Air conditioning#Automobile air conditioners

Air conditioning dates back to prehistory.{{Cite book |last=Laub |first=Julian M. |url=https://books.google.com/books?id=8c9SAAAAMAAJ&q=Air+conditioning+dates+back+to+prehistory. |title=Air Conditioning & Heating Practice |date=1963 |publisher=Holt, Rinehart and Winston |isbn=978-0-03-011225-6 |pages=367 |language=en}} 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.{{Cite web |date=2000-06-24 |title=Air-conditioning found at 'oldest city in the world' |url= https://www.independent.co.uk/news/world/middle-east/airconditioning-found-at-oldest-city-in-the-world-5370557.html |access-date=2023-12-09 |work=The Independent |archive-date=2023-12-08 |archive-url=https://web.archive.org/web/20231208053155/https://www.independent.co.uk/news/world/middle-east/airconditioning-found-at-oldest-city-in-the-world-5370557.html |url-status=live}} Ancient Egyptian buildings also used a wide variety of passive air-conditioning techniques. These became widespread from the Iberian Peninsula through North Africa, the Middle East, and Northern India.

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.{{cite journal |last1=Ford |first1=Brian |title=Passive downdraught evaporative cooling: principles and practice |doi-access=free |journal=Architectural Research Quarterly |date=September 2001 |volume=5 |issue=3 |pages=271–280 |doi=10.1017/S1359135501001312}}

File:ACFujitsu2.jpg

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.{{cite web |date=2014-10-17 |title=Heating, Ventilation and Air-Conditioning Systems, Part of Indoor Air Quality Design Tools for Schools |url=https://www.epa.gov/iaq-schools/heating-ventilation-and-air-conditioning-systems-part-indoor-air-quality-design-tools |access-date=2022-07-05 |website=US EPA |archive-date=2022-07-05 |archive-url=https://web.archive.org/web/20220705191247/https://www.epa.gov/iaq-schools/heating-ventilation-and-air-conditioning-systems-part-indoor-air-quality-design-tools |url-status=live}}

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.{{cite book|last1=Shachtman |first1=Tom|url=https://books.google.com/books?id=B8PvAAAAMAAJ |title=Absolute zero and the conquest of cold |date=1999 |publisher=Houghton Mifflin Harcourt |isbn=978-0395938881 |location=Boston |chapter=Winter in Summer|oclc=421754998 |access-date=May 12, 2021 |chapter-url=https://archive.nytimes.com/www.nytimes.com/books/first/s/shachtman-zero.html |archive-date=May 13, 2021 |archive-url=https://web.archive.org/web/20210513150237/https://books.google.com/books?id=B8PvAAAAMAAJ |url-status=live}}{{cite book|last=Porta |first=Giambattista Della |url=https://tile.loc.gov/storage-services/service/rbc/rbc0001/2009/2009pre23451/2009pre23451.pdf |title=Magiae naturalis|year=1584 |location=London |lccn=09023451 |quote=In our method I shall observe what our ancestors have said; then I shall show by my own experience, whether they be true or false|access-date=May 12, 2021 |archive-date=May 13, 2021|archive-url=https://web.archive.org/web/20210513150230/https://tile.loc.gov/storage-services/service/rbc/rbc0001/2009/2009pre23451/2009pre23451.pdf |url-status=live}}{{cite journal|last=Beck |first=Leonard D. |date=October 1974 |title=Things Magical in the collections of the Rare Book and Special Collections Division|url=https://www.loc.gov/rr/rarebook/pdf/thingsmagical.pdf |journal=Library of Congress Quarterly Journal |volume=31 |pages=208–234 |access-date=May 12, 2021 |archive-date=March 24, 2021 |archive-url=https://web.archive.org/web/20210324072531/https://www.loc.gov/rr/rarebook/pdf/thingsmagical.pdf |url-status=live}} 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.{{cite book|last=Laszlo |first=Pierre |url=https://archive.org/details/saltgrainoflife00laszrich |title=Salt: Grain of Life |publisher=Columbia University Press |year=2001 |isbn=978-0231121989 |page=[https://archive.org/details/saltgrainoflife00laszrich/page/117 117] |oclc=785781471 |quote=Cornelius Drebbel air conditioning. |url-access=registration}} 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."

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 {{convert|-14|C|0}} while the ambient temperature was {{convert|18|C|0}}. Franklin noted that soon after they passed the freezing point of water {{convert|0|C|0}}, a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about {{convert|1/4|in|mm|abbr=on|0|order=flip}} thick when they stopped the experiment upon reaching {{convert|-14|C|0}}. Franklin concluded: "From this experiment, one may see the possibility of freezing a man to death on a warm summer's day."{{cite letter|subject=Cooling by Evaporation |first=Benjamin |last=Franklin |recipient=John Lining |date=June 17, 1758|url=http://www.waughfamily.ca/Montgomery/benfranklin1758.htm |access-date=May 12, 2021|title=The Montgomery Family: An historical and photographic perspective |archive-date=February 25, 2021 |archive-url=https://web.archive.org/web/20210225102815/http://www.waughfamily.ca/Montgomery/benfranklin1758.htm |url-status=live}}

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.{{cite magazine|last=Green |first=Amanda |date=January 1, 2015 |title=The Cool History of the Air Conditioner |url=https://www.popularmechanics.com/home/a7951/history-of-air-conditioning/ |magazine=Popular Mechanics |access-date=May 12, 2021 |archive-date=April 10, 2021 |archive-url=https://web.archive.org/web/20210410032417/https://www.popularmechanics.com/home/a7951/history-of-air-conditioning/ |url-status=live}} 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.{{cite encyclopedia |title=John Gorrie |encyclopedia=Encyclopædia Britannica |url=https://www.britannica.com/biography/John-Gorrie |access-date=May 12, 2021 |date=September 29, 2020 |archive-date=March 13, 2021 |archive-url=https://web.archive.org/web/20210313143440/https://www.britannica.com/biography/John-Gorrie |url-status=live}} He envisioned centralized air conditioning that could cool entire cities. Gorrie was granted a patent in 1851,Gorrie, John [https://ppubs.uspto.gov/dirsearch-public/print/downloadPdf/0008080 "Improved process for the artificial production of ice"] U.S. Patent no. 8080 (Issued: May 6, 1851). but following the death of his main backer, he was not able to realize his invention.{{cite book|first=E. Lynne |last=Wright |title=It Happened in Florida: Remarkable Events That Shaped History|url=https://books.google.com/books?id=RqKU1t8Js4cC&pg=PA13 |year=2009 |publisher=Rowman & Littlefield |isbn=978-0762761692 |pages=13–}} 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.{{Australian Dictionary of Biography |first=L. G.|last=Bruce-Wallace|title=Harrison, James (1816–1893) |id2=harrison-james-2165|accessdate=May 12, 2021 |year=1966 |volume=1}} 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.

File:Willis Carrier 1915.jpg, who is credited with building the first modern electrical air conditioning unit]]

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.{{cite web|last=Palermo |first=Elizabeth |date=May 1, 2014 |title=Who Invented Air Conditioning? |url=https://www.livescience.com/45268-who-invented-air-conditioning.html|url-status=live|access-date=May 12, 2021 |website=livescience.com |archive-date=January 16, 2021 |archive-url=https://web.archive.org/web/20210116122721/https://www.livescience.com/45268-who-invented-air-conditioning.html}}{{cite web|last=Varrasi|first=John|date=June 6, 2011 |title=Global Cooling: The History of Air Conditioning |url=https://www.asme.org/topics-resources/content/global-cooling-the-history-of-air-conditioning |url-status=live|access-date=May 12, 2021 |publisher=American Society of Mechanical Engineers |archive-date=March 8, 2021 |archive-url=https://web.archive.org/web/20210308223349/https://www.asme.org/topics-resources/content/global-cooling-the-history-of-air-conditioning}}{{cite journal|last=Simha |first=R. V.|date=February 2012 |title=Willis H Carrier |journal=Resonance |volume=17 |issue=2|pages=117–138 |doi=10.1007/s12045-012-0014-y| issn=0971-8044 |s2cid=116582893}}{{cite web|last1=Gulledge III |first1=Charles|last2=Knight|first2=Dennis|date=11 February 2016 |title=Heating, Ventilating, Air-Conditioning, And Refrigerating Engineering|url=https://www.wbdg.org/design-disciplines/hvac-refrigerating-engineering |url-status=live|access-date=May 12, 2021 |publisher=National Institute of Building Sciences |quote=Though he did not actually invent air-conditioning nor did he take the first documented scientific approach to applying it, Willis Carrier is credited with integrating the scientific method, engineering, and business of this developing technology and creating the industry we know today as air-conditioning.|archive-date=20 April 2021 |archive-url=https://web.archive.org/web/20210420233536/https://www.wbdg.org/design-disciplines/hvac-refrigerating-engineering}} In 1902, he installed his first air-conditioning system, in the Sackett-Wilhelms Lithographing & Publishing Company in Brooklyn, New York.{{cite web|date=|title=Willis Carrier – 1876–1902 |url=http://www.williscarrier.com/m/1876-1902.php |url-status=live |access-date=May 12, 2021 |work=Carrier Global |archive-date=February 27, 2021 |archive-url=https://web.archive.org/web/20210227041543/http://www.williscarrier.com/m/1876-1902.php}} 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.{{cite press release|url=https://www.corporate.carrier.com/news/news-articles/carrier_reports_first_quarter_2020_earnings.html |title=Carrier Reports First Quarter 2020 Earnings |date=May 8, 2020 |work=Carrier Global |access-date=May 12, 2021|archive-date=January 24, 2021 |archive-url=https://web.archive.org/web/20210124002914/https://www.corporate.carrier.com/news/news-articles/carrier_reports_first_quarter_2020_earnings.html |url-status=live}}{{cite press release|title=Carrier Becomes Independent, Publicly Traded Company, Begins Trading on New York Stock Exchange |work=Carrier Global |date=April 3, 2020 |url=https://www.corporate.carrier.com/news/news-articles/carrier_becomes_independent_publicly_traded_company__begins_trading_on_new_york_stock_exchange.html |access-date=May 12, 2021 |archive-date=February 25, 2021 |archive-url=https://web.archive.org/web/20210225083334/https://www.corporate.carrier.com/news/news-articles/carrier_becomes_independent_publicly_traded_company__begins_trading_on_new_york_stock_exchange.html |url-status=live}}

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.Cramer, Stuart W. [https://ppubs.uspto.gov/dirsearch-public/print/downloadPdf/0852823 "Humidifying and air conditioning apparatus"] U.S. Patent no. 852,823 (filed: April 18, 1906; issued: May 7, 1907).

• See also: Cramer, Stuart W. (1906) [https://babel.hathitrust.org/cgi/pt?id=nyp.33433066400650&view=1up&seq=220 "Recent development in air conditioning"] in: Proceedings of the Tenth Annual Convention of the American Cotton Manufacturers Association Held at Asheville, North Carolina May 16–17, 1906. Charlotte, North Carolina, USA: Queen City Publishing Co. pp. 182-211. 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.{{cite patent |country=US |number=US808897A |title=Apparatus for treating air |status=patent |pubdate=January 2, 1906 |fdate=September 16, 1904 |pridate=September 16, 1904 |gdate=January 2, 1906 |invent1=Carrier |inventor1-first=Willis H. |assign2=Buffalo Forge Company |url=https://patentimages.storage.googleapis.com/65/90/66/df3edc322cbd28/US808897.pdf}}{{cite web |url=https://patentimages.storage.googleapis.com/65/90/66/df3edc322cbd28/US808897.pdf |title=No. 808,897 Patented Jan. 2, 1906: H. W. Carrier: Apparatus for Treating Air |access-date=May 12, 2021 |archive-date=December 5, 2019 |archive-url=https://web.archive.org/web/20191205075346/https://patentimages.storage.googleapis.com/65/90/66/df3edc322cbd28/US808897.pdf |url-status=live}}

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. {{cvt|7x6x20|ft|m|1|order=flip|disp=semicolon}}) was never used, as the house remained uninhabited (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 ${{Inflation|US|10000|1932|r=-5|fmt=c}} to ${{Inflation|US|50000|1932|r=-5|fmt=c}} in {{Inflation/year|US}}.) A year later, the first air conditioning systems for cars were offered for sale.{{cite magazine|date=Nov 1933|title=First Air-Conditioned Auto|url=https://books.google.com/books?id=7CcDAAAAMBAJ&pg=PA30 |magazine=Popular Science |volume=123 |issue=5 |page=30|issn=0161-7370 |access-date=May 12, 2021 |archive-date=April 26, 2021 |archive-url=https://web.archive.org/web/20210426234603/https://books.google.com/books?id=7CcDAAAAMBAJ&pg=PA30|url-status=live}} Chrysler Motors introduced the first practical semi-portable air conditioning unit in 1935,{{cite magazine|date=June 1935|title=Room-size air conditioner fits under window sill |url=https://books.google.com/books?id=uN4DAAAAMBAJ&pg=PA885|magazine=Popular Mechanics |volume=63|issue=6 |page=885 |issn=0032-4558|access-date=May 12, 2021|archive-date=November 22, 2016 |archive-url=https://web.archive.org/web/20161122120919/https://books.google.com/books?id=uN4DAAAAMBAJ&pg=PA885 |url-status=live}} and Packard became the first automobile manufacturer to offer an air conditioning unit in its cars in 1939.{{cite web|title=Michigan Fast Facts and Trivia |url=https://www.50states.com/facts/michigan.htm |url-status=live|access-date=May 12, 2021 |website=50states.com |archive-date=June 18, 2017 |archive-url=https://web.archive.org/web/20170618225101/http://www.50states.com/facts/michigan.htm}}

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.{{cite patent |country=US |number=US2433960A |title=Air conditioning apparatus |status=patent |pubdate=January 6, 1948 |fdate=February 13, 1945 |pridate=February 13, 1945 |gdate=January 6, 1948 |invent1=Sherman |inventor1-first=Robert S. |url=https://patentimages.storage.googleapis.com/16/28/e4/3eb554d15411f0/US2433960.pdf}} The first inverter air conditioners were released in 1980–1981.{{cite web |url=http://www.ieee-jp.org/japancouncil/jchc/adm/milestone/39Inverter_Air_Conditioners.pdf |title=IEEE milestones (39) Inverter Air Conditioners, 1980–1981 |date=March 2021 |access-date=February 9, 2024 |archive-date=2024-01-21 |archive-url=https://web.archive.org/web/20240121175703/http://www.ieee-jp.org/japancouncil/jchc/adm/milestone/39Inverter_Air_Conditioners.pdf |url-status=live}}{{Cite web |url=https://ethw.org/w/images/8/8b/IEEE_Milestone_Leaflet_ENG.PDF |title=Inverter Air Conditioners, 1980–1981 IEEE Milestone Celebration Ceremony |date=March 16, 2021 |access-date=February 9, 2024 |archive-date=2024-01-21 |archive-url= https://web.archive.org/web/20240121175740/https://ethw.org/w/images/8/8b/IEEE_Milestone_Leaflet_ENG.PDF |url-status=live}}

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.{{Cite web |last=Seale |first=Avrel |date=2023-08-07 |title=Texas alumnus and his alma mater central to air-conditioned homes |url=https://news.utexas.edu/2023/08/07/texas-alumnus-and-his-alma-mater-central-to-air-conditioned-homes/ |access-date=2024-11-13 |website=UT News |language=en-US}}{{Cite web |title=Air Conditioned Village |url=https://www.atlasobscura.com/places/austin-air-conditioned-village |access-date=2024-11-13 |website=Atlas Obscura |language=en}}

Air conditioner adoption tends to increase above around $10,000 annual household income in warmer areas.{{cite journal |last1=Davis |first1=Lucas |last2=Gertler |first2=Paul |last3=Jarvis |first3=Stephen |last4=Wolfram |first4=Catherine |title=Air conditioning and global inequality |journal=Global Environmental Change |date=July 2021 |volume=69 |pages=102299 |doi=10.1016/j.gloenvcha.2021.102299 |bibcode=2021GEC....6902299D}} Global GDP growth explains around 85% of increased air condition adoption by 2050, while the remaining 15% can be explained by climate change.

As of 2016 an estimated 1.6 billion air conditioning units were used worldwide, with over half of them in China and USA, and a total cooling capacity of 11,675 gigawatts.{{cite news|last=Pierre-Louis |first=Kendra |date=May 15, 2018 |title=The World Wants Air-Conditioning. That Could Warm the World. |newspaper=The New York Times |url=https://www.nytimes.com/2018/05/15/climate/air-conditioning.html |url-access=limited|access-date=May 12, 2021 |archive-date=February 16, 2021 |archive-url=https://web.archive.org/web/20210216124538/https://www.nytimes.com/2018/05/15/climate/air-conditioning.html |url-status=live}} 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. Between 1995 and 2004, the proportion of urban households in China with air conditioners increased from 8% to 70%.{{cite news|last=Carroll |first=Rory |date=October 26, 2015|title=How America became addicted to air conditioning |newspaper=The Guardian |location=Los Angeles |url=https://www.theguardian.com/environment/2015/oct/26/how-america-became-addicted-to-air-conditioning |access-date=May 12, 2021|archive-date=March 13, 2021 |archive-url=https://web.archive.org/web/20210313201201/https://www.theguardian.com/environment/2015/oct/26/how-america-became-addicted-to-air-conditioning|url-status=live}} As of 2015, nearly 100 million homes, or about 87% of US households, had air conditioning systems.{{cite web|last=Lester|first=Paul|date=July 20, 2015 |title=History of Air Conditioning |url=https://www.energy.gov/articles/history-air-conditioning|url-status=live|access-date=May 12, 2021 |publisher=United States Department of Energy |archive-date=June 5, 2020|archive-url=https://web.archive.org/web/20200605141658/https://www.energy.gov/articles/history-air-conditioning}} 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).{{cite report|url=https://www.census.gov/construction/chars/highlights.html |title=Characteristics of New Housing |last1=Cornish |first1=Cheryl |last2=Cooper |first2=Stephen |date=|publisher=United States Census Bureau |last3=Jenkins |first3=Salima |access-date=May 12, 2021 |archive-date=April 11, 2021 |archive-url=https://web.archive.org/web/20210411005738/https://www.census.gov/construction/chars/highlights.html|url-status=live}}{{cite web|date=March 3, 2021 |title=Central Air Conditioning Buying Guide |url=https://www.consumerreports.org/cro/central-air-conditioning/buying-guide/index.htm |url-status=live |access-date=May 12, 2021 |work=Consumer Reports |archive-date=May 9, 2021 |archive-url=https://web.archive.org/web/20210509034728/https://www.consumerreports.org/cro/central-air-conditioning/buying-guide/index.htm}}

{{main|Vapor-compression refrigeration}}

File:Heatpump.svg, 2) expansion valve, 3) evaporator coil, 4) compressor]]

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.{{Cite book |last=Petchers |first=Neil |url=https://books.google.com/books?id=eYJIv2wZtXEC&dq=Cooling+in+traditional+air+conditioner+systems+is+accomplished+using+the+vapor-compression+cycle&pg=PA737 |title=Combined Heating, Cooling & Power Handbook: Technologies & Applications : an Integrated Approach to Energy Resource Optimization |date=2003 |publisher=The Fairmont Press |isbn=978-0-88173-433-1 |pages=737}}{{Cite book |last=Krarti |first=Moncef |url=https://books.google.com/books?id=6AoHEAAAQBAJ&dq=Cooling+in+traditional+air+conditioner+systems+is+accomplished+using+the+vapor-compression+cycle&pg=PA370 |title=Energy Audit of Building Systems: An Engineering Approach, Third Edition |date=2020-12-01 |publisher=CRC Press |isbn=978-1-000-25967-4 |pages=370 |language=en}} 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.{{cite web |title=What is a Reversing Valve |url=https://www.samsung.com/in/support/home-appliances/what-is-a-reversing-valve/ |url-status=live |archive-url=https://web.archive.org/web/20190222153359/https://www.samsung.com/in/support/home-appliances/what-is-a-reversing-valve/ |archive-date=February 22, 2019 |access-date=May 12, 2021 |work=Samsung India}}

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.{{cite web |title=Humidity and Comfort |url=http://www.dristeem-media.com/literature/Web_HumidityAndComfort.pdf |url-status=dead |archive-url=https://web.archive.org/web/20180516235855/http://www.dristeem-media.com/literature/Web_HumidityAndComfort.pdf |archive-date=May 16, 2018 |access-date=May 12, 2021 |work=DriSteem}}

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{{cite web |last=Perryman |first=Oliver |date=April 19, 2021 |title=Dehumidifier vs Air Conditioning |url=https://dehumidifiercritic.com/dehumidifier-vs-air-conditioning/ |url-status=live |archive-url=https://web.archive.org/web/20210513150241/https://dehumidifiercritic.com/dehumidifier-vs-air-conditioning/ |archive-date=May 13, 2021 |access-date=May 12, 2021 |work=Dehumidifier Critic}} and dehumidified before passing over the condenser coil, where it is warmed again before it is released back into the room.{{Citation needed|date=May 2021}}

Free cooling can sometimes be selected when the external air is cooler than the internal air. Therefore, 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.{{cite web |last=Snijders |first=Aart L. |date=July 30, 2008 |title=Aquifer Thermal Energy Storage (ATES) Technology Development and Major Applications in Europe |url=http://trca.on.ca/dotAsset/16551.pdf |url-status=live |archive-url=https://web.archive.org/web/20210308043800/http://trca.on.ca/dotAsset/16551.pdf |archive-date=March 8, 2021 |access-date=May 12, 2021 |work=Toronto and Region Conservation Authority |publisher=IFTech International |location=Arnhem}}

{{Main|Heat pump}}

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.{{cite web |title=Cold Climate Air Source Heat Pump |url=https://mn.gov/commerce-stat/pdfs/card-air-source-heat-pump.pdf |access-date=2022-03-29 |work=Minnesota Department of Commerce, Division of Energy Resources |archive-date=2022-01-02 |archive-url=https://web.archive.org/web/20220102201057/https://mn.gov/commerce-stat/pdfs/card-air-source-heat-pump.pdf |url-status=live}} 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 {{Cvt|-14|F|C}}.{{cite web |title=Even in Frigid Temperatures, Air-Source Heat Pumps Keep Homes Warm From Alaska Coast to U.S. Mass Market |url=https://www.nrel.gov/news/features/2021/even-in-frigid-temperatures-air-source-heat-pumps-keep-homes-warm-from-alaska-coast-to-us-mass-market.html |access-date=2022-03-29 |website=nrel.gov |archive-date=2022-04-10 |archive-url=https://web.archive.org/web/20220410124219/https://www.nrel.gov/news/features/2021/even-in-frigid-temperatures-air-source-heat-pumps-keep-homes-warm-from-alaska-coast-to-us-mass-market.html |url-status=live}}{{cite web |date=2020-12-10 |title=Heat Pumps: A Practical Solution for Cold Climates |url=https://rmi.org/heat-pumps-a-practical-solution-for-cold-climates/ |access-date=2022-03-28 |work=RMI |archive-date=2022-03-31 |archive-url=https://web.archive.org/web/20220331011727/https://rmi.org/heat-pumps-a-practical-solution-for-cold-climates/ |url-status=live}} 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.

{{main|coefficient of performance|Seasonal energy efficiency ratio|European seasonal energy efficiency ratio}}

The coefficient of performance (COP) of an air conditioning system is a ratio of useful heating or cooling provided to the work required.{{cite web |date=March 14, 2012 |title=TEM Instruction Sheet |url=http://www.tetech.com/temodules/graphs/instructions.pdf |url-status=dead |archive-url=https://web.archive.org/web/20130124080037/http://www.tetech.com/temodules/graphs/instructions.pdf |archive-date=January 24, 2013 |access-date=May 12, 2021 |work=TE Technology}}{{cite web |date=November 18, 2020 |title=Coefficient of Performance (COP) heat pumps |url=https://www.grundfos.com/solutions/learn/research-and-insights/coefficient-of-system-performance |url-status=live |archive-url=https://web.archive.org/web/20210503013223/https://www.grundfos.com/solutions/learn/research-and-insights/coefficient-of-system-performance |archive-date=May 3, 2021 |access-date=May 12, 2021 |work=Grundfos}} 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.{{cite web |title=Unpotted HP-199-1.4-0.8 at a hot-side temperature of 25 °C |url=http://www.tetech.com/temodules/graphs/HP-199-1.4-0.8.pdf |url-status=dead |archive-url=https://web.archive.org/web/20090107132318/http://www.tetech.com/temodules/graphs/HP-199-1.4-0.8.pdf |archive-date=January 7, 2009 |access-date=February 9, 2024 |work=TE Technology}} 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 ({{convert|2000|lb|kg}} of ice melting in a 24-hour period. The value is equal to 12,000 BTU_IT per hour, or 3,517 watts.{{cite book |url=https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf |title=The International System of Units (SI) |date=August 2019 |publisher=National Institute of Standards and Technology |editor-last=Newell |editor-first=David B. |doi=10.6028/NIST.SP.330-2019 |access-date=May 13, 2021 |editor-last2=Tiesinga |editor-first2=Eite |archive-url=https://web.archive.org/web/20210422022620/https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf |archive-date=April 22, 2021 |url-status=live |doi-access=free}} Residential central air systems are usually from 1 to 5 tons (3.5 to 18 kW) in capacity.{{Citation needed|date=May 2021}}

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.{{cite book |url=https://www.ahrinet.org/App_Content/ahri/files/standards%20pdfs/ANSI%20standards%20pdfs/ANSI.AHRI%20Standard%20210.240%20with%20Addenda%201%20and%202.pdf |title=ANSI/AHRI 210/240-2008: 2008 Standard for Performance Rating of Unitary Air-Conditioning & Air-Source Heat Pump Equipment |publisher=Air Conditioning, Heating and Refrigeration Institute |year=2012 |access-date=May 13, 2021 |archive-url=https://web.archive.org/web/20180329072533/http://www.ahrinet.org/App_Content/ahri/files/standards |archive-date=March 29, 2018 |url-status=live}} A similar standard is the European seasonal energy efficiency ratio (ESEER).{{Citation needed|date=May 2021}}

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.{{Cite magazine |last=Baraniuk |first=Chris |title=Cutting-Edge Technology Could Massively Reduce the Amount of Energy Used for Air Conditioning |url=https://www.wired.com/story/cutting-edge-technology-could-massively-reduce-the-amount-of-energy-used-for-air-conditioning/ |access-date=2024-07-18 |magazine=Wired |language=en-US |issn=1059-1028}}

{{Main article|Remote control|Infrared blaster}}{{Multiple image

| direction = horizontal

| image1 = Daikin ARC480A48 remote control.jpg

| image2 = Daikin ARC480A48 IR transmitter.jpg

| image3 = The receiver on the aircon.jpg

| total_width = 200

| caption1 = A wireless remote controller

| caption2 = The infrared transmitting LED on the remote

| perrow = 2 / 2

| image4 = IR receiver sensor.jpg

| footer = 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.

{{Main|Thermostat}}{{Multiple image

| total_width = 300

| image1 = Daikin wired controller.jpg

| image2 = Honeywell wired thermostat.jpg

| image3 = Fujiaire wired thermostat.jpg

| image4 = Midea wired controller.jpg

| direction = horizontal

| perrow = 2

| footer = 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

File:GALANZ II.jpg

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.{{cite web|title=M-Series Contractor Guide |url=https://www.mitsubishipro.com/pdfs/m-series-catalog.pdf |url-status=live |archive-url=https://web.archive.org/web/20210318225607/https://www.mitsubishipro.com/pdfs/m-series-catalog.pdf |archive-date=March 18, 2021 |access-date=May 12, 2021|website=Mitsubishipro.com |page=19}} 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.{{Cite web|url=https://shouene-kaden.net/try/kaden/air-con.html#:~:text=1953%E5%B9%B4%20%E6%97%A5%E6%9C%AC%E5%88%9D%E3%81%AE,%E5%88%9D%E3%82%81%E3%81%A6%E7%94%9F%E7%94%A3%E3%81%95%E3%82%8C%E3%81%BE%E3%81%97%E3%81%9F%E3%80%82 |title=エアコンの歴史とヒミツ {{!}} 調べよう家電と省エネ {{!}} キッズ版 省エネ家電 de スマートライフ（一般財団法人 家電製品協会） 学ぼう！スマートライフ |website=shouene-kaden.net |access-date=2024-01-21 |archive-date=2022-09-07|archive-url=https://web.archive.org/web/20220907192752/https://shouene-kaden.net/try/kaden/air-con.html#:~:text=1953%E5%B9%B4%20%E6%97%A5%E6%9C%AC%E5%88%9D%E3%81%AE,%E5%88%9D%E3%82%81%E3%81%A6%E7%94%9F%E7%94%A3%E3%81%95%E3%82%8C%E3%81%BE%E3%81%97%E3%81%9F%E3%80%82 |url-status=live}}{{cite web|date=April 2016 |title=Air conditioner {{!}} History|url=https://www.toshiba-carrier.co.jp/global/about/history.htm |url-status=live|access-date=May 12, 2021 |work=Toshiba Carrier |archive-date=March 9, 2021 |archive-url=https://web.archive.org/web/20210309011149/https://www.toshiba-carrier.co.jp/global/about/history.htm}}{{cite web|title=1920s–1970s {{!}} History |url=https://www.mitsubishielectric.com/en/about/history/1920s-70s/index.page|url-status=live|access-date=May 12, 2021 |work=Mitsubishi Electric |archive-date=March 8, 2021 |archive-url=https://web.archive.org/web/20210308111609/https://www.mitsubishielectric.com/en/about/history/1920s-70s/index.page}} In 1969, the first mini-split air conditioner was sold in the US.{{cite web |last1=Wagner |first1=Gerry |title=The Duct Free Zone: History of the Mini Split |url=https://www.hpacmag.com/features/the-duct-free-zone-history-of-the-mini-split/#:~:text=The%20Twin%20Pac%2C%20the%20first,and%2016000%20(230%20V) |work=HPAC Magazine |access-date=February 9, 2024 |date=November 30, 2021}} 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.{{cite web|title=History of Daikin Innovation |url=https://www.daikin.com/corporate/overview/summary/history/digest/ |url-status=live|access-date=May 12, 2021 |work=Daikin |archive-date=June 5, 2020 |archive-url=https://web.archive.org/web/20200605045033/https://www.daikin.com/corporate/overview/summary/history/digest/}} 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.{{cite web|last=Feit |first=Justin |date=December 20, 2017 |title=The Emergence of VRF as a Viable HVAC Option |website=buildings.com |url=https://www.buildings.com/articles/28170/emergence-vrf-viable-hvac-option |access-date=May 12, 2021 |archive-date=December 3, 2020|archive-url=https://web.archive.org/web/20201203234715/https://www.buildings.com/articles/28170/emergence-vrf-viable-hvac-option|url-status=live}} Variable refrigerant flow indoor units can also be turned off individually in unused spaces.{{Citation needed|date=May 2021}} 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. Floor standing air conditioners are similar to this type of air conditioner but sit within spaces that need cooling.

{{See also|Chiller}}

File:Industrial air conditioning unit (DFdB).JPG

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.{{cite book |url=https://books.google.com/books?id=KX9XDwAAQBAJ&q=chiller+air+conditioning+system |title=Air Conditioning and Refrigeration Engineering |isbn=978-1-351-46783-4 |last1=Kreith |first1=Frank |last2=Wang |first2=Shan K. |last3=Norton |first3=Paul |date=April 20, 2018 |publisher=CRC Press}}{{cite book |url=https://books.google.com/books?id=0tVSAAAAMAAJ |title=Handbook of Air Conditioning and Refrigeration |isbn=978-0-07-068167-5 |last1=Wang |first1=Shan K. |date=November 7, 2000 |publisher=McGraw-Hill Education}}

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.{{cite web|last=Hleborodova |first=Veronika |date=August 14, 2018 |title=Portable Vs Split System Air Conditioning {{!}} Pros & Cons |url=https://www.canstarblue.com.au/appliances/portable-or-split-system-air-conditioning-the-pros-and-cons/ |url-status=live |access-date=May 12, 2021 |work=Canstar Blue |archive-date=March 9, 2021 |archive-url=https://web.archive.org/web/20210309052103/https://www.canstarblue.com.au/appliances/portable-or-split-system-air-conditioning-the-pros-and-cons/}}

{{main|Packaged terminal air conditioner}}

File:A649, University Motor Inn, Schuylkill River, Philadelphia, 2018.jpg

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.{{cite web|last=Kamins |first=Toni L. |date=July 15, 2013 |title=Through-the-Wall Versus PTAC Air Conditioners: A Guide for New Yorkers |url=https://www.brickunderground.com/blog/2013/07/go_a_through_the_wall_or_ptac_ac_system_were_here_to_help|url-status=live |access-date=May 12, 2021 |work=Brick Underground |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115235022/https://www.brickunderground.com/blog/2013/07/go_a_through_the_wall_or_ptac_ac_system_were_here_to_help}}

Packaged air conditioners (also known as self-contained units){{cite web|date=2015 |title=Self-Contained Air Conditioning Systems |url=https://oslo.daikinapplied.com/api/daikindocument/DownloadDocumentByName/Doc100/Daikin_CAT_860-10_LR_Self-Contained_SWP-H_Catalog.pdf/ |url-status=live |access-date=May 12, 2021 |work=Daikin Applied Americas |archive-date=October 30, 2020 |archive-url=https://web.archive.org/web/20201030103405/https://oslo.daikinapplied.com/api/daikindocument/DownloadDocumentByName/Doc100/Daikin_CAT_860-10_LR_Self-Contained_SWP-H_Catalog.pdf/}}{{cite web|date=April 6, 2018 |title=LSWU/LSWD Vertical Water-Cooled Self-Contained Unit Engineering Guide|url=https://cgproducts.johnsoncontrols.com/yorkdoc/145.05-EG2.pdf |url-status=live |access-date=May 12, 2021 |work=Johnson Controls |archive-date=May 13, 2021|archive-url=https://web.archive.org/web/20210513150232/https://cgproducts.johnsoncontrols.com/yorkdoc/145.05-EG2.pdf}} 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),{{cite web|date=2016 |title=Packaged Rooftop Unit |url=https://www.carrier.com/commercial/en/ae/media/DesertMasterPKG-50TCMborchure_tcm478-51454.pdf |url-status=live |access-date=May 12, 2021 |work=Carrier Global |archive-date=May 13, 2021 |archive-url=https://web.archive.org/web/20210513150232/https://www.carrier.com/commercial/en/ae/media/DesertMasterPKG-50TCMborchure_tcm478-51454.pdf}}{{cite web|date=November 2006 |title=Packaged Rooftop Air Conditioners |url=https://www.trane.com/Commercial/Uploads/Pdf/1102/rtprc010en.pdf |url-status=live |access-date=May 12, 2021 |work=Trane Technologies |archive-date=May 13, 2021 |archive-url=https://web.archive.org/web/20210513150233/https://www.trane.com/Commercial/Uploads/Pdf/1102/rtprc010en.pdf}} 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.{{cite web|title=Central Air Conditioning |url=https://www.energy.gov/energysaver/central-air-conditioning|url-status=live|access-date=May 12, 2021 |publisher=United States Department of Energy |archive-date=January 30, 2021 |archive-url=https://web.archive.org/web/20210130034016/https://www.energy.gov/energysaver/central-air-conditioning}}{{cite web|date=January 13, 2010 |title=What is Packaged Air Conditioner? Types of Packged Air Condtioners |url=https://www.brighthubengineering.com/hvac/61457-packaged-air-conditioners-types-of-packaged-ac/ |url-status=live|access-date=May 12, 2021 |work=Bright Hub Engineering |archive-date=February 22, 2018 |archive-url=https://web.archive.org/web/20180222112931/http://www.brighthubengineering.com/hvac/61457-packaged-air-conditioners-types-of-packaged-ac/}}{{cite web|last=Evans |first=Paul |date=November 11, 2018 |title=RTU Rooftop Units explained |url=https://theengineeringmindset.com/rtu-rooftop-units-explained/ |url-status=live |access-date=May 12, 2021 |work=The Engineering Mindset |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115090022/https://theengineeringmindset.com/rtu-rooftop-units-explained/}}{{cite web|date=2000|title=water-cooled – Johnson Supply |url=https://studylib.net/doc/18423029/water-cooled---johnson-supply |url-status=live |access-date=May 12, 2021 |website=studylib.net |archive-date=May 13, 2021 |archive-url=https://web.archive.org/web/20210513150304/https://studylib.net/doc/18423029/water-cooled---johnson-supply}}{{cite web|date=May 2, 2003 |title=Water Cooled Packaged Air Conditioners |url=https://www.daikin.com.sg/resources/ck/files/catalogue/Water-cooled_Type_PL97-6A.pdf|url-status=live|access-date=May 12, 2021 |publisher=Daikin |location=Japan |archive-date=June 19, 2018 |archive-url=https://web.archive.org/web/20180619070815/http://www.daikin.com.sg/resources/ck/files/catalogue/Water-cooled_Type_PL97-6A.pdf}}{{cite web|title=Water Cooled Packaged Unit |url=https://www.daikin.com.sg/resources/ck/files/catalogue/Watercooled%20package_UCCP.pdf |url-status=live |access-date=May 12, 2021 |publisher=Daikin |archive-date=May 13, 2021 |archive-url=https://web.archive.org/web/20210513150236/https://www.daikin.com.sg/resources/ck/files/catalogue/Watercooled%20package_UCCP.pdf}}

:{{main|Reciprocating compressor}}

This compressor consists of a crankcase, crankshaft, piston rod, piston, piston ring, cylinder head and valves. {{citation needed|date=November 2023}}

:{{main|Scroll compressor}}

This compressor uses two interleaving scrolls to compress the refrigerant.{{Cite book |last1=Lun |first1=Y. H. Venus |url=https://books.google.com/books?id=6iy-DwAAQBAJ&dq=This+compressor+uses+two+interleaving+scrolls+to+compress+the+refrigerant+.&pg=PA25 |title=Heat Pumps for Sustainable Heating and Cooling |last2=Tung |first2=S. L. Dennis |date=2019-11-13 |publisher=Springer Nature |isbn=978-3-030-31387-6 |pages=25 |language=en}} it consists of one fixed and one orbiting scrolls. This type of compressor is more efficient because it has 70 percent less moving parts than a reciprocating compressor. {{citation needed|date=November 2023}}

{{Main article|Rotary-screw compressor}}

This compressor use two very closely meshing spiral rotors to compress the gas. The gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws. The working area is the inter-lobe volume between the male and female rotors. It is larger at the intake end, and decreases along the length of the rotors until the exhaust port. This change in volume is the compression. {{citation needed|date=November 2023}}

There are several ways to modulate the cooling capacity in refrigeration or air conditioning and heating systems. The most common in air conditioning are: on-off cycling, hot gas bypass, use or not of liquid injection, manifold configurations of multiple compressors, mechanical modulation (also called digital), and inverter technology. {{citation needed|date=November 2023}}

Hot gas bypass involves injecting a quantity of gas from discharge to the suction side. The compressor will keep operating at the same speed, but due to the bypass, the refrigerant mass flow circulating with the system is reduced, and thus the cooling capacity. This naturally causes the compressor to run uselessly during the periods when the bypass is operating. The turn down capacity varies between 0 and 100%.{{cite journal| url=http://www.pipelineandgasjournal.com/bypass-method-recip-compressor-capacity-control |archive-date=12 August 2014 |archive-url=https://web.archive.org/web/20140812130223/http://www.pipelineandgasjournal.com/bypass-method-recip-compressor-capacity-control |date=June 2012 |journal=Pipeline and Gas Journal |volume=239 |issue=6 |title=Bypass Method For Recip Compressor Capacity Control |first1=Ali |last1=Ghanbariannaeeni |first2=Ghazalehsadat |last2=Ghazanfarihashemi |access-date=February 9, 2024}}

Several compressors can be installed in the system to provide the peak cooling capacity. Each compressor can run or not in order to stage the cooling capacity of the unit. The turn down capacity is either 0/33/66 or 100% for a trio configuration and either 0/50 or 100% for a tandem.{{citation needed|date=May 2019}}

This internal mechanical capacity modulation is based on periodic compression process with a control valve, the two scroll set move apart stopping the compression for a given time period. This method varies refrigerant flow by changing the average time of compression, but not the actual speed of the motor. Despite an excellent turndown ratio – from 10 to 100% of the cooling capacity, mechanically modulated scrolls have high energy consumption as the motor continuously runs.{{citation needed|date=May 2023}}

:{{main|Inverter compressor}}

This system uses a variable-frequency drive (also called an Inverter) to control the speed of the compressor. The refrigerant flow rate is changed by the change in the speed of the compressor. The turn down ratio depends on the system configuration and manufacturer. It modulates from 15 or 25% up to 100% at full capacity with a single inverter from 12 to 100% with a hybrid tandem. This method is the most efficient way to modulate an air conditioner's capacity. It is up to 58% more efficient than a fixed speed system.{{citation needed|date=November 2023}}

File:Osaka Municipal Subway 10 series EMU 021.JPG unit fitted on top of an Osaka Municipal Subway 10 series subway carriage. Air conditioning has become increasingly prevalent on public transport vehicles as a form of climate control, and to ensure passenger comfort and drivers' occupational safety and health.]]

In hot weather, air conditioning can prevent heat stroke, dehydration due to excessive sweating, electrolyte imbalance, kidney failure, and other issues due to hyperthermia.{{cite web|date=January 2, 2019 |title=Heat Stroke (Hyperthermia) |url=https://www.health.harvard.edu/a_to_z/heat-stroke-hyperthermia-a-to-z |url-access=subscription |url-status=live |access-date=May 13, 2021 |work=Harvard Health |archive-date=January 29, 2021 |archive-url=https://web.archive.org/web/20210129121907/https://www.health.harvard.edu/a_to_z/heat-stroke-hyperthermia-a-to-z}} Heat waves are the most lethal type of weather phenomenon in the United States.{{cite web |date=2021 |title=Weather Related Fatality and Injury Statistics |work=National Weather Service |url=https://www.weather.gov/hazstat/ |access-date=2022-08-24 |archive-date=2022-08-24 |archive-url=https://web.archive.org/web/20220824211305/https://www.weather.gov/hazstat/ |url-status=live}}{{cite journal |date=2012-10-19 |title=Extreme Weather: A Guide to Surviving Flash Floods, Tornadoes, Hurricanes, Heat Waves, Snowstorms Tsunamis and Other Natural Disasters |url=http://dx.doi.org/10.1108/09504121211278322 |journal=Reference Reviews |volume=26 |issue=8 |pages=41 |doi=10.1108/09504121211278322 |issn=0950-4125 |access-date=2023-12-09 |archive-date=2024-01-21 |archive-url=https://web.archive.org/web/20240121175825/https://www.emerald.com/insight/content/doi/10.1108/09504121211278322/full/html |url-status=live}} A 2020 study found that areas with lower use of air conditioning correlated with higher rates of heat-related mortality and hospitalizations. The August 2003 France heatwave resulted in approximately 15,000 deaths, where 80% of the victims were over 75 years old. In response, the French government required all retirement homes to have at least one air-conditioned room at {{cvt|25|C|F|0}} per floor during heatwaves.

Air conditioning (including filtration, humidification, cooling and disinfection) can be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating rooms and other environments where proper atmosphere is critical to patient safety and well-being. It is sometimes recommended for home use by people with allergies, especially mold.{{cite journal |last1=Spiegelman |first1=Jay |last2=Friedman |first2=Herman |last3=Blumstein |first3=George I. |date=1963-09-01 |title=The effects of central air conditioning on pollen, mold, and bacterial concentrations |url=https://dx.doi.org/10.1016/0021-8707%2863%2990007-8 |journal=Journal of Allergy |volume=34 |issue=5 |pages=426–431 |doi=10.1016/0021-8707(63)90007-8 |pmid=14066385 |issn=0021-8707}}{{Cite journal |last1=Portnoy |first1=Jay M. |last2=Jara |first2=David |date=2015-02-01 |title=Mold allergy revisited |journal=Annals of Allergy, Asthma & Immunology |volume=114 |issue=2 |pages=83–89 |doi=10.1016/j.anai.2014.10.004 |pmid=25624128 |issn=1081-1206|doi-access=free}} However, poorly maintained water cooling towers can promote the growth and spread of microorganisms such as Legionella pneumophila, the infectious agent responsible for Legionnaires' disease. As long as the cooling tower is kept clean (usually by means of a chlorine treatment), these health hazards can be avoided or reduced. The state of New York has codified requirements for registration, maintenance, and testing of cooling towers to protect against Legionella.{{cite web|date=June 7, 2016 |title=Subpart 4-1 – Cooling Towers |url=https://regs.health.ny.gov/content/subpart-4-1-cooling-towers |url-status=live |access-date=May 13, 2021 |work=New York Codes, Rules and Regulations |archive-date=May 13, 2021 |archive-url=https://web.archive.org/web/20210513150257/https://regs.health.ny.gov/content/subpart-4-1-cooling-towers}}

First designed to benefit targeted industries such as the press as well as large factories, the invention quickly spread to public agencies and administrations with studies with claims of increased productivity close to 24% in places equipped with air conditioning.{{cite journal |last=Nordhaus |first=William D. |date=February 10, 2010 |title=Geography and macroeconomics: New data and new findings |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=10 |pages=3510–3517 |doi=10.1073/pnas.0509842103 |issn=0027-8424 |pmc=1363683 |pmid=16473945 |doi-access=free}}

Air conditioning caused various shifts in demography, notably that of the United States starting from the 1970s. In the US, the birth rate was lower in the spring than during other seasons until the 1970s but this difference then declined since then.{{cite journal |last1=Barreca |first1=Alan |last2=Deschenes |first2=Olivier |last3=Guldi |first3=Melanie |year=2018 |title=Maybe next month? Temperature shocks and dynamic adjustments in birth rates |journal=Demography |doi=10.1007/s13524-018-0690-7 |volume=55 |issue=4 |pages=1269–1293|pmid=29968058 |pmc=7457515}} As of 2007, the Sun Belt contained 30% of the total US population while it was inhabited by 24% of Americans at the beginning of the 20th century.{{cite journal |last1=Glaeser |first1=Edward L. |last2=Tobio |first2=Kristina |title=The Rise of the Sunbelt |journal=Southern Economic Journal |date=January 2008 |volume=74 |issue=3 |pages=609–643 |doi=10.1002/j.2325-8012.2008.tb00856.x}} Moreover, the summer mortality rate in the US, which had been higher in regions subject to a heat wave during the summer, also evened out.{{cite journal |last1=Barreca |first1=Alan |last2=Clay |first2=Karen |last3=Deschenes |first3=Olivier |last4=Greenstone |first4=Michael |last5=Shapiro |first5=Joseph S. |title=Adapting to Climate Change: The Remarkable Decline in the US Temperature-Mortality Relationship over the Twentieth Century |journal=Journal of Political Economy |date=February 2016 |volume=124 |issue=1 |pages=105–159 |doi=10.1086/684582 |url=https://escholarship.org/uc/item/08c6t2hs}}

The spread of the use of air conditioning acts as a main driver for the growth of global demand of electricity.{{cite journal |last1=Sherman |first1=Peter |last2=Lin |first2=Haiyang |last3=McElroy |first3=Michael |date=2018 |title=Projected global demand for air conditioning associated with extreme heat and implications for electricity grids in poorer countries |journal=Energy and Buildings |volume=268 |pages=112198 |doi=10.1016/j.enbuild.2022.112198 |s2cid=248979815 |issn=0378-7788|doi-access=free}} According to a 2018 report from the International Energy Agency (IEA), it was revealed that the energy consumption for cooling in the United States, involving 328 million Americans, surpasses the combined energy consumption of 4.4 billion people in Africa, Latin America, the Middle East, and Asia (excluding China). A 2020 survey found that an estimated 88% of all US households use AC, increasing to 93% when solely looking at homes built between 2010 and 2020.{{cite tech report |type=Withdrawn Standard |number=BS 6540-1:1985 |title=Air Filters Used in Air Conditioning and General Ventilation Part 1: Methods of Test for Atmospheric Dust Spot Efficiency and Synthetic Dust Weight Arrestance |publisher=British Standards Institution |date=29 March 1985 |url=https://knowledge.bsigroup.com/products/air-filters-used-in-air-conditioning-and-general-ventilation-methods-of-test-for-atmospheric-dust-spot-efficiency-and-synthetic-dust-weight-arrestance}}

File:External units of air conditioners in Singapore.jpg

Space cooling including air conditioning accounted globally for 2021 terawatt-hours of energy usage in 2016 with around 99% in the form of electricity, according to a 2018 report on air-conditioning efficiency by the International Energy Agency. The report predicts an increase of electricity usage due to space cooling to around 6200 TWh by 2050,

{{cite report |date=May 15, 2018 |title=The Future of Cooling - Opportunities for energy-efficient air conditioning |author=International Energy Agency |url=https://iea.blob.core.windows.net/assets/0bb45525-277f-4c9c-8d0c-9c0cb5e7d525/The_Future_of_Cooling.pdf |access-date=2024-07-01 |language=en-GB |archive-date=2024-06-26 |archive-url=https://web.archive.org/web/20240626023624/https://iea.blob.core.windows.net/assets/0bb45525-277f-4c9c-8d0c-9c0cb5e7d525/The_Future_of_Cooling.pdf |url-status=live}}{{cite journal |last1=Mutschler |first1=Robin |last2=Rüdisüli |first2=Martin |last3=Heer |first3=Philipp |last4=Eggimann |first4=Sven |date=April 15, 2021 |title=Benchmarking cooling and heating energy demands considering climate change, population growth and cooling device uptake |journal=Applied Energy |volume=288 |issue=|pages=116636 |doi=10.1016/j.apenergy.2021.116636 |issn=0306-2619 |doi-access=free|bibcode=2021ApEn..28816636M}} and that with the progress currently seen, greenhouse gas emissions attributable to space cooling will double: 1,135 million tons (2016) to 2,070 million tons. There is some push to increase the energy efficiency of air conditioners. United Nations Environment Programme (UNEP) and the IEA found that if air conditioners could be twice as effective as now, 460 billion tons of GHG could be cut over 40 years.{{Cite journal|url=http://dx.doi.org/10.1163/9789004322714_cclc_2020-0252-0973|title=Climate-friendly cooling could cut years of Greenhouse Gas Emissions and save US$ trillions: UN|journal=Climate Change and Law Collection|doi=10.1163/9789004322714_cclc_2020-0252-0973}} The UNEP and IEA also recommended legislation to decrease the use of hydrofluorocarbons, better building insulation, and more sustainable temperature-controlled food supply chains going forward.

Refrigerants have also caused and continue to cause serious environmental issues, including ozone depletion and climate change, as several countries have not yet ratified the Kigali Amendment to reduce the consumption and production of hydrofluorocarbons.{{cite web |last=Gerretsen |first=Isabelle |date=December 8, 2020 |title=How your fridge is heating up the planet |url=https://www.bbc.com/future/article/20201204-climate-change-how-chemicals-in-your-fridge-warm-the-planet |url-status=live |archive-url=https://web.archive.org/web/20210510055033/https://www.bbc.com/future/article/20201204-climate-change-how-chemicals-in-your-fridge-warm-the-planet |archive-date=May 10, 2021 |access-date=May 13, 2021 |work=BBC Future}} CFCs and HCFCs refrigerants such as R-12 and R-22, respectively, used within air conditioners have caused damage to the ozone layer,{{cite book |url=https://books.google.com/books?id=Jo1UAAAAMAAJ&q=CFC+and+HCFC+refrigerants+such+as+R-12+and+R-22,+respectively,+used+within+air+conditioners+have+caused+damage+to+the+ozone+layer |title=Encyclopedia of Energy: Ph-S |date=2004 |publisher=Elsevier |isbn=978-0121764821}} and hydrofluorocarbon refrigerants such as R-410A and R-404A, which were designed to replace CFCs and HCFCs, are instead exacerbating climate change.{{cite book |doi=10.1016/B978-0-08-100311-4.00013-3 |chapter=New trends and developments in ground-source heat pumps |title=Advances in Ground-Source Heat Pump Systems |date=2016 |last1=Corberan |first1=J.M. |pages=359–385 |isbn=978-0-08-100311-4}} Both issues happen due to the venting of refrigerant to the atmosphere, such as during repairs. HFO refrigerants, used in some if not most new equipment, solve both issues with an ozone damage potential (ODP) of zero and a much lower global warming potential (GWP) in the single or double digits vs. the three or four digits of hydrofluorocarbons.{{cite book |last1=Roselli |first1=Carlo |url=https://books.google.com/books?id=YCZAEAAAQBAJ&dq=HFO+refrigerants,+used+in+some+if+not+most+new+equipment,+solve+both+issues+with+an+ozone+damage+potential+(ODP)+of+zero&pg=PA90 |title=Geothermal Energy Utilization and Technologies 2020 |last2=Sasso |first2=Maurizio |date=2021 |publisher=MDPI |isbn=978-3036507040}}

Hydrofluorocarbons would have raised global temperatures by around {{cvt|0.3-0.5|C-change|F-change|1}} by 2100 without the Kigali Amendment. With the Kigali Amendment, the increase of global temperatures by 2100 due to hydrofluorocarbons is predicted to be around {{cvt|0.06|C-change|F-change|1}}.{{Cite web|url=https://iea.blob.core.windows.net/assets/71c8db7e-1137-41ef-99c3-8f2c8d3a5d86/Cooling_Emissions_and_Policy_Synthesis_Report.pdf|title=Cooling Emissions and Policy Synthesis Report: Benefits of cooling efficiency and the Kigali Amendment, United Nations Environment Programme - International Energy Agency, 2020}}

Alternatives to continual air conditioning include passive cooling, passive solar cooling, natural ventilation, operating shades to reduce solar gain, using trees, architectural shades, windows (and using window coatings) to reduce solar gain.{{Citation needed|date=May 2021}}

Socioeconomic groups with a household income below around $10,000 tend to have a low air conditioning adoption, which worsens heat-related mortality. The lack of cooling can be hazardous, as areas with lower use of air conditioning correlate with higher rates of heat-related mortality and hospitalizations.{{cite journal |last1=Gamarro |first1=Harold |last2=Ortiz |first2=Luis |last3=González |first3=Jorge E. |date=2020-08-01 |title=Adapting to Extreme Heat: Social, Atmospheric, and Infrastructure Impacts of Air-Conditioning in Megacities—The Case of New York City |journal=Journal of Engineering for Sustainable Buildings and Cities |volume=1 |issue=3 |doi=10.1115/1.4048175 |s2cid=222121944 |issn=2642-6641|doi-access=free}} Premature mortality in NYC is projected to grow between 47% and 95% in 30 years, with lower-income and vulnerable populations most at risk. Studies on the correlation between heat-related mortality and hospitalizations and living in low socioeconomic locations can be traced in Phoenix, Arizona,{{cite journal |last1=Harlan |first1=Sharon L. |last2=Declet-Barreto |first2=Juan H. |last3=Stefanov |first3=William L. |last4=Petitti |first4=Diana B. |date=February 2013 |title=Neighborhood Effects on Heat Deaths: Social and Environmental Predictors of Vulnerability in Maricopa County, Arizona |journal=Environmental Health Perspectives |language=en |volume=121 |issue=2 |pages=197–204 |doi=10.1289/ehp.1104625 |issn=0091-6765 |pmc=3569676 |pmid=23164621|bibcode=2013EnvHP.121..197H}} Hong Kong,{{Cite journal |last1=Chan |first1=Emily Ying Yang |last2=Goggins |first2=William B |last3=Kim |first3=Jacqueline Jakyoung |last4=Griffiths |first4=Sian M |date=April 2012 |title=A study of intracity variation of temperature-related mortality and socioeconomic status among the Chinese population in Hong Kong |journal=Journal of Epidemiology and Community Health |language=en |volume=66 |issue=4 |pages=322–327 |doi=10.1136/jech.2008.085167 |issn=0143-005X |pmc=3292716 |pmid=20974839}} China, Japan,{{cite journal |last1=Ng |first1=Chris Fook Sheng |last2=Ueda |first2=Kayo |last3=Takeuchi |first3=Ayano |last4=Nitta |first4=Hiroshi |last5=Konishi |first5=Shoko |last6=Bagrowicz |first6=Rinako |last7=Watanabe |first7=Chiho |last8=Takami |first8=Akinori |title=Sociogeographic Variation in the Effects of Heat and Cold on Daily Mortality in Japan |journal=Journal of Epidemiology |date=2014 |volume=24 |issue=1 |pages=15–24 |doi=10.2188/jea.JE20130051 |pmid=24317342 |pmc=3872520}} and Italy.{{cite journal |last1=Stafoggia |first1=Massimo |last2=Forastiere |first2=Francesco |last3=Agostini |first3=Daniele |last4=Biggeri |first4=Annibale |last5=Bisanti |first5=Luigi |last6=Cadum |first6=Ennio |last7=Caranci |first7=Nicola |last8=de'Donato |first8=Francesca |last9=De Lisio |first9=Sara |last10=De Maria |first10=Moreno |last11=Michelozzi |first11=Paola |last12=Miglio |first12=Rossella |last13=Pandolfi |first13=Paolo |last14=Picciotto |first14=Sally |last15=Rognoni |first15=Magda |date=2006 |title=Vulnerability to Heat-Related Mortality: A Multicity, Population-Based, Case-Crossover Analysis |journal=Epidemiology |volume=17 |issue=3 |pages=315–323 |doi=10.1097/01.ede.0000208477.36665.34 |jstor=20486220 |pmid=16570026 |s2cid=20283342 |issn=1044-3983|doi-access=free}}{{cite journal |last1=Gronlund |first1=Carina J. |title=Racial and Socioeconomic Disparities in Heat-Related Health Effects and Their Mechanisms: a Review |journal=Current Epidemiology Reports |date=September 2014 |volume=1 |issue=3 |pages=165–173 |doi=10.1007/s40471-014-0014-4 |pmid=25512891 |pmc=4264980}} Additionally, costs concerning health care can act as another barrier, as the lack of private health insurance during a 2009 heat wave in Australia, was associated with heat-related hospitalization.

Disparities in socioeconomic status and access to air conditioning are connected by some to institutionalized racism, which leads to the association of specific marginalized communities with lower economic status, poorer health, residing in hotter neighborhoods, engaging in physically demanding labor, and experiencing limited access to cooling technologies such as air conditioning. A study overlooking Chicago, Illinois, Detroit, and Michigan found that black households were half as likely to have central air conditioning units when compared to their white counterparts.{{cite journal |last1=O'Neill |first1=M. S. |title=Disparities by Race in Heat-Related Mortality in Four US Cities: The Role of Air Conditioning Prevalence |journal=Journal of Urban Health: Bulletin of the New York Academy of Medicine |date=11 May 2005 |volume=82 |issue=2 |pages=191–197 |doi=10.1093/jurban/jti043 |pmid=15888640 |pmc=3456567}} Especially in cities, Redlining creates heat islands, increasing temperatures in certain parts of the city. This is due to materials heat-absorbing building materials and pavements and lack of vegetation and shade coverage.{{cite journal |last1=Sampson |first1=Natalie R. |last2=Gronlund |first2=Carina J. |last3=Buxton |first3=Miatta A. |last4=Catalano |first4=Linda |last5=White-Newsome |first5=Jalonne L. |last6=Conlon |first6=Kathryn C. |last7=O’Neill |first7=Marie S. |last8=McCormick |first8=Sabrina |last9=Parker |first9=Edith A. |date=2013-04-01 |title=Staying cool in a changing climate: Reaching vulnerable populations during heat events |journal=Global Environmental Change |volume=23 |issue=2 |pages=475–484 |doi=10.1016/j.gloenvcha.2012.12.011 |pmid=29375195 |pmc=5784212 |bibcode=2013GEC....23..475S |issn=0959-3780}} There have been initiatives that provide cooling solutions to low-income communities, such as public cooling spaces.

Buildings designed with passive air conditioning are generally less expensive to construct and maintain than buildings with conventional HVAC systems with lower energy demands.{{cite conference|last1=Niktash|first1=Amirreza|last2=Huynh|first2=B. Phuoc |date=July 2–4, 2014 |title=Simulation and Analysis of Ventilation Flow Through a Room Caused by a Two-sided Windcatcher Using a LES Method|journal=Lecture Notes in Engineering and Computer Science|url=http://www.iaeng.org/publication/WCE2014/WCE2014_pp1294-1297.pdf |conference=World Congress on Engineering |location=London |volume=2 |isbn=978-9881925350 |issn=2078-0958 |access-date=May 13, 2021 |eissn=2078-0966 |archive-date=April 26, 2018 |archive-url=https://web.archive.org/web/20180426210916/http://www.iaeng.org/publication/WCE2014/WCE2014_pp1294-1297.pdf |url-status=live}} While tens of air changes per hour, and cooling of tens of degrees, can be achieved with passive methods, site-specific microclimate must be taken into account, complicating building design.

Many techniques can be used to increase comfort and reduce the temperature in buildings. These include evaporative cooling, selective shading, wind, thermal convection, and heat storage.{{cite journal |last1=Zhang |first1=Chen |last2=Kazanci |first2=Ongun Berk |last3=Levinson |first3=Ronnen |last4=Heiselberg |first4=Per |last5=Olesen |first5=Bjarne W. |last6=Chiesa |first6=Giacomo |last7=Sodagar |first7=Behzad |last8=Ai |first8=Zhengtao |last9=Selkowitz |first9=Stephen |last10=Zinzi |first10=Michele |last11=Mahdavi |first11=Ardeshir |date=2021-11-15 |title=Resilient cooling strategies – A critical review and qualitative assessment |journal=Energy and Buildings |volume=251 |pages=111312 |doi=10.1016/j.enbuild.2021.111312 |issn=0378-7788 |doi-access=free |bibcode=2021EneBu.25111312Z |hdl=2117/363031 |hdl-access=free}}

{{Excerpt|Passive ventilation}}

{{Excerpt|passive cooling}}

File:malqaf.svgs ({{transliteration|ar|malqaf}}) used in traditional architecture; wind is forced down on the windward side and leaves on the leeward side (cross-ventilation). In the absence of wind, the circulation can be driven with evaporative cooling in the inlet (which is also designed to catch dust). In the center, a {{transliteration|ar|shuksheika}} (roof lantern vent), used to shade the qa'a below while allowing hot air rise out of it (stack effect).{{cite conference|last1=Mohamed|first1=Mady A.A.|date=January 2010|editor-last=Lehmann|editor-first=S. |editor2=Waer |editor2-first=H.A. |editor3=Al-Qawasmi |editor3-first=J.|title=Traditional Ways of Dealing with Climate in Egypt |url=https://www.researchgate.net/publication/273122348 |conference=The Seventh International Conference of Sustainable Architecture and Urban Development (SAUD 2010) |location=Amman, Jordan |publisher=The Center for the Study of Architecture in Arab Region (CSAAR Press)|pages=247–266|access-date=May 12, 2021 |archive-date=May 13, 2021 |archive-url=https://web.archive.org/web/20210513150247/https://www.researchgate.net/publication/273122348_Traditional_Ways_of_Dealing_with_Climate_in_Egypt |url-status=live}}]]

File:Passive daytime radiative cooling diagram.jpg

Passive daytime radiative cooling (PDRC) surfaces reflect incoming solar radiation and heat back into outer space through the infrared window for cooling during the daytime. Daytime radiative cooling became possible with the ability to suppress solar heating using photonic structures, which emerged through a study by Raman et al. (2014).{{cite journal |last1=Raman |first1=Aaswath P. |last2=Anoma |first2=Marc Abou |last3=Zhu |first3=Linxiao |last4=Rephaeli |first4=Eden |last5=Fan |first5=Shanhui |title=Passive radiative cooling below ambient air temperature under direct sunlight |journal=Nature |date=November 2014 |volume=515 |issue=7528 |pages=540–544 |doi=10.1038/nature13883 |pmid=25428501 |bibcode=2014Natur.515..540R}} PDRCs can come in a variety of forms, including paint coatings and films, that are designed to be high in solar reflectance and thermal emittance.{{cite journal |last1=Bijarniya |first1=Jay Prakash |last2=Sarkar |first2=Jahar |last3=Maiti |first3=Pralay |title=Review on passive daytime radiative cooling: Fundamentals, recent researches, challenges and opportunities |journal=Renewable and Sustainable Energy Reviews |date=November 2020 |volume=133 |pages=110263 |doi=10.1016/j.rser.2020.110263 |bibcode=2020RSERv.13310263B |s2cid=224874019}}

PDRC applications on building roofs and envelopes have demonstrated significant decreases in energy consumption and costs. In suburban single-family residential areas, PDRC application on roofs can potentially lower energy costs by 26% to 46%.{{cite journal |last1=Mokhtari |first1=Reza |last2=Ulpiani |first2=Giulia |last3=Ghasempour |first3=Roghayeh |title=The Cooling Station: Combining hydronic radiant cooling and daytime radiative cooling for urban shelters |journal=Applied Thermal Engineering |date=July 2022 |volume=211 |pages=118493 |doi=10.1016/j.applthermaleng.2022.118493 |bibcode=2022AppTE.21118493M}} PDRCs are predicted to show a market size of ~$27 billion for indoor space cooling by 2025 and have undergone a surge in research and development since the 2010s.{{cite journal |last1=Yang |first1=Yuan |last2=Zhang |first2=Yifan |title=Passive daytime radiative cooling: Principle, application, and economic analysis |journal=MRS Energy & Sustainability |date=July 2020 |volume=7 |issue=1 |doi=10.1557/mre.2020.18 |doi-access=free}}{{cite journal |last1=Miranda |first1=Nicole D. |last2=Renaldi |first2=Renaldi |last3=Khosla |first3=Radhika |last4=McCulloch |first4=Malcolm D. |title=Bibliometric analysis and landscape of actors in passive cooling research |journal=Renewable and Sustainable Energy Reviews |date=October 2021 |volume=149 |pages=111406 |doi=10.1016/j.rser.2021.111406 |bibcode=2021RSERv.14911406M |url=https://ora.ox.ac.uk/objects/uuid:2266ea3e-4463-4554-9272-6a041888b60b}}

{{main|Ceiling fan}}

Hand fans have existed since prehistory. Large human-powered fans built into buildings include the punkah.

The 2nd-century Chinese inventor Ding Huan of the Han dynasty invented a rotary fan for air conditioning, with seven wheels {{convert|3|m|ft|abbr=on|0}} in diameter and manually powered by prisoners.{{cite book|last1=Needham|first1=Joseph|title=Science and Civilisation in China, Volume 4: Physics and Physical Technology, Part 2, Mechanical Engineering |last2=Wang |first2=Ling |publisher=Cambridge University Press |year=1991 |isbn=978-0521058032 |pages= |oclc=468144152}}{{Rp|99, 151, 233}} In 747, Emperor Xuanzong (r. 712–762) of the Tang dynasty (618–907) had the Cool Hall (Liang Dian {{lang|zh|涼殿}}) built in the imperial palace, which the Tang Yulin describes as having water-powered fan wheels for air conditioning as well as rising jet streams of water from fountains. During the subsequent Song dynasty (960–1279), written sources mentioned the air conditioning rotary fan as even more widely used.{{Rp|134, 151}}

In areas that are cold at night or in winter, heat storage is used. Heat may be stored in earth or masonry; air is drawn past the masonry to heat or cool it.{{cite conference|last1=Attia|first1=Shady|last2=Herde|first2=André de|date=22–24 June 2009|title=Designing the Malqaf for Summer Cooling in Low-Rise Housing, an Experimental Study |url=https://www.researchgate.net/publication/242550698 |via=ResearchGate |conference=26th Conference on Passive and Low Energy Architecture (PLEA2009) |location=Quebec City |archive-url=https://web.archive.org/web/20210513150322/https://www.researchgate.net/publication/242550698_Designing_the_Malqaf_for_Summer_Cooling_in_Low-Rise_Housing_an_Experimental_Study |archive-date=13 May 2021 |access-date=May 12, 2021|url-status=live}}

In areas that are below freezing at night in winter, snow and ice can be collected and stored in ice houses for later use in cooling. This technique is over 3,700 years old in the Middle East.{{cite book|last=Dalley |first=Stephanie |url=https://books.google.com/books?id=Qe41vgAACAAJ |title=Mari and Karana: Two Old Babylonian Cities |date=2002 |publisher=Gorgias Press |isbn=978-1931956024 |edition=2nd |location=Piscataway, New Jersey |page=91 |oclc=961899663 |author-link=Stephanie Dalley |access-date=2021-05-13 |archive-date=2021-01-29 |archive-url=https://web.archive.org/web/20210129171612/https://books.google.com/books?id=Qe41vgAACAAJ|url-status=live}} Harvesting outdoor ice during winter and transporting and storing for use in summer was practiced by wealthy Europeans in the early 1600s, and became popular in Europe and the Americas towards the end of the 1600s.{{cite journal|last=Nagengast |first=Bernard |date=February 1999 |title=Comfort from a Block of Ice: A History of Comfort Cooling Using Ice |url=https://www.ashrae.org/File%20Library/About/Mission%20and%20Vision/ASHRAE%20and%20Industry%20History/A-History-of-Comfort-Cooling-Using-Ice.pdf |url-status=live |journal=ASHRAE Journal |volume=41 |issue=2 |page=49 |issn=0001-2491 |archive-url=https://web.archive.org/web/20210513150238/https://www.ashrae.org/File%20Library/About/Mission%20and%20Vision/ASHRAE%20and%20Industry%20History/A-History-of-Comfort-Cooling-Using-Ice.pdf |archive-date=May 13, 2021|access-date=May 13, 2021}} This practice was replaced by mechanical compression-cycle icemakers.

{{Main|Evaporative cooler}}

File:Evaporative cooler, CO, IMG 5681.JPG

In dry, hot climates, the evaporative cooling effect may be used by placing water at the air intake, such that the draft draws air over water and then into the house. For this reason, it is sometimes said that the fountain, in the architecture of hot, arid climates, is like the fireplace in the architecture of cold climates. Evaporative cooling also makes the air more humid, which can be beneficial in a dry desert climate.{{cite journal |last1=Bahadori |first1=Mehdi N. |title=Passive Cooling Systems in Iranian Architecture |journal=Scientific American |date=February 1978 |volume=238 |issue=2 |pages=144–154 |doi=10.1038/SCIENTIFICAMERICAN0278-144 |bibcode=1978SciAm.238b.144B}}

Evaporative coolers tend to feel as if they are not working during times of high humidity, when there is not much dry air with which the coolers can work to make the air as cool as possible for dwelling occupants. Unlike other types of air conditioners, evaporative coolers rely on the outside air to be channeled through cooler pads that cool the air before it reaches the inside of a house through its air duct system; this cooled outside air must be allowed to push the warmer air within the house out through an exhaust opening such as an open door or window.{{cite book|last=Smith|first=Shane|url=https://books.google.com/books?id=Onv60-c6iEIC |title=Greenhouse Gardener's Companion: Growing Food and Flowers in Your Greenhouse Or Sunspace |publisher=Fulcrum Publishing |others=Illustrated by Marjorie C. Leggitt |year=2000 |isbn=978-1555914509 |edition=illustrated, revised |location=Golden, Colorado |page=62 |oclc=905564174 |access-date=2020-08-25 |archive-date=2021-05-13 |archive-url=https://web.archive.org/web/20210513150323/https://books.google.com/books?id=Onv60-c6iEIC|url-status=live}}

• Air filter
• Air purifier
• Cleanroom
• Crankcase heater
• Energy recovery ventilation
• Indoor air quality
• Particulates

{{Reflist|colwidth=30em}}

{{Commons category|Air conditioners}}

{{Wiktionary|Cassette air conditioner}}

{{Wikiversity|Refrigeration and air conditioning}}

• {{US patent|808897}} Carrier's original patent
• {{US patent|1172429}}
• {{US patent|2363294}}
• Scientific American, "[https://books.google.com/books?id=6ok9AQAAIAAJ Artificial Cold]", 28 August 1880, p. 138
• Scientific American, "[https://books.google.com/books?id=YIE9AQAAIAAJ&pg=PA84 The Presidential Cold Air Machine]", 6 August 1881, p. 84

{{HVAC}}

{{Home appliances}}

{{Roofs}}

{{Electronic systems}}

{{Authority control}}

Category:1902 introductions

Category:American inventions

Category:Ancient Egyptian technology

Category:Ancient Roman technology

Category:Building automation

Category:Chinese inventions

Category:Cooling technology

Category:Dutch inventions

Category:Gas technologies

Category:Home appliances