Air source heat pump

2025-07-01T15:25:19Z

82.9.76.72: added sentence about merging ASHP and underfloor and added citation

{{Short description|Most common type of heat pump}}
{{About|details of the most common type of [[heat pump]]|more general information|heat pump}}
{{Use dmy dates|date=July 2022}}
[[File:Heat pump on balcony.jpg|thumb|[[Heat pump]] on balcony of apartment]]
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 conditioning|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.<ref name=":9">{{Cite news |title=Why Britain's homes will need different types of heat pump |url=https://www.economist.com/britain/2023/11/30/why-britains-homes-will-need-different-types-of-heat-pump?utm_medium=cpc.adword.pd&utm_source=google&ppccampaignID=18151738051&ppcadID=&utm_campaign=a.22brand_pmax&utm_content=conversion.direct-response.anonymous&gad_source=1&gclid=EAIaIQobChMIzquM9uy2hAMVV4toCR2ZYA9FEAAYASAAEgKJm_D_BwE&gclsrc=aw.ds |access-date=2024-02-19 |newspaper=The Economist |issn=0013-0613}}</ref>

''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 [[Radiator (heating)|radiators]] or [[underfloor heating]] to heat a whole house and are often also used to provide [[domestic hot water]]. Air source heat pumps are particularly effective when combined with low-temperature heat distribution systems, such as underfloor heating. The large surface area of underfloor systems allows rooms to be heated efficiently at lower flow temperatures, improving the overall coefficient of performance ([[Coefficient of performance|COP]]) of air source heat pumps, especially in well-insulated homes.<ref>{{Cite web |last=Poyser |first=Jack |date=2025-01-01 |title=Air Source Heat Pumps with Underfloor Heating: How To Merge |url=https://www.underfloorheatingtradesupplies.co.uk/blog/air-source-heat-pumps-with-underfloor-heating/ |access-date=2025-07-01 |language=en-GB}}</ref>

An ASHP can typically gain 4 kWh thermal energy from 1 kWh electric energy. They are optimized for flow temperatures between {{convert|30|and|40|C|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 {{convert|80|C}}.<ref>{{cite journal |last1=Le |first1=Khoa |last2=Huang |first2=M.J. |last3=Hewitt |first3=Neil |date=2018 |title=Domestic High Temperature Air Source Heat Pump: Performance Analysis Using TRNSYS Simulations |url=https://docs.lib.purdue.edu/ihpbc/315 |journal=International High Performance Buildings Conference |location=West Lafayette, IN, USA |publisher=5th International High Performance Buildings Conference at Purdue University |page=1 |access-date=20 February 2022}}</ref>

{{As of|2023}} about 10% of building heating worldwide is from ASHPs. They are the main way to [[Phase-out of gas boilers|phase out gas boilers]] (also known as "furnaces") from houses, to avoid their [[greenhouse gas emissions]].<ref name=":0" />

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.<ref>{{Cite web |last=Lawrence |first=Karen |title=Air source heat pumps explained |url=https://www.which.co.uk/reviews/ground-and-air-source-heat-pumps/article/air-source-heat-pumps-explained-al5MC4f773Zq |access-date=2022-10-04 |website=Which? |language=en |archive-date=2022-10-04 |archive-url=https://web.archive.org/web/20221004170246/https://www.which.co.uk/reviews/ground-and-air-source-heat-pumps/article/air-source-heat-pumps-explained-al5MC4f773Zq |url-status=live }}</ref>

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 {{convert|-8|C}} an air-source heat pump may still achieve a COP of 1 to 4.<ref>{{Cite web |last=Canada |first=Natural Resources |date=2009-04-22 |title=Heating and Cooling With a Heat Pump |url=https://natural-resources.canada.ca/energy-efficiency/energy-star-canada/about/energy-star-announcements/publications/heating-and-cooling-heat-pump/6817 |access-date=2024-02-22 |website=natural-resources.canada.ca}}</ref>

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.<ref>{{Cite web |date=2022-05-09 |title=Heat pumps do work in the cold – Americans just don't know it yet |url=https://grist.org/housing/heat-pumps-do-work-in-the-cold-americans-just-dont-know-it-yet/ |url-status=live |archive-url=https://web.archive.org/web/20220509105325/https://grist.org/housing/heat-pumps-do-work-in-the-cold-americans-just-dont-know-it-yet/ |archive-date=2022-05-09 |access-date=2022-05-09 |website=Grist |language=en-us}}</ref>

==Technology==

[[File:Outunit of heat pump.jpg|thumb|Air source heat pump]]

[[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.<ref name=":1">{{Cite web |title=The 9 Types of Heat Pumps |url=https://www.achrnews.com/articles/152628-the-9-types-of-heat-pumps |access-date=2023-09-15 |website=ACHR News |language=en}}</ref> An ''air-to-water'' system only pumps heat inwards, and can provide space heating and hot water.<ref>{{Cite web |first1=Karen|last1=Lawrence|first2=Jake|last2=Massey|date=2023-07-14 |title=Air source heat pumps explained |url=https://www.which.co.uk/reviews/ground-and-air-source-heat-pumps/article/air-source-heat-pumps-explained-al5MC4f773Zq|website=Which? |language=en}}</ref> 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<ref>{{Cite web |title=Ductless Mini-Split Heat Pumps |url=https://www.energy.gov/energysaver/ductless-mini-split-heat-pumps |access-date=2023-09-14 |website=Energy.gov |language=en}}</ref> indoor [[Condenser (heat transfer)|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.<ref>{{Cite web |title=Air-Source Heat Pumps |url=https://www.energy.gov/energysaver/air-source-heat-pumps |access-date=2023-09-14 |website=Energy.gov |language=en}}</ref> These are also called monobloc and are useful for keeping flammable propane outside the house.<ref name=":0" />

An ASHP can provide three or four times as much heat as an electric resistance heater using the same amount of electricity.<ref name=":5" /> Burning gas or oil will emit carbon dioxide and also [[NOx]], which can be harmful to health.<ref name=GBN_1>{{cite web| title=Gas boilers and NOx: the hidden emitter| url=https://ca1-eci.edcdn.com/Gas-boilers-and-NOx-the-hidden-emitter.pdf?v=1603351014| publisher=Energy & Climate Intelligence Unit| date=October 2020| access-date=20 January 2024}}</ref> 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<ref>{{Cite report|url=https://assets.publishing.service.gov.uk/media/5a7d8cc4ed915d3fb95946eb/renewable-energy-reversible-air-to-air-heat-pumps.pdf|publisher=Delta Energy & Environment|title=The Contribution of Reversible Air-to-Air Heat Pumps to the UK's Obligation under the Renewable Energy Directive (2009/28/EC) - A Delta-ee Report for the Department for Business, Energy and Industrial Strategy|year=2017|quote=virtually all air-air heat pumps sold today are reversible (p.7)}}</ref> and in some cases also provide [[water heating|domestic hot water]]. The use of an ''air-to-water'' heat pump for house cooling has been criticised.<ref>{{cite web | last=Hendra | first=Graham | title=Four reasons not cool your house using a heat pump | website=Renewable Heating Hub | date=5 May 2021 | url=https://renewableheatinghub.co.uk/hard-to-cool-your-house-with-a-heat-pump | access-date=23 December 2023}}</ref>

[[File:Ecodan outdoor unit Internal view.jpg|thumb|An internal view of the outdoor unit of an air source heat pump]]
[[File:Refrigerator-cycle.svg|thumb|A: indoor compartment, B: outdoor compartment, I: insulation, 1: condenser, 2: expansion valve, 3: evaporator, 4: compressor]]
Heating and cooling is accomplished by pumping a [[refrigerant]] through the heat pump's indoor and outdoor coils. Like in a refrigerator, a [[gas compressor|compressor]], [[Condenser (heat transfer)|condenser]], [[expansion valve (steam engine)|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 [[Ideal gas law|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 (mechanical)|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.<ref name=":5">{{Cite web |title=Everything you need to know about the wild world of heat pumps |url=https://www.technologyreview.com/2023/02/14/1068582/everything-you-need-to-know-about-heat-pumps/ |access-date=2023-09-19 |website=MIT Technology Review |language=en}}</ref>

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.<ref>{{Cite web |title=Annex 53 Advanced Cooling/Refrigeration Technologies 2 page summary |url=https://heatpumpingtechnologies.org/publications/annex-53-advanced-cooling-refrigeration-technologies-2-page-summary/ |access-date=2024-02-19 |website=HPT - Heat Pumping Technologies |language=en}}</ref>

==Usage==
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.<ref name=":9" /> In dense city centres heat networks may be better than ASHP.<ref name=":9" /> 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 [[Geothermal heat pump|''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.<ref>{{Cite news |last=Ambrose |first=Jillian |date=2023-08-14 |title=UK homes install 'record number' of solar panels and heat pumps |language=en-GB |work=The Guardian |url=https://www.theguardian.com/environment/2023/aug/14/uk-homes-install-record-number-of-solar-panels-and-heat-pumps |access-date=2023-09-16 |issn=0261-3077}}</ref> Some ASHPs can be coupled to [[solar panel]]s as primary energy source, with a conventional electric grid as backup source.{{Citation needed|date=February 2024}}

[[Thermal energy storage|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;<ref>{{cite web|author1=Franklin Energy Services, LLC |title=Air Source Heat Pump Efficiency Gains from Low Ambient Temperature Operation Using Supplemental Electric Heating: Thermal Storage Supplemental Heating Systems |url=http://mn.gov/commerce/energy/images/CIP-AirSource-Pump-Report.pdf |publisher=Minnesota Division of Energy Resources; Minnesota Department of Commerce |access-date=15 October 2014 |page=9 |date=2011 |url-status=dead |archive-url=https://web.archive.org/web/20140611123108/http://mn.gov/commerce/energy/images/CIP-AirSource-Pump-Report.pdf |archive-date=11 June 2014 }}</ref> [[storage heater]]s 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 ===
Good [[home insulation]] is important.<ref name=":10" /> {{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.<ref name=":0">{{Cite news |title=Heat pumps show how hard decarbonisation will be |newspaper=The Economist |url=https://www.economist.com/leaders/2023/09/06/heat-pumps-show-how-hard-decarbonisation-will-be |access-date=2023-09-14 |issn=0013-0613}}</ref><ref name=":4">{{cite web| title=Heat pumps| author=Harris, B.| url=https://researchbriefings.files.parliament.uk/documents/POST-PN-0699/POST-PN-0699.pdf| publisher=[[UK Parliament ]]| date=14 July 2023| access-date=20 January 2024}}</ref> If running costs are important choosing the right size is important because an ASHP which is too large will be more expensive to run.<ref>{{Cite web |title=Here's How to Pick the Right Size Heat Pump for Your Home |url=https://www.cnet.com/home/energy-and-utilities/heres-how-to-pick-the-right-size-heat-pump-for-your-home/ |access-date=2023-09-18 |website=CNET |language=en}}</ref>

It can be more complicated to retrofit conventional heating systems that use radiators/[[Radiant heating and cooling|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.<ref>{{Cite web |title=Do Heat Pumps Work with Radiators? {{!}} Heat Pump House |url=https://heatpumphouse.com/guides/do-heat-pumps-work-with-radiators/ |access-date=2024-02-19 |website=heatpumphouse.com |language=en-GB}}</ref>

Alternatively, a high temperature heat pump can be installed and existing heat emitters can be retained, however {{As of|2023|lc=y}} 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.<ref>{{Cite web |last=Jackman |first=Josh |date=2022-01-28 |title=High Temperature Heat Pumps {{!}} Are They Worth It? |url=https://www.theecoexperts.co.uk/heat-pumps/high-temperature-air-source-heat-pumps |access-date=2023-09-17 |website=The Eco Experts |language=en-GB}}</ref>

ASHP are claimed to be healthier than fossil-fuelled heating such as [[gas heater]]s by maintaining a more even temperature and avoiding harmful fumes risk.<ref name=":10">{{Cite news |date=2023-02-28 |title=Advice {{!}} I tried, and failed, to install a heat pump. Here's how to do it right. |url=https://www.washingtonpost.com/climate-environment/2023/02/28/how-to-get-heat-pump-home/ |access-date=2024-02-19 |newspaper=Washington Post |language=en}}</ref> By filtering the air and reducing humidity in hot humid summer climates, they are also said to reduce dust, [[allergen]]s, and [[mold]], which poses a health risk.<ref name=":11" />

===In cold climates===
[[File:Ecodan_outdoor_unit_in_the_snow.jpg|thumb|The outdoor unit of an air source heat pump operating in freezing conditions]]
Operation of normal ASHPs is generally not recommended below −10 °C.<ref name=":3" /> However, ASHPs designed specifically for very cold climates (in the US, these are certified under Energy Star<ref name=":2">{{Cite web |date=2022-08-02 |title=Can Heat Pumps Actually Work in Cold Climates? |url=https://www.consumerreports.org/heat-pumps/can-heat-pumps-actually-work-in-cold-climates-a4929629430/ |access-date=2023-09-15 |website=Consumer Reports |language=en-US}}</ref>) can extract useful heat from ambient air as cold as {{convert|-30|°C|°F|abbr=on}} but electric resistance heating may be more efficient below −25 °C.<ref name=":3">{{Cite web |title=Cold climate air source heat pumps |url=https://www.hydro.mb.ca/your_home/heating_and_cooling/cold_climate_air_source_heat_pumps/ |access-date=2023-09-15 |website=www.hydro.mb.ca |language=en}}</ref> This is made possible by the use of variable-speed compressors, powered by inverters.<ref name=":2" /> Although air source heat pumps are less efficient than well-installed [[ground source heat pump]]s (GSHPs) in cold conditions, air source heat pumps have lower initial costs and may be the most economic or practical choice.<ref>{{cite web|title=Are Air Source Heat Pumps A Threat To Geothermal Heat Pump Suppliers?|url=https://www.forbes.com/sites/tomkonrad/2014/01/15/are-air-source-heat-pumps-a-threat-to-geothermal-heat-pump-suppliers/|work=Forbes|access-date=15 October 2014}}</ref> 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.<ref name=":8">{{Cite web |title=Hybrid heat pumps |url=https://energysavingtrust.org.uk/advice/hybrid-heat-pumps/ |access-date=2023-09-30 |website=Energy Saving Trust |language=en-GB}}</ref> Alternatively multiple heat pumps or a high temperature heat pump may be considered.<ref name=":8" />

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;<ref>{{cite web |date=February 2018 |title=How to defrost a heat pump in winter |url=https://www.evergreenenergy.co.uk/heat-pumps/how-to-defrost-a-heat-pump-in-winter/ |access-date=14 September 2021 |work=Evergreen Energy}}</ref> for air-to-air systems, heat is either taken from the air in the building or using an electrical heater.<ref>{{cite web |title=Defrost Cycle of a Heat Pump |url=https://www.nachi.org/defrost-cycle-heat-pump.htm |access-date=14 September 2021 |work=International Association of Home Inspectors}}</ref> 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),<ref>{{Cite journal |last1=Olabi |first1=Abdul-Ghani |last2=Mahmoud |first2=Montaser |last3=Obaideen |first3=Khaled |last4=Sayed |first4=Enas Taha |last5=Ramadan |first5=Mohamad |last6=Abdelkareem |first6=Mohammad Ali |date=2023-06-01 |title=Ground source heat pumps: Recent progress, applications, challenges, barriers, and role in achieving sustainable development goals based on bibliometric analysis |url=http://dx.doi.org/10.1016/j.tsep.2023.101851 |journal=Thermal Science and Engineering Progress |volume=41 |pages=101851 |doi=10.1016/j.tsep.2023.101851 |bibcode=2023TSEP...4101851O |issn=2451-9049|url-access=subscription |doi-access=free }}</ref> offer higher efficiency but are expensive to install due to labor and material costs.<ref name=":12">{{Cite journal |last1=Mattinen |first1=Maija K. |last2=Nissinen |first2=Ari |last3=Hyysalo |first3=Sampsa |last4=Juntunen |first4=Jouni K. |date=2014-07-27 |title=Energy Use and Greenhouse Gas Emissions of Air-Source Heat Pump and Innovative Ground-Source Air Heat Pump in a Cold Climate |url=http://dx.doi.org/10.1111/jiec.12166 |journal=Journal of Industrial Ecology |volume=19 |issue=1 |pages=61–70 |doi=10.1111/jiec.12166 |issn=1088-1980|url-access=subscription }}</ref> A ground source air heat pump (GSAHP)—or water-to-refrigerant type GSHPs <ref name=":13">{{Cite journal |last1=Kim |first1=Euiyoung |last2=Lee |first2=Jaekeun |last3=Jeong |first3=Youngman |last4=Hwang |first4=Yujin |last5=Lee |first5=Sangheon |last6=Park |first6=Naehyun |date=2012-07-01 |title=Performance evaluation under the actual operating condition of a vertical ground source heat pump system in a school building |url=https://linkinghub.elsevier.com/retrieve/pii/S0378778812000813 |journal=Energy and Buildings |volume=50 |pages=1–6 |doi=10.1016/j.enbuild.2012.02.006 |bibcode=2012EneBu..50....1K |issn=0378-7788|url-access=subscription }}</ref>—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.<ref>{{Cite journal |last1=Koopman |first1=Tim |last2=Zhu |first2=Tingting |last3=Rohlfs |first3=Wilko |date=2024-02-29 |title=Performance evaluation of air-source heat pump based on a pressure drop embedded model |journal=Heliyon |volume=10 |issue=4 |pages=e24634 |doi=10.1016/j.heliyon.2024.e24634 |doi-access=free |issn=2405-8440 |pmc=10877192 |pmid=38380015|bibcode=2024Heliy..1024634K }}</ref> 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,<ref name=":13" /> 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,<ref>{{Cite journal |last1=Greening |first1=Benjamin |last2=Azapagic |first2=Adisa |date=2012-03-01 |title=Domestic heat pumps: Life cycle environmental impacts and potential implications for the UK |url=https://linkinghub.elsevier.com/retrieve/pii/S0360544212000333 |journal=Energy |series=Sustainable Energy and Environmental Protection 2010 |volume=39 |issue=1 |pages=205–217 |doi=10.1016/j.energy.2012.01.028 |bibcode=2012Ene....39..205G |issn=0360-5442|url-access=subscription }}</ref> 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,<ref name=":12" /> 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===
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.<ref>{{Cite web |title=HEAT PUMPS & NOISE: A NEIGHBOURLY INSTALLATION GUIDE |url=https://vancouver.ca/files/cov/heat-pump-noise-guide.pdf}}</ref>

In the United States, the allowed night-time noise level is 45 [[A-weighting|A-weighted decibels (dBA)]].<ref>{{cite web|url=http://ehp.niehs.nih.gov/1307272/|title=Monica S. Hammer, Tracy K. Swinburn, and Richard L. Neitzel "Environmental Noise Pollution in the United States: Developing an Effective Public Health Response" Environmental Health Perspectives V122,I2,2014|access-date=25 January 2016|archive-url=https://web.archive.org/web/20160702095821/http://ehp.niehs.nih.gov/1307272/|archive-date=2 July 2016|url-status=dead}}</ref> In the UK the limit is set at 42 dB measured from the nearest neighbour<ref>{{Cite web |last= |date=2022-04-11 |title=How Noisy Are Heat Pumps? |url=https://heat-pumps.org.uk/how-noisy-are-heat-pumps/ |access-date=2023-09-14 |website=Heat Pumps UK |language=en-GB}}</ref> according to the MCS 020 standard<ref>{{Cite web |title=Microgeneration Installation Standard MCS 020 |url=https://mcscertified.com/wp-content/uploads/2021/10/MCS-020.pdf |access-date=17 March 2024 |website=MCS}}</ref> or equivalent.<ref>{{Cite web |title=Noise and Neighbours: The rules regarding noise from heat pump outdoor units - MCS 020 or equivalent |url=https://check-mark.co.uk/info/plan/planning-permission/noise-neighbours/ |access-date=17 March 2024 |website=Checkmark}}</ref> In Germany the limit in residential areas is 35, which is usually measured by [[European Committee for Standardization|European Standard]] EN 12102.<ref>{{Cite web |title=Petition No 0922/2020 by F.B. (German) on low-frequency noise limit values |url=https://www.europarl.europa.eu/doceo/document/PETI-CM-703148_EN.pdf}}</ref>

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.<ref>{{cite web|url=http://www.hiil.org/bestpractices/How%20to%20determine%20acceptable%20levels%20of%20noise%20nuisance%20%28France%29|title=Hiil innovating Justice "How to determine acceptable levels of noise nuisance (France)|access-date=25 January 2016|archive-url=https://web.archive.org/web/20170212105148/http://www.hiil.org/bestpractices/How%20to%20determine%20acceptable%20levels%20of%20noise%20nuisance%20%28France%29|archive-date=12 February 2017|url-status=dead}}</ref><ref>{{cite web|url=http://legifrance.gouv.fr/affichCodeArticle.do?cidTexte=LEGITEXT000006072665&idArticle=LEGIARTI000006910540&dateTexte=&categorieLien=cid |title=Code de la santé publique – Article R1334-33 (in French)|access-date=8 February 2016}}</ref> 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 {{convert|-3|C}} to {{convert|10|C}}, the COP for many machines is fairly stable. Approximately TheoreticalMaxCOP = (desiredIndoorTempC + 273) ÷ (desiredIndoorTempC - outsideTempC).{{Citation needed|date=February 2024}}<ref>{{Cite web |title=Is there some theoretical maximum coefficient of performance (COP) for heat pumps and chillers? |url=https://physics.stackexchange.com/questions/350074/is-there-some-theoretical-maximum-coefficient-of-performance-cop-for-heat-pump |access-date=2024-02-19 |website=Physics Stack Exchange |language=en}}</ref>{{Better source needed|reason=The current source is insufficiently reliable ([[WP:NOTRS]]).|date=February 2024}}

In mild weather with an outside temperature of {{convert|10|C}}, the COP of efficient air source heat pumps ranges from 4 to 6.<ref>{{cite web |title=Wärmepumpen mit Prüf- / Effizienznachweis (heat pumps with efficiency validation) |url= https://www.bafa.de/SharedDocs/Downloads/DE/Energie/ee_waermepumpen_anlagenliste_bis_2020.html| access-date=20 February 2022 |publisher=BAFA (Federal Office for Economic Affairs and Export Control in Germany)}}</ref> However, on a cold winter day, it takes more work to move the same amount of heat indoors than on a mild day.<ref name=EH_1>{{cite web| title=Air Source Heat Pumps| url=http://www.icax.co.uk/Air_Source_Heat_Pumps.html| publisher=ICAX| access-date=20 January 2024}}</ref> 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 {{convert|-18|C}}.{{Citation needed|date=February 2024}}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 {{convert|-30|C}}.{{Citation needed|date=February 2024}} 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 unit]]s, 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.<ref name=":2" />

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.<ref name=":11">{{Cite web |date=2018-04-04 |title=Single Stage vs. Two Stage vs. Variable Speed For Heat Pump/Air Conditioner |url=https://www.pickhvac.com/heat-pump/basics/single-stage-vs-two-stage/ |access-date=2024-02-19 |website=How to Choose Best HVAC Systems |language=en-US}}</ref> Maintenance such as changing filters can improve performance by 10% to 25%.<ref>{{Cite web |title=Operating and Maintaining Your Heat Pump |url=https://www.energy.gov/energysaver/operating-and-maintaining-your-heat-pump |access-date=2024-02-19 |website=Energy.gov |language=en}}</ref>

==Refrigerant types==
{{excerpt|Heat pump#Refrigerant choice}}

==Impact on decarbonization and electricity supply==

Heat pumps are key to [[Decarbonising|decarbonizing]] home energy use by [[phasing out gas boilers]].<ref name=":4" /><ref name=":5" /> As of 2024 the IEA says that 500 million tonnes of CO2 emissions could be cut by 2030.<ref>{{Cite web |title=Heat Pumps - Energy System |url=https://www.iea.org/energy-system/buildings/heat-pumps |access-date=2024-02-19 |website=IEA |language=en-GB}}</ref>

As [[wind farm]]s 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 turbine]]s, and calmer days result in decreased heating load for most houses even if the air temperature is low.<ref>{{cite web|title=An Evaluation of Air Source Heat Pump Technology in Yukon|url=http://www.energy.gov.yk.ca/pdf/air_source_heat_pumps_final_may2013_v04.pdf|publisher=Government of Yukon's Energy Solution Centre and Yukon Energy, Mines and Resources|access-date=15 October 2014|date=31 May 2013}}</ref>

Heat pumps could help stabilize grids through [[demand response]].<ref>{{Cite web |title=The added value of heatpumps for grid stability via demand response |url=https://heatpumpingtechnologies.org/news/1/54881 |access-date=2023-09-15 |website=HPT - Heat Pumping Technologies |language=en}}</ref> 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.<ref>{{Cite web |title=How heat pumps can keep homes warm without frying the power grid |url=https://eng.ox.ac.uk/news/how-heat-pumps-can-keep-homes-warm-without-frying-the-power-grid |access-date=2023-09-15 |website=eng.ox.ac.uk |language=en-gb}}</ref> In some countries, such as Australia, integration of this thermal storage with [[Rooftop solar power|rooftop solar]] would also help.<ref>{{Cite journal |last1=Li |first1=Yuanyuan |last2=Rosengarten |first2=Gary |last3=Stanley |first3=Cameron |last4=Mojiri |first4=Ahmad |date=2022-12-10 |title=Electrification of residential heating, cooling and hot water: Load smoothing using onsite photovoltaics, heat pump and thermal batteries |url=https://www.sciencedirect.com/science/article/pii/S2352152X22018618 |journal=Journal of Energy Storage |volume=56 |pages=105873 |doi=10.1016/j.est.2022.105873 |bibcode=2022JEnSt..5605873L |s2cid=253858807 |issn=2352-152X|url-access=subscription }}</ref>

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."<ref>{{Cite journal |last1=Olympios |first1=Andreas V. |last2=Hoseinpoori |first2=Pooya |last3=Markides |first3=Christos N. |date=2024-02-14 |title=Toward optimal designs of domestic air-to-water heat pumps for a net-zero carbon energy system in the UK |journal=Cell Reports Sustainability |volume=1 |issue=2 |pages=100021 |doi=10.1016/j.crsus.2024.100021 |issn=2949-7906|doi-access=free }}</ref>

==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.<ref name=":7">{{Cite web |title=How Much Does Heat Pump Installation Cost? (2023 Guide) |url=https://www.homeinspector.org/consumers/hvac/heat-pump-installation-cost |access-date=2023-09-30 |website=ASHI |language=en-US}}</ref><ref name=WS_1>{{cite web| title=Will switching to a heat pump save you money? Here's how to find out| author=Chung, E.| url=https://www.cbc.ca/news/science/heat-pump-cost-savings-1.6975426| publisher=CBC/Radio-Canada| date=26 September 2023| access-date=20 January 2024}}</ref> This is generally also true if cooling is not required, as the ASHP will likely last longer if only heating.<ref>{{Cite web |title=Heat pumps vs. AC — Why Upfront Costs Can Be Misleading |url=https://carbonswitch.com/heat-pump-vs-ac/ |access-date=2023-09-30 |website=carbonswitch.com}}</ref> 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.<ref name=":7" /> The IEA recommends governments subsidize the purchase price of residential heat pumps, and some countries do so.<ref name=":6" />

===Market===

In Norway,<ref>{{Cite journal |last1=Rosenow |first1=Jan |last2=Gibb |first2=Duncan |last3=Nowak |first3=Thomas |last4=Lowes |first4=Richard |date=October 2022 |title=Heating up the global heat pump market |journal=Nature Energy |language=en |volume=7 |issue=10 |pages=901–904 |doi=10.1038/s41560-022-01104-8 |s2cid=252141783 |issn=2058-7546|doi-access=free |bibcode=2022NatEn...7..901R }}</ref> Australia and New Zealand most heating is from heat pumps. In 2022 heat pumps outsold fossil fuel based heating in the US and France.<ref name=":6">{{Cite web |title=Global heat pump sales continue double-digit growth – Analysis |url=https://www.iea.org/commentaries/global-heat-pump-sales-continue-double-digit-growth |access-date=2023-09-15 |website=IEA |date=31 March 2023 |language=en-GB}}</ref> 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.<ref>{{Cite web |title=Source: Heat Pump Association |url=https://www.heatpumps.org.uk/resources/statistics/ |access-date=2024-06-14 |website=Heat Pumps |language=en-GB}}</ref> ASHPs can be helped to compete by increasing the [[price of fossil gas]] compared to [[Electricity pricing|that of electricity]] and using suitable [[flexible electricity pricing]].<ref name=":4" /> In the US air-to-air is the most common type.<ref>{{Cite news |date=2023-06-06 |title=A Heat Pump Might Be Right for Your Home. Here's Everything to Know. |language=en-US |work=The New York Times |url=https://www.nytimes.com/wirecutter/guides/heat-pump-buying-guide/ |access-date=2023-09-18 |issn=0362-4331}}</ref> {{As of|2023}} over 80% of heat pumps are air source.<ref name=":5" /> In 2023 the IEA appealed for better data - especially on air-to-air.<ref name=":6" />

==Maintenance and reliability==

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.<ref>{{Cite web |title=AIR SOURCE HEAT PUMP MAINTENANCE TIPS |url=https://www.nyserda.ny.gov/-/media/Project/Nyserda/Files/Publications/Fact-Sheets/CHC-CON-ASHP-main-fs-1-v1.pdf |website=New York State Clean Heat}}</ref><ref>{{Cite web |title=AIR SOURCE HEAT PUMPS. MAINTENANCE CONSIDERATIONS FOR HOMES |url=https://www.saveonenergy.ca/-/media/Files/SaveOnEnergy/training-and-support/hvac/ASHP-Regular-Maintenance-and-Timely-Repairs-Guide.pdf |website=Save on Energy. Independent Electricity System Operator.}}</ref><ref>{{Cite web |title=YOUR GUIDE TO Air-Source Heat Pumps |url=https://nextzero.org/wp-content/uploads/MassCEC_ASHP_GUIDE_V2-1.pdf |website=Clean Energy Lives Here. Massachusetts Clean Energy Center.}}</ref> 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.<ref>{{Cite web |title=Heating, Ventilation and Air Conditioning. What Kind of HVAC System You Have and How to Use It |url=https://www.midohioenergy.com/sites/midohioenergy/files/documents/energy_saving_cards/hvac_id_041614.pdf |website=Energy Saving Recipe. Southface Energy Institute.}}</ref><ref>{{Cite web |date=2019 |title=HEAT PUMP BEST PRACTICES. INSTALLATION GUIDE FOR EXISTING HOMES. |url=https://www.homeperformance.ca/wp-content/uploads/2019/12/ASHP_QI_Best_Practice_Guide_20191209.pdf |website=HOME PERFORMANCE STAKEHOLDER COUNCIL {{!}} HEAT PUMP BEST PRACTICES GUIDE.}}</ref>

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.<ref>{{Cite journal |last1=Milev |first1=George |last2=Al-Habaibeh |first2=Amin |last3=Fanshawe |first3=Simon |last4=Luke Siena |first4=Francesco |date=1 December 2023 |title=Investigating the effect of the defrost cycles of air-source heat pumps on their electricity demand in residential buildings |journal=Energy and Buildings |volume=300 |doi=10.1016/j.enbuild.2023.113656|doi-access=free |bibcode=2023EneBu.30013656M }}</ref> 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.<ref>{{Cite journal |last1=Jingdong |first1=Liu |last2=Wei |first2=Wang |last3=Yuying |first3=Sun |last4=Shiming |first4=Deng |date=April 2018 |title=Operating performances of a space heating ASHP unit with a foul outdoor coil following a prolonged cooling operation |url=https://www.sciencedirect.com/science/article/pii/S014070071830046X |journal=International Journal of Refrigeration |volume=88 |pages=614–625 |doi=10.1016/j.ijrefrig.2018.01.013|url-access=subscription }}</ref> 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.<ref>{{Cite web |last1=Nawaz |first1=Kashif |last2=Elatar |first2=Ahmed |last3=Fricke |first3=Brian |date=March 31, 2018 |title=A Critical Literature Review of Defrost Technologies for Heat Pumps and Refrigeration Systems |url=https://info.ornl.gov/sites/publications/Files/Pub117976.pdf |publisher=Energy and Transportation Sciences Division. Oak Ridge National Laboratory. US DEPARTMENT OF ENERGY under contract DE-AC05-00OR22725}}</ref><ref>{{Cite web |title=HEAT PUMP 101: HOW TO TAKE CARE OF YOUR HEAT PUMP |url=https://www.nspower.ca/docs/default-source/pdf-to-upload/jc0006_ashley_heatpump101onepager-(3).pdf?sfvrsn=811939d7_0#:~:text=Keeping%20the%20filters%20and%20coils,and%20can%20be%20performed%20easily.&text=Keeping%20the%20outdoor%20unit%20clear,and%20debris%20is%20also%20important.&text=An%20annual%20maintenance%20check%20of,recommended%20by%20your%20installing%20contractor. |website=Nova Scotia Power}}</ref> 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.<ref>{{Cite web |last=Johnson |first=R.K. |date=September 2013 |title=Measured Performance of a Low Temperature Air Source Heat Pump |url=https://www.nrel.gov/docs/fy13osti/56393.pdf |publisher=The National Renewable Energy Laboratory. On behalf of the U.S. Department of Energy's Building America Program. Office of Energy Efficiency and Renewable Energy. NREL Contract No. DE-AC36-08GO28308}}</ref> These defrost cycles can cause pressure fluctuations in the refrigerant lines that lead to refrigerant leaks and diminish performance.<ref>{{Cite journal |last1=Jianhui |first1=Niu |last2=Guoyuan |first2=Ma |last3=Shuxue |first3=Xu |date=20 January 2020 |title=Experimental study on the performance of air source heat pump system with multiple parallel outdoor units |journal=International Journal of Energy Research |volume=44 |issue=4 |pages=2819–2832 |doi=10.1002/er.5098|bibcode=2020IJER...44.2819J |doi-access=free }}</ref><ref>{{Cite journal |last1=Long |first1=Zhang |last2=Jiankai |first2=Dong |last3=Yiqiang |first3=Jiang |last4=Yang |first4=Yao |date=15 November 2014 |title=A novel defrosting method using heat energy dissipated by the compressor of an air source heat pump |url=https://www.sciencedirect.com/science/article/pii/S0306261914007211 |journal=Applied Energy |volume=133 |pages=101–111 |doi=10.1016/j.apenergy.2014.07.039|bibcode=2014ApEn..133..101L |url-access=subscription }}</ref>

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.<ref>{{Cite journal |last1=Higa |first1=Randall |last2=Horwitz |first2=Matt |last3=Buendia |first3=Jose |last4=McHugh |first4=Jon |last5=Haile |first5=James |last6=Heinemeier |first6=Kristin |date=2024 |title=Yeah, But It's a Dry Cold: Applicability of Cold Climate Heat Pumps in California |url=https://www.aceee.org/sites/default/files/proceedings/ssb24/pdfs/Applicability%20of%20Cold%20Climate%20Heat%20Pumps%20in%20California.pdf |journal=Summer Study on Energy Efficiency in Buildings |publisher=ACEE}}</ref><ref>{{Cite journal |last1=Mendon |first1=Vrushali |last2=Keene |first2=Kevin |last3=Rosenberg |first3=Sam |last4=Rotondo |first4=Julia |last5=Brambley |first5=Dr. Michael |last6=Young |first6=Jim |last7=Kazmi |first7=Ali Akber |last8=Delgoshaei |first8=Dr. Payam |date=2024 |title=Rising Up to the Challenge: Cold Climate Heat Pumps in the Field |url=https://www.aceee.org/sites/default/files/proceedings/ssb24/pdfs/Rising%20up%20to%20the%20Challenge%20-%20Cold%20Climate%20Heat%20Pumps%20in%20the%20Field.pdf |journal=Summer Study on Energy Efficiency in Buildings |publisher=ACEE}}</ref>

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).<ref>{{Cite web |title=Which Heat Pump is the Best for Reliability & Efficiency 2023? |url=https://www.ecohome.net/guides/3699/heat-pump-buying-guide-which-are-the-best-heat-pumps/ |access-date=2023-09-18 |website=www.ecohome.net |language=en}}</ref> However others say that boilers require less maintenance than ASHPs.<ref>{{Cite web |title=Heat Pump vs. Furnace: Which Heat Source Is Right for Your Home? |url=https://www.cnet.com/home/energy-and-utilities/heat-pump-vs-furnace-which-heat-source-is-right-for-your-home/ |access-date=2023-09-18 |website=CNET |language=en}}</ref> 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".<ref>{{Cite web |date=2023-04-07 |title=Most and Least Reliable Heat Pump Brands |url=https://www.consumerreports.org/appliances/heat-pumps/most-and-least-reliable-heat-pumps-a2741062924/ |access-date=2023-09-18 |website=Consumer Reports |language=en-US}}</ref>

== History ==
Modern chemical refrigeration techniques developed after the proposal of the [[Carnot cycle]] in 1824. [[Jacob Perkins]] invented an [[ice]]-making machine that used [[diethyl ether|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.<ref>[https://www.chuden.co.jp/resource/seicho_kaihatsu/kaihatsu/kai_library/news/news_2010/news_100_02_N09627.pdf Fusanosuke Kuhara used CO2 gas compressed Cryocooler in 1917.] {{Webarchive|url=https://web.archive.org/web/20220710110245/https://www.chuden.co.jp/resource/seicho_kaihatsu/kaihatsu/kai_library/news/news_2010/news_100_02_N09627.pdf
|date=2022-07-10}} 2nd Page numbered 28, right side line 3-6. [[Fusanosuke Kuhara]] attached CO2 gas compressed [[Cryocooler]] with approx. 6,400[[Calorie|kcal]]/[[Hour|h]] in his home in 1917. In Japanese: 冷凍機が冷房用として使用されたのは1917年久原房之助が神戸の私邸に約6400kcal/h炭酸ガス圧縮機を取り付け、室内を冷やしたのが最初といわれています。</ref>

In 1930 [[Thomas Midgley Jr.]] discovered [[dichlorodifluoromethane]], a chlorinated [[fluorocarbon]] ([[Chlorofluorocarbon|CFC]]) known as [[freon]]. CFCs rapidly replaced traditional refrigerant substances, including CO2 (which proved hard to compress for domestic use<ref name=discoverCO2>[http://www.shecco.com/about/history.php The rediscovery of {{CO2}}] {{webarchive|url=https://web.archive.org/web/20071007040239/http://www.shecco.com/about/history.php |date=2007-10-07 }} SHECCO</ref>), 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 ([[Hydrofluorocarbon|HFCs]]) and hydrochlorofluorocarbons ([[Hydrochlorofluorocarbons#Development of alternatives for CFCs|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 (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.<ref name=discoverCO2 />

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.<ref>[https://ci.nii.ac.jp/ncid/BA54218170 Natural Working Fluids '98, IIR - Gustav Lorentzen Conference]: [[CiNii]] {{Webarchive|url=https://web.archive.org/web/20240227080144/https://ci.nii.ac.jp/ncid/BA54218170|date=2024-02-27}}</ref> 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 Prefecture|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.<ref>{{Cite web |last=Kuwajima |first=Hiroto |date=2023-09-08 |title=Panasonic Amps Up Production of Eco Cute Water Heaters {{!}} JAPAN Forward |url=https://japan-forward.com/panasonic-amps-up-production-of-eco-cute-water-heaters/ |access-date=2024-05-28 |website=japan-forward.com |language=en-US}}</ref>

==Manufacturing==
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.<ref>{{Cite news |title=Who Wants to Become a Heat-Pump Billionaire? |newspaper=[[The Washington Post]] |url=https://www.washingtonpost.com/business/energy/2023/05/17/energy-transition-who-wants-to-become-a-heat-pump-billionaire/393c2a9e-f46a-11ed-918d-012572d64930_story.html}}</ref>

== See also ==
* [[:Category:Heating, ventilation, and air conditioning companies]]
* [[Transcritical cycle]]

==References==
{{reflist|25em}}

===Sources===
====IPCC reports====
*{{Cite book |ref= {{harvid|IPCC AR6 WG1|2021}}
|author= IPCC |author-link= IPCC
|year= 2021
|title= Climate Change 2021: The Physical Science Basis
|series= Contribution of Working Group I to the [[IPCC Sixth Assessment Report|Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change
|display-editors= 4
|editor1-first= V. |editor1-last= Masson-Delmotte
|editor2-first= P. |editor2-last= Zhai
|editor3-first= A. |editor3-last= Pirani
|editor4-first= S. L. |editor4-last= Connors
|editor5-first= C. |editor5-last= Péan
|editor6-first= S. |editor6-last= Berger
|editor7-first= N. |editor7-last= Caud
|editor8-first= Y. |editor8-last= Chen
|editor9-first= L. |editor9-last= Goldfarb
|editor10-first= M. I. |editor10-last= Gomis
|publisher= Cambridge University Press (In Press)
|place=
|isbn=
|url= https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf
}}
**{{Cite book
|ref= {{harvid|IPCC AR6 WG1 Ch7|2021}}
|chapter=Chapter 7: The Earth's energy budget, climate feedbacks, and climate sensitivity Supplementary Material
|chapter-url= https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07_SM.pdf
|last1=Forster |first1=P.
|last2=Storelvmo |first2=T.
|last3=Armour|first3=K.
|last4=Collins|first4=W.
|title= {{Harvnb|IPCC AR6 WG1|2021}}
|year=2021
}}

{{HVAC}}

{{DEFAULTSORT:Air Source Heat Pumps}}
[[Category:Consumer electronics]]
[[Category:Heating, ventilation, and air conditioning]]
[[Category:Energy conservation]]
[[Category:Building engineering]]
[[Category:Construction]]
[[Category:Energy economics]]
[[Category:Environmental design]]
[[Category:Heating]]
[[Category:Heat pumps]]
[[Category:Sustainable technologies]]

wiki143 - User contributions [en]

Air source heat pump