air source 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.  
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 (86–104°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).As of 2023 about 10% of home 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.

Technology
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.  An air-to-water system only pumps heat inwards, and can provide space heating and hot water. 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 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.These are also called monobloc and are useful for keeping flamable propane outside the house.An ASHP can provide three or four times as much heat as an electric resistance heater using the same amount of electricity.  Burning gas or oil will emit carbon dioxide and also NOx, which can be harmful to health. 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.
Unlike an air conditioning unit, most ASHPs are reversible and are able to either warm or cool buildings and in some cases also provide domestic hot water.

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.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.

Usage
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. Some ASHPs can be coupled to solar panels as primary energy source, with a conventional electric grid as backup source.
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; 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
As of 2023 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. If running costs are important choosing the right size is important because an ASHP which is too large will be more expensive to run.It is difficult 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 would mean radiators would have to be increased in size or a low temperature underfloor heating system be installed instead. 
Alternatively, a high temperature heat pump can be installed and existing heat emitters can be retained, however as of 2023 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.

In cold climates
Operation of normal ASHPs is generally not recommended below −10°C. However ASHPs designed specifically for very cold climates (in the US these are certified under Energy Star) can extract useful heat from ambient air as cold as −30 °C (−22 °F), however below −25°C electric resistance heating may be more efficient. This is made possible by the use of variable-speed compressors, powered by inverters. Although air source heat pumps are less efficient than well-installed ground source heat pumps in cold conditions, air source heat pumps have lower initial costs and may be the most economic or practical choice. 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. Alternatively multiple heat pumps or a high temperature heat pump may be considered.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 the unit operates a defrost cycle, switching to cooling mode for a few minutes, 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; for air-to-air systems heat is either taken from the air in the building or using an electrical heater. 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.

Noise
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.
In the United States, the allowed nighttime noise level is 45 A-weighted decibels (dBA), and in the UK 42 measured from the nearest neighbour. In Germany the limit in residential areas is 35, which is usually measured by European Standard EN 12102.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 nighttime 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.By contrast a ground source heat pump has no need for an outdoor unit with moving mechanical components.

Efficiency ratings
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.
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. However, on a cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. 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). 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). 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.

Refrigerant types
Impact on decarbonization and electricity supply
Heat pumps are key to decarbonizing home energy use by phasing out gas boilers.While heat pumps with backup systems other than electrical resistance heating are often encouraged by electric utilities, air source heat pumps are a concern for winter-peaking utilities if electrical resistance heating is used as the supplemental or replacement heat source when the temperature drops below the point that the heat pump can meet all of the home's heat requirement. Even if there is a non-electric backup system, the fact that efficiencies of ASHPs decrease with outside temperatures is a concern to electric utilities. The drop in efficiency means their electrical load increases steeply as temperatures drop. 
A study in Canada's Yukon Territory, where diesel generators are used for peaking capacity, noted that widespread adoption of air source heat pumps could lead to increased diesel consumption if the increased electrical demand due to ASHP use exceeds available hydroelectric capacity. Notwithstanding those concerns, the study did conclude that ASHPs are a cost-effective heating alternative for Yukon residents. As wind farms are increasingly used to supply electricity to the grid, 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.
Heat pumps could help stabilize grids through demand response. 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. In some countries, such as Australia, integration of this thermal storage with rooftop solar would also help.

Economics
Cost
As of 2023 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. This is generally also true if cooling is not required, as the ASHP will likely last longer if only heating. 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. The IEA recommends governments subsidize the purchase price of residential heat pumps, and some countries do so.

Market
In Norway, Australia and New Zealand most heating is from heat pumps. In 2022 heat pumps outsold fossil fuel based heating in the US and France. ASHPs can be helped to compete by increasing the price of fossil gas compared to that of electricity and using suitable flexible electricity pricing. In the US air-to-air is the most common type. As of 2023 over 80% of heat pumps are air source.

Maintenance and reliability
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). However others say that boilers require less maintenance than ASHPs. 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".

References
Sources
IPCC reports
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.