditional refrigerant substances, including CO₂ (which proved hard to compress for domestic use^[100]), for use in heat pumps and refrigerators. But from the 1980s CFCs began to lose favor as refrigerant when their damaging effects on the ozone layer were discovered. Two alternative types of refrigerant, hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), also lost favor when they were identified as greenhouse gases (additionally, HCFCs were found to be more damaging to the ozone layer than originally thought). The Vienna Convention for the Protection of the Ozone Layer, the Montreal Protocol and the Kyoto Protocol call for the complete abandonment of such refrigerants by 2030.

In 1989, amid international concern about the effects of chlorofluorocarbons and hydrochlorofluorocarbons on the ozone layer, scientist Gustav Lorentzen and SINTEF patented a method for using CO₂ as a refrigerant in heating and cooling. Further research into CO₂ refrigeration was then conducted at Shecco (Sustainable HEating and Cooling with CO₂) in Brussels, Belgium, leading to increasing use of CO₂ refrigerant technology in Europe.^[100]

In 1993 the Japanese company Denso, in collaboration with Gustav Lorentzen, developed an automobile air conditioner using CO₂ as a refrigerant. They demonstrated the invention at the June 1998 International Institute of Refrigeration/Gustav Lorentzen Conference.^[101] After the conference, CRIEPI (Central Research Institute of Electric Power Industry) and TEPCO (The Tokyo Electric Power Company) approached Denso about developing a prototype air conditioner using natural refrigerant materials instead of freon. Together they produced 30 prototype units for a year-long experimental installation at locations throughout Japan, from the cold climate of Hokkaidō to hotter Okinawa. After this successful feasibility study, Denso obtained a patent to compress CO₂ refrigerant for use in a heat pump from SINTEF in September 2000. During the early 21st century CO₂ heat pumps, under the EcoCute patent, became popular for new-build housing in Japan but were slower to take off elsewhere.^[102]

Demand for heat pumps increased in the first quarter of the 21st century in the US and Europe, with governments subsidizing them to increase energy security and decarbonisation. Europeans tend to use air-to-water (also called hydronic) systems which utilize radiators, rather than the air-to-air systems more common elsewhere. Asian countries made three-quarters of heat pumps globally in 2021.^[103]

• Category:Heating, ventilation, and air conditioning companies
• Transcritical cycle

• IPCC (2021). Masson-Delmotte, V.; Zhai, P.; Pirani, A.; Connors, S. L.; et al. (eds.). Climate Change 2021: The Physical Science Basis (PDF). Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press (In Press).
  • Forster, P.; Storelvmo, T.; Armour, K.; Collins, W. (2021). "Chapter 7: The Earth's energy budget, climate feedbacks, and climate sensitivity Supplementary Material" (PDF). IPCC AR6 WG1 2021.

There are various types of air conditioners. Popular examples include: Window-mounted air conditioner (China, 2023); Ceiling-mounted cassette air conditioner (China, 2023); Wall-mounted air conditioner (Japan, 2020); Ceiling-mounted console (Also called ceiling suspended) air conditioner (China, 2023); and portable air conditioner (Vatican City, 2018).

Air conditioning, often abbreviated as A/C (US) or air con (UK),^[1] is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air conditioning can be achieved using a mechanical 'air conditioner' or through other methods, such as passive cooling and ventilative cooling.^[2][3] Air conditioning is a member of a family of systems and techniques that provide heating, ventilation, and air conditioning (HVAC).^[4] Heat pumps are similar in many ways to air conditioners but use a reversing valve, allowing them to both heat and cool an enclosed space.^[5]

Air conditioners, which typically use vapor-compression refrigeration, range in size from small units used in vehicles or single rooms to massive units that can cool large buildings.^[6] Air source heat pumps, which can be used for heating as well as cooling, are becoming increasingly common in cooler climates.

Air conditioners can reduce mortality rates due to higher temperature.^[7] According to the International Energy Agency (IEA) 1.6 billion air conditioning units were used globally in 2016.^[8] The United Nations called for the technology to be made more sustainable to mitigate climate change and for the use of alternatives, like passive cooling, evaporative cooling, selective shading, windcatchers, and better thermal insulation.

Air conditioning dates back to prehistory.^[9] Double-walled living quarters, with a gap between the two walls to encourage air flow, were found in the ancient city of Hamoukar, in modern Syria.^[10] Ancient Egyptian buildings also used a wide variety of passive air-conditioning techniques.^[11] These became widespread from the Iberian Peninsula through North Africa, the Middle East, and Northern India.^[12]

Passive techniques remained widespread until the 20th century when they fell out of fashion and were replaced by powered air conditioning. Using information from engineering studies of traditional buildings, passive techniques are being revived and modified for 21st-century architectural designs.^[13][12]

Air conditioners allow the building's indoor environment to remain relatively constant, largely independent of changes in external weather conditions and internal heat loads. They also enable deep plan buildings to be created and have allowed people to live comfortably in hotter parts of the world.^[14]

In 1558, Giambattista della Porta described a method of chilling ice to temperatures far below its freezing point by mixing it with potassium nitrate (then called "nitre") in his popular science book Natural Magic.^[15][16][17] In 1620, Cornelis Drebbel demonstrated "Turning Summer into Winter" for James I of England, chilling part of the Great Hall of Westminster Abbey with an apparatus of troughs and vats.^[18] Drebbel's contemporary Francis Bacon, like della Porta a believer in science communication, may not have been present at the demonstration, but in a book published later the same year, he described it as "experiment of artificial freezing" and said that "Nitre (or rather its spirit) is very cold, and hence nitre or salt when added to snow or ice intensifies the cold of the latter, the nitre by adding to its cold, but the salt by supplying activity to the cold of the snow."^[15]

In 1758, Benjamin Franklin and John Hadley, a chemistry professor at the University of Cambridge, conducted experiments applying the principle of evaporation as a means to cool an object rapidly. Franklin and Hadley confirmed that the evaporation of highly volatile liquids (such as alcohol and ether) could be used to drive down the temperature of an object past the freezing point of water. They experimented with the bulb of a mercury-in-glass thermometer as their object. They used a bellows to speed up the evaporation. They lowered the temperature of the thermometer bulb down to −14 °C (7 °F) while the ambient temperature was 18 °C (64 °F). Franklin noted that soon after they passed the freezing point of water 0 °C (32 °F), a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about 6 mm (1⁄4 in) thick when they stopped the experiment upon reaching −14 °C (7 °F). Franklin concluded: "From this experiment, one may see the possibility of freezing a man to death on a warm summer's day."^[19]

The 19th century included many developments in compression technology. In 1820, English scientist and inventor Michael Faraday discovered that compressing and liquefying ammonia could chill air when the liquefied ammonia was allowed to evaporate.^[20] In 1842, Florida physician John Gorrie used compressor technology to create ice, which he used to cool air for his patients in his hospital in Apalachicola, Florida. He hoped to eventually use his ice-making machine to regulate the temperature of buildings.^[20][21] He envisioned centralized air conditioning that could cool entire cities. Gorrie was granted a patent in 1851,^[22] but following the death of his main backer, he was not able to realize his invention.^[23] In 1851, James Harrison created the first mechanical ice-making machine in Geelong, Australia, and was granted a patent for an ether vapor-compression refrigeration system in 1855 that produced three tons of ice per day.^[24] In 1860, Harrison established a second ice company. He later entered the debate over competing against the American advantage of ice-refrigerated beef sales to the United Kingdom.^[24]

Electricity made the development of effective units possible. In 1901, American inventor Willis H. Carrier built what is considered the first modern electrical air conditioning unit.^{[25][26][27][28]} In 1902, he installed his first air-conditioning system in the Sackett-Wilhelms Lithographing & Publishing Company in Brooklyn, New York.^[29] His invention controlled both the temperature and humidity, which helped maintain consistent paper dimensions and ink alignment at the printing plant. Later, together with six other employees, Carrier formed The Carrier Air Conditioning Company of America, a business that in 2020, employed 53,000 people and was valued at $18.6 billion.^[30][31]

In 1906, Stuart W. Cramer of Charlotte, North Carolina, was exploring ways to add moisture to the air in his textile mill. Cramer coined the term "air conditioning" in a patent claim which he filed that year, where he suggested that air conditioning was analogous to "water conditioning", then a well-known process for making textiles easier to process.^[32] He combined moisture with ventilation to "condition" and change the air in the factories, thus controlling the humidity that is necessary in textile plants. Willis Carrier adopted the term and incorporated it into the name of his company.^[33]

Domestic air conditioning soon took off. In 1914, the first domestic air conditioning was installed in Minneapolis in the home of Charles Gilbert Gates. It is, however, possible that the considerable device (c. 2.1 m × 1.8 m × 6.1 m; 7 ft × 6 ft × 20 ft) was never used, as the house remained uninhabited^[20] (Gates had already died in October 1913.)

In 1931, H.H. Schultz and J.Q. Sherman developed what would become the most common type of individual room air conditioner: one designed to sit on a window ledge. The units went on sale in 1932 at US$10,000 to $50,000 (the equivalent of $200,000 to $1,200,000 in 2024.)^[20] A year later, the first air conditioning systems for cars were offered for sale.^[34] Chrysler Motors introduced the first practical semi-portable air conditioning unit in 1935,^[35] and Packard became the first automobile manufacturer to offer an air conditioning unit in its cars in 1939.^[36]

Innovations in the latter half of the 20th century allowed more ubiquitous air conditioner use. In 1945, Robert Sherman of Lynn, Massachusetts, invented a portable, in-window air conditioner that cooled, heated, humidified, dehumidified, and filtered the air.^[37] The first inverter air conditioners were released in 1980–1981.^[38][39]

In 1954, Ned Cole, a 1939 architecture graduate from the University of Texas at Austin, developed the first experimental "suburb" with inbuilt air conditioning in each house. 22 homes were developed on a flat, treeless track in northwest Austin, Texas, and the community was christened the 'Austin Air-Conditioned Village.' The residents were subjected to a year-long study of the effects of air conditioning led by the nation’s premier air conditioning companies, builders, and social scientists. In addition, researchers from UT’s Health Service and Psychology Department studied the effects on the "artificially cooled humans." One of the more amusing discoveries was that each family reported being troubled with scorpions, the leading theory being that scorpions sought cool, shady places. Other reported changes in lifestyle were that mothers baked more, families ate heavier foods, and they were more apt to choose hot drinks.^[40][41]

Air conditioner adoption tends to increase above around $10,000 annual household income in warmer areas.^[42] Global GDP growth explains around 85% of increased air condition adoption by 2050, while the remaining 15% can be explained by climate change.^[42]

As of 2016, an estimated 1.6 billion air conditioning units were used worldwide, with over half of them in China and the United States, and with a total cooling capacity of 11,675 gigawatts.^[8][43] The International Energy Agency predicted in 2018 that the number of air conditioning units would grow to around 4 billion units by 2050 and that the total cooling capacity would grow to around 23,000 GW, with the biggest increases in India and China.^[8] Between 1995 and 2004, the proportion of urban households in China with air conditioners increased from 8% to 70%.^[44] As of 2015, nearly 100 million homes, or about 87% of US households, had air conditioning systems.^[45] In 2019, it was estimated that 90% of new single-family homes constructed in the US included air conditioning, ranging from 99% in the South to 62% in the West.^[46][47]

Cooling in traditional air conditioner systems is accomplished using the vapor-compression cycle, which uses a refrigerant's forced circulation and phase change between gas and liquid to transfer heat.^[48][49] The vapor-compression cycle can occur within a unitary, or packaged piece of equipment, or within a chiller that is connected to terminal cooling equipment (such as a fan coil unit in an air handler) on its evaporator side and heat rejection equipment such as a cooling tower on its condenser side. An air source heat pump shares many components with an air conditioning system, but includes a reversing valve, which allows the unit to be used to heat as well as cool a space.^[50]

Air conditioning equipment will reduce the absolute humidity of the air processed by the system if the surface of the evaporator coil is significantly cooler than the dew point of the surrounding air. An air conditioner designed for an occupied space will typically achieve a 30% to 60% relative humidity in the occupied space.^[51]

Most modern air-conditioning systems feature a dehumidification cycle during which the compressor runs. At the same time, the fan is slowed to reduce the evaporator temperature and condense more water. A dehumidifier uses the same refrigeration cycle but incorporates both the evaporator and the condenser into the same air path; the air first passes over the evaporator coil, where it is cooled^[52] and dehumidified, before passing over the condenser coil, where it is warmed again before it is released back into the room.^{[citation needed]}

Free cooling can sometimes be selected when the external air is cooler than the internal air. In this case, the compressor does not need to be used, resulting in high cooling efficiencies for these times. This may also be combined with seasonal thermal energy storage.^[53]

Some air conditioning systems can reverse the refrigeration cycle and act as an air source heat pump, thus heating instead of cooling the indoor environment. They are also commonly referred to as "reverse cycle air conditioners". The heat pump is significantly more energy-efficient than electric resistance heating, because it moves energy from air or groundwater to the heated space and the heat from purchased electrical energy. When the heat pump is in heating mode, the indoor evaporator coil switches roles and becomes the condenser coil, producing heat. The outdoor condenser unit also switches roles to serve as the evaporator and discharges cold air (colder than the ambient outdoor air).

Most air source heat pumps become less efficient in outdoor temperatures lower than 4 °C or 40 °F.^[54] This is partly because ice forms on the outdoor unit's heat exchanger coil, which blocks air flow over the coil. To compensate for this, the heat pump system must temporarily switch back into the regular air conditioning mode to switch the outdoor evaporator coil back to the condenser coil, to heat up and defrost. Therefore, some heat pump systems will have electric resistance heating in the indoor air path that is activated only in this mode to compensate for the temporary indoor air cooling, which would otherwise be uncomfortable in the winter.

Newer models have improved cold-weather performance, with efficient heating capacity down to −14 °F (−26 °C).^[55][54][56] However, there is always a chance that the humidity that condenses on the heat exchanger of the outdoor unit could freeze, even in models that have improved cold-weather performance, requiring a defrosting cycle to be performed.

The icing problem becomes much more severe with lower outdoor temperatures, so heat pumps are sometimes installed in tandem with a more conventional form of heating, such as an electrical heater, a natural gas, heating oil, or wood-burning fireplace or central heating, which is used instead of or in addition to the heat pump during harsher winter temperatures. In this case, the heat pump is used efficiently during milder temperatures, and the system is switched to the conventional heat source when the outdoor temperature is lower.

The coefficient of performance (COP) of an air conditioning system is a ratio of useful heating or cooling provided to the work required.^[57][58] Higher COPs equate to lower operating costs. The COP usually exceeds 1; however, the exact value is highly dependent on operating conditions, especially absolute temperature and relative temperature between sink and system, and is often graphed or averaged against expected conditions.^[59] Air conditioner equipment power in the U.S. is often described in terms of "tons of refrigeration", with each approximately equal to the cooling power of one short ton (2,000 pounds (910 kg) of ice melting in a 24-hour period. The value is equal to 12,000 BTU_IT per hour, or 3,517 watts.^[60] Residential central air systems are usually from 1 to 5 tons (3.5 to 18 kW) in capacity.^{[citation needed]}

The efficiency of air conditioners is often rated by the seasonal energy efficiency ratio (SEER), which is defined by the Air Conditioning, Heating and Refrigeration Institute in its 2008 standard AHRI 210/240, Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.^[61] A similar standard is the European seasonal energy efficiency ratio (ESEER).^{[citation needed]}

Efficiency is strongly affected by the humidity of the air to be cooled. Dehumidifying the air before attempting to cool it can reduce subsequent cooling costs by as much as 90 percent. Thus, reducing dehumidifying costs can materially affect overall air conditioning costs.^[62]

A wireless remote controller

The infrared transmitting LED on the remote

The infrared receiver on the air conditioner

This type of controller uses an infrared LED to relay commands from a remote control to the air conditioner. The output of the infrared LED (like that of any infrared remote) is invisible to the human eye because its wavelength is beyond the range of visible light (940 nm). This system is commonly used on mini-split air conditioners because it is simple and portable. Some window and ducted central air conditioners uses it as well.

Several wired controllers (Indonesia, 2024)

A wired controller, also called a "wired thermostat," is a device that controls an air conditioner by switching heating or cooling on or off. It uses different sensors to measure temperatures and actuate control operations. Mechanical thermostats commonly use bimetallic strips, converting a temperature change into mechanical displacement, to actuate control of the air conditioner. Electronic thermostats, instead, use a thermistor or other semiconductor sensor, processing temperature change as electronic signals to control the air conditioner.

These controllers are usually used in hotel rooms because they are permanently installed into a wall and hard-wired directly into the air conditioner unit, eliminating the need for batteries.

* where the typical capacity is in kilowatt as follows:

• very small: <1.5 kW
• small: 1.5–3.5 kW
• medium: 4.2–7.1 kW
• large: 7.2–14 kW
• very large: >14 kW

Ductless systems (often mini-split, though there are now ducted mini-split) typically supply conditioned and heated air to a single or a few rooms of a building, without ducts and in a decentralized manner.^[63] Multi-zone or multi-split systems are a common application of ductless systems and allow up to eight rooms (zones or locations) to be conditioned independently from each other, each with its indoor unit and simultaneously from a single outdoor unit.

The first mini-split system was sold in 1961 by Toshiba in Japan, and the first wall-mounted mini-split air conditioner was sold in 1968 in Japan by Mitsubishi Electric, where small home sizes motivated their development. The Mitsubishi model was the first air conditioner with a cross-flow fan.^[64][65][66] In 1969, the first mini-split air conditioner was sold in the US.^[67] Multi-zone ductless systems were invented by Daikin in 1973, and variable refrigerant flow systems (which can be thought of as larger multi-split systems) were also invented by Daikin in 1982. Both were first sold in Japan.^[68] Variable refrigerant flow systems when compared with central plant cooling from an air handler, eliminate the need for large cool air ducts, air handlers, and chillers; instead cool refrigerant is transported through much smaller pipes to the indoor units in the spaces to be conditioned, thus allowing for less space above dropped ceilings and a lower structural impact, while also allowing for more individual and independent temperature control of spaces. The outdoor and indoor units can be spread across the building.^[69] Variable refrigerant flow indoor units can also be turned off individually in unused spaces.^{[citation needed]} The lower start-up power of VRF's DC inverter compressors and their inherent DC power requirements also allow VRF solar-powered heat pumps to be run using DC-providing solar panels.

Split-system central air conditioners consist of two heat exchangers, an outside unit (the condenser) from which heat is rejected to the environment and an internal heat exchanger (the evaporator, or Fan Coil Unit, FCU) with the piped refrigerant being circulated between the two. The FCU is then connected to the spaces to be cooled by ventilation ducts.^[70] Floor standing air conditioners are similar to this type of air conditioner but sit within spaces that need cooling.

Large central cooling plants may use intermediate coolant such as chilled water pumped into air handlers or fan coil units near or in the spaces to be cooled which then duct or deliver cold air into the spaces to be conditioned, rather than ducting cold air directly to these spaces from the plant, which is not done due to the low density and heat capacity of air, which would require impractically large ducts. The chilled water is cooled by chillers in the plant, which uses a refrigeration cycle to cool water, often transferring its heat to the atmosphere even in liquid-cooled chillers through the use of cooling towers. Chillers may be air- or liquid-cooled.^[71][72]

A portable system has an indoor unit on wheels connected to an outdoor unit via flexible pipes, similar to a permanently fixed installed unit (such as a ductless split air conditioner).

Hose systems, which can be monoblock or air-to-air, are vented to the outside via air ducts. The monoblock type collects the water in a bucket or tray and stops when full. The air-to-air type re-evaporates the water, discharges it through the ducted hose, and can run continuously. Many but not all portable units draw indoor air and expel it outdoors through a single duct, negatively impacting their overall cooling efficiency.

Many portable air conditioners come with heat as well as a dehumidification function.^[73]

The packaged terminal air conditioner (PTAC), through-the-wall, and window air conditioners are similar. These units are installed on a window frame or on a wall opening. The unit usually has an internal partition separating its indoor and outdoor sides, which contain the unit's condenser and evaporator, respectively. PTAC systems may be adapted to provide heating in cold weather, either directly by using an electric strip, gas, or other heaters, or by reversing the refrigerant flow to heat the interior and draw heat from the exterior air, converting the air conditioner into a heat pump. They may be installed in a wall opening with the help of a special sleeve on the wall and a custom grill that is flush with the wall and window air conditioners can also be installed in a window, but without a custom grill.^[74]

Packaged air conditioners (also known as self-contained units)^[75][76] are central systems that integrate into a single housing all the components of a split central system, and deliver air, possibly through ducts, to the spaces to be cooled. Depending on their construction they may be outdoors or indoors, on roofs (rooftop units),^[77][78] draw the air to be conditioned from inside or outside a building and be water or air-cooled. Often, outdoor units are air-cooled while indoor units are liquid-cooled using a cooling tower.^{[70][79][80][81][82][83]}