Air Conditioning

Air conditioning is the process of controlling the temperature, humidity, air movement and cleanliness of an enclosed space, most often by removing heat from it. Its basic physics rests on thermodynamics: heat does not flow from a cold region to a warm one without work, so a cooling system must move heat against its natural direction using a refrigeration cycle, usually powered by electricity. Modern systems range from small window units to large building-wide plants, and they are central to thermal comfort, to working and sleeping conditions in hot climates, and to processes that need stable environments, such as printing, data processing and medicine. Its history runs from ice-based cooling in the nineteenth century to the 1902 system designed by Willis Carrier to control humidity, and then to the mass adoption of residential and vehicle air conditioning after 1945.

Air conditioning matters for reasons beyond comfort. It shapes how cities are built, how buildings are designed, how electricity grids are planned, and how populations cope with heat. It is also a major source of energy demand and of greenhouse gases, both through electricity use and through refrigerants that leak from equipment. For these reasons, the field has been shaped by international treaties on ozone-depleting and climate-warming substances, by efficiency standards, and by new work on heat pumps and passive cooling.

This entry explains the physical principles and the vapor-compression refrigeration cycle, then compares other cooling methods. It traces the history from early attempts to Carrier's invention and mid-twentieth-century expansion, and it classifies system types, components and refrigerants. The later sections cover the ozone and climate regulation that followed the switch between refrigerants, efficiency measurement, electricity and climate effects, building design, health and economic consequences, and current technological directions and open questions.

Definition and Basic Principles

Air conditioning is the control of the temperature, humidity, air movement and air quality of an enclosed space. Cooling is its best-known function, but the term covers the whole set of conditions. Ventilation only exchanges indoor air with outdoor air, and it does not necessarily change that air's temperature or moisture. Heating adds thermal energy but does not manage humidity or air purity. Modern buildings usually combine these functions in a heating, ventilation and air-conditioning (HVAC) system, and the same equipment often provides both heating and cooling, as a heat pump does.

Temperature alone does not determine whether people feel comfortable. Thermal comfort depends on four environmental variables: air temperature, relative humidity, air speed and radiant heat exchange with surrounding surfaces. Two personal factors, clothing and metabolic rate, also matter. Humidity is central because the human body sheds heat largely by evaporating sweat, and moist air slows that evaporation. Humidity control was in fact the original motive for mechanical air conditioning. Early systems were built to stabilise moisture in factories such as printing works, textile mills and tobacco plants, where dryness or dampness damaged the product. Comfort for people came later.

Cooling systems are governed by the second law of thermodynamics. Heat flows spontaneously from a hotter body to a colder one, and the reverse does not happen without outside intervention. A refrigeration or air-conditioning device therefore has to pump heat from the cool interior to the warmer exterior, and this requires external work, which in most devices is supplied as electrical energy driving a compressor. The minimum work needed is set by thermodynamic limits. An ideal reversed Carnot cycle sets the upper bound on efficiency, and real machines fall well short of it. The electricity input therefore represents a real energy cost, and the energy balance of the building is always part of the design.

Cooling air involves two distinct kinds of heat. Sensible heat changes the air temperature and can be measured with an ordinary thermometer. For dry air at constant pressure the sensible load is roughly $Q_s = m c_p \Delta T$, with $c_p \approx 1.005\ \text{kJ/(kg·K)}$. Latent heat is absorbed or released when water changes phase without a change in temperature. Removing moisture from air means condensing water vapour, and the latent heat of vaporisation of water, about 2.45 MJ/kg near room temperature, is roughly $Q_L = m_w h_{fg}$. Because this figure is far larger than the sensible heat needed to lower the air temperature by a few degrees, latent load can dominate the cooling demand. This is why humid climates, where the air carries a large amount of water vapour, require much more cooling energy than dry climates at the same temperature. A system that cools air quickly without running long enough to condense moisture may also leave a room clammy, which is a common problem with oversized equipment.

Humidity, moving air and radiant heat each shape how much the air temperature must be lowered to achieve comfort. For this reason, air conditioning is best understood as an engineering problem of managing the full thermal and moisture balance of a space, not simply a matter of lowering the thermometer reading.

The Vapor-Compression Refrigeration Cycle

Most room, residential and commercial air conditioners use the vapor-compression cycle. A working fluid called the refrigerant circulates in a closed loop and carries heat from a cool indoor space to warmer outdoor air. Four components make up the loop: the compressor, the condenser, the expansion valve and the evaporator. Because heat is moved against its natural direction, the cycle needs an external energy input, which is the work done by the compressor's electric-motor. This follows from the second-law-of-thermodynamics.

Stage 1: Evaporation indoors

The cycle begins in the indoor coil, the evaporator. There the refrigerant is at low pressure and low temperature, so it is colder than the room air. Heat from the room flows into the fluid, and the refrigerant boils. At this stage the refrigerant is at a lower temperature than its surroundings, so it evaporates and absorbs latent heat of vaporization. Most of the cooling effect therefore comes from the energy absorbed during evaporation rather than from a change in the refrigerant's temperature. This is the same latent-heat principle that governs moisture removal.

Stage 2: Compression

The compressor draws in the low-pressure vapor and raises its pressure. It takes in a low temperature, low pressure refrigerant vapor, and compresses it into a high temperature, high pressure vapor. The rise in pressure also lifts the vapor's temperature well above that of the outdoor air. Electrical work is consumed in this stage, and it is the only stage that requires significant energy input.

Stage 3: Condensation outdoors

The hot, pressurized vapor enters the outdoor coil, the condenser. Because the vapor is much hotter than the outside air, heat flows out of it into the air, and the vapor turns back into liquid. The condenser cools the high-pressure and high-temperature gas allowing it to condense to a high-pressure liquid by transferring heat to a lower temperature medium, usually ambient air. The heat removed here is the sum of the indoor heat absorbed earlier plus the work added by the compressor. This is why an outdoor unit feels hot in operation.

Stage 4: Expansion

The high-pressure liquid then passes through a restriction, such as a thermostatic expansion valve or a capillary tube. The restriction lowers the pressure sharply and returns the refrigerant to the low-pressure state needed for evaporation. The refrigerant leaves the throttle valve as a liquid vapor mixture, typically in proportions of around 75 % and 25 % respectively. The flash vapor produced here cools the mixture, so it enters the indoor coil cold enough to absorb heat. The valve also meters how much liquid reaches the evaporator.

Pressure, boiling point and a single working fluid

The cycle works with one fluid because the refrigerant's boiling point depends strongly on pressure. At low pressure, the liquid boils at a temperature below indoor air, so it absorbs heat from the room. At high pressure, the same fluid condenses at a temperature above outdoor air, so it releases heat outside. Common refrigerants such as R-134a boil below 0 °C at atmospheric pressure, which is why the evaporator pressure is set well below the atmospheric level. The compressor and expansion valve together maintain the pressure difference that separates the cold and hot sides of the loop.

Coefficient of performance

The efficiency of the cycle is expressed as the coefficient of performance (COP), the ratio of cooling delivered to the work put in:

Here $Q_{\text{cool}}$ is the heat removed indoors and $W$ is the compressor work. Most air conditioners have a COP of 3.5 to 5. The theoretical ceiling is set by the Carnot limit, $\text{COP}_{\text{Carnot}} = T_c / (T_h - T_c)$, where both temperatures must be expressed on an absolute scale. Both temperatures must be measured using a thermodynamic temperature scale based at absolute zero such as Kelvin or Rankine.

The COP falls as outdoor temperature rises, because the temperature gap between the condenser and evaporator widens. A larger gap means a greater pressure ratio across the compressor. In experiments on a split-unit air conditioner, researchers found that the system cooling capacity decreases as outdoor air temperature increases, mainly due to the decrease in the cooling effect resulting from increasing the pressure ratio, and the compressor power increased. The result is that the air conditioner delivers less cooling while using more electricity on the hottest days, which is an important point for energy planning.

Reversing the cycle for heating

The same loop can run in reverse to supply heat. A heat-pump uses a reversing valve to swap the roles of the two heat exchangers. In the heating mode, the refrigerant flow through the system is essentially reversed. The indoor heat exchanger becomes the condenser and releases heat into the environment to be conditioned, and the outdoor heat exchanger serves as the evaporator. Heating efficiency follows the same thermodynamic logic, but it is sensitive to cold weather. In one example, a heat pump using ambient air as a heat source may have a coefficient of performance of 3.0 or 4.0 at an outside temperature of 10° C, but this can drop to 1.0 when the outside temperature drops to −18° C. This link between cooling and heating hardware is why reversible systems are increasingly common in temperate climates.

Other Cooling Methods

Vapor compression is the dominant technology, but several other methods cool air or remove moisture without a mechanical compressor, either by exploiting evaporation, by running on heat, by using sorbent materials, or by means of electrical junctions. Each has a narrow range of conditions where it performs well.

Evaporative cooling

Evaporative, or "swamp," coolers lower air temperature by evaporating water into the airstream. Water absorbs a large amount of heat when it changes to vapor, so dry air can drop noticeably in temperature with very little energy input, mainly for a fan and a water pump. The achievable cooling depends on the wet-bulb depression, the gap between dry-bulb and wet-bulb temperature, which is large only when relative humidity is low. In arid regions the process can save considerable energy compared with compressor-based cooling. In humid regions it works poorly. Direct evaporative units add moisture as they cool, so the room can become muggy and uncomfortable, and the air leaving the unit approaches the dew point of the incoming air rather than a comfortable temperature. Indirect and two-stage designs first pre-cool air through a heat exchanger that is itself chilled by evaporation, which avoids adding moisture in the first stage and allows a better result, though there is still a practical limit in humid climates. Pads must also be cleaned or replaced regularly.

Absorption refrigeration

Absorption systems replace the mechanical compressor with a thermochemical cycle driven by heat. A refrigerant is absorbed into a liquid, which is then heated to drive the refrigerant back out as vapor, so the thermal energy input, rather than shaft work, does the work of compression. The most common air-conditioning pair is lithium bromide and water: water is the refrigerant, and the lithium bromide salt solution, which has a strong affinity for water vapor, is the absorbent. Because water freezes, this pair suits comfort cooling above 0 °C. For industrial refrigeration that must reach sub-zero temperatures, ammonia and water is used instead, with ammonia as the refrigerant. The heat source can be steam, hot water, a direct gas burner, waste heat from engines or industrial processes, or solar and geothermal heat. Absorption chillers have lower coefficients of performance than electric compressor machines, so they make economic sense mainly where heat is cheap or otherwise wasted, such as in combined heat and power plants.

Desiccant systems

Desiccant systems remove moisture directly rather than by lowering temperature until water condenses. A hygroscopic material takes up water vapor from the air, and the material is then regenerated by heating it so that the water is driven off to a separate airstream. Solid desiccants include silica gel, activated alumina, zeolites and molecular sieves, often coated on a rotating wheel so that one sector dries incoming air while another is regenerated. Liquid desiccants, usually concentrated salt solutions such as lithium chloride or lithium bromide, can be sprayed through air to absorb moisture and then reconcentrated by heating. Because latent load is removed separately from sensible cooling, desiccant units are often paired with an evaporative cooler or a conventional cooling coil. This pairing is especially useful in humid climates, where separating dehumidification from temperature control can reduce the work a compressor would otherwise do.

Thermoelectric (Peltier) cooling

Thermoelectric cooling uses the Peltier effect, in which an electric current driven through a junction of dissimilar semiconductor materials transfers heat from one side to the other. The devices have no moving parts, are silent, and can be switched in polarity to heat instead of cool. They are used for small enclosures such as portable coolers, instrument housings, and localized cooling of electronic components. Their drawback is efficiency. Their coefficient of performance is generally well below that of vapor-compression machines, and the heat they reject must be removed from the hot side, which adds further load. This makes them unsuitable for cooling whole buildings, which is why they remain a niche technology.

Pre-industrial and passive techniques

Long before mechanical cooling, builders in hot regions used a set of passive strategies. Thick masonry or earth walls delayed heat flow so that interior temperatures peaked after the hottest part of the day. Shading through deep eaves, courtyards, latticed screens and orientation reduced solar gain. Night ventilation let cool air flush out stored heat. In Iran, Egypt and Pakistan, wind towers (badgirs) captured breezes and channelled them down into living spaces, and some were combined with evaporative pools or wet materials. Underground qanat water systems and ice-storage structures such as the Persian yakhchal, which stored winter ice in insulated pits for summer use, extended these approaches further. Modern designers still borrow these ideas through thermal mass, night-purge ventilation, shading devices and passive ventilation design, often reducing cooling loads before any equipment is specified.

Early Attempts: Ice, Evaporation and Mechanical Refrigeration

Ice, ice-houses and evaporative cooling

Before machines, cooling depended on what nature supplied. Civilizations relied on natural ice cut from lakes and mountains, a practice that was costly, unreliable and geographically limited. In winter, ice was cut from frozen ponds and packed into insulated ice-houses, where sawdust or straw slowed melting so that some stock lasted into summer. Similar stores existed in Persia and other hot regions, where ice pits and cellars were used to keep food and drink cold.

Evaporation offered a second route. Porous clay vessels, wet mats and cloth screens cooled rooms in dry climates because water drawing heat from the air lowered its temperature. In South Asia, hand-pulled fans were hung over rooms with screens of wet grass, so that moving air passed over water before it reached the occupants. The principle is the same one later used in evaporative coolers, which depend on evaporation and therefore work poorly in humid air.

In the nineteenth century, natural ice became a traded commodity. Boston merchant Frederic Tudor shipped cut ice to Cuba and planned to expand to India, which made him a direct competitor to any machine-made ice.

Cullen, Faraday and the physics of evaporation

The scientific basis of mechanical cooling came from Scotland. In 1748, William Cullen at the University of Glasgow pumped air out of a vessel containing diethyl ether. The ether boiled away at room temperature, drawing so much heat from its surroundings that the water around it froze into a crust of ice. The experiment showed that lowering the pressure lowers the boiling point of a liquid, so that the liquid can absorb latent heat from its surroundings. Cullen built no practical machine, and his result remained largely unused for decades. Some accounts also date a small refrigerating apparatus to 1755, so the exact chronology varies between sources.

Early in the 1820s, Michael Faraday showed that gases could be turned into liquids. He liquefied ammonia and other gases by applying high pressures and low temperatures. Liquid ammonia became one of the most important working fluids in later refrigeration because it boils at a low temperature and carries a large amount of heat per kilogram when it evaporates.

Perkins's 1834 machine and the nineteenth-century working fluids

The step from laboratory demonstration to working machine came with Jacob Perkins, an American engineer working in London. Perkins is credited with the first patent for the vapor-compression refrigeration cycle, assigned on August 14, 1834. His apparatus was a closed loop. It used diethyl ether as the refrigerant, with a hand-cranked compressor and a condenser cooled by water. The ether boiled in an evaporator, absorbing heat from the surrounding water, and the compressor drew off the vapor and raised its pressure so that it could condense again. Because the device was a closed cycle, it could operate continuously.

Other designers quickly experimented with different fluids. Alexander Twining, an American engineer, received a British patent in 1850 for a vapor-compression system using ether, ammonia and carbon dioxide. The Scottish-born James Harrison, working in Australia, was the next major figure. He built a mechanical ice-making machine in 1851 near Geelong in Victoria, followed by his first commercial ice-making machine in 1854, and by 1861 a dozen of his systems were in operation, including in breweries and meat-packing houses. His patent of 1856 covered ether, alcohol or ammonia as working fluids. These early fluids were chosen for their boiling behaviour rather than for safety, a problem that would shape later refrigerant development.

John Gorrie and the ice machine of Florida

John Gorrie, a physician in Apalachicola, Florida, was motivated by public health. He believed that cooling the air could ease the suffering of patients with yellow fever and malaria. His machine did not rely on evaporating a volatile liquid. Instead, it compressed air, cooled the compressed air in water-cooled coils, and then released it into a lower-pressure chamber, where its temperature dropped sharply. The cold air chilled a brine bath, and water-filled containers placed in the brine froze into ice blocks.

Gorrie's work progressed through several stages. He submitted his patent petition on February 27, 1848, and by April of that year had one of his ice machines built in Cincinnati, Ohio. He obtained a London patent in 1850 and U.S. Patent No. 8080 on May 6, 1851. Accounts of his prototype date to the early 1840s, which is why some sources give an earlier date for his invention.

Gorrie's machine failed commercially for several reasons. Because of the limitations of nineteenth-century engineering knowledge, his machine was bulky, inefficient and difficult to operate. His partner died, which halted the venture. Opposition from the ice trade also mattered: his plan to manufacture the machine for the general public was reportedly thwarted by a smear campaign led by Frederic Tudor, along with the death of his partner. Humiliated by criticism and financially ruined, Gorrie died in seclusion in 1855.

Willis Carrier and the Birth of Modern Air Conditioning

Modern air conditioning is usually traced to Willis Haviland Carrier, a Cornell-trained engineer who joined the Buffalo Forge Company in 1901. Cooling was already practised in various forms by then, but the problem Carrier was given in 1902 concerned humidity rather than temperature.

The commission came from the Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn. The printing process required precise humidity control because paper would expand and contract with moisture changes, throwing off color registration on multi-pass print jobs. Carrier's answer was a system installed that summer. Chilled coils, fans, ducts, heaters and humidification pipes held the plant at a constant 55 percent humidity year-round. Some accounts date the design differently: one source gives a finalised design date of July 17 of that year, while another states that the Buffalo Forge Company installed the first scientific system at the plant in 1903. The 1902 date is the one most commonly cited.

Carrier's central insight was that moisture, not only heat, had to be governed. Authorities in the field came to recognise that air conditioning must perform four basic functions: control temperature, control humidity, control air circulation and ventilation, and cleanse the air. Dew point, the temperature at which water vapour begins to condense as air cools, became the key variable. Carrier found that by passing air through a spray of water he could set its dew point, and by 1906 he had noted that "constant dew-point depression provided practically constant relative humidity," a relationship later called the law of constant dew-point depression. He built an automatic control system on this principle and filed a patent claim for it in 1907.

This work turned air conditioning into a calculable discipline. On 3 December 1911 Carrier presented his "Rational Psychrometric Formulae" to the American Society of Mechanical Engineers. This document tied together the concepts of relative humidity, absolute humidity, and dew-point temperature, thus making it possible to design air-conditioning systems to precisely fit the requirements at hand. Earlier he had developed the psychrometric chart, the ASHRAE-style psychrometric chart, shown here, was pioneered by Willis Carrier in 1904. Engineers still use it to read the relationships between temperature, moisture content and enthalpy, and it underpins the sizing of cooling coils and dehumidifiers. His first patent, "Apparatus for Treating Air," was granted on 2 January 1906, and Carrier had invented the world's first spray-type air conditioning equipment, able to both wash and humidify or dehumidify air.

Carrier's career moved from employee to entrepreneur through the company that employed him. He had worked at Buffalo Forge since 1901, and by 1908 a Carrier-named subsidiary existed under that parent company. With the onset of World War I in late 1914, the Buffalo Forge Company, where Carrier had been employed for 12 years, decided to confine its activities entirely to manufacturing. Rather than abandon the new field, seven young engineers pooled together their life savings of $32,600 to form the Carrier Engineering Corporation in New York on June 26, 1915. The founders included Carrier and J. Irvine Lyle, who had earlier commissioned the Brooklyn work. The Carrier Engineering Corporation was a center of both research and manufacturing. The firm later settled in Newark, New Jersey, and was renamed Carrier Corporation in 1930.

Adoption spread first through industry. Carrier's systems served printing, candy and textile manufacturing, and later health care. Commercial use then reached public venues: the market for air conditioning developed first in public venues, such as department stores, sports venues, and movie theaters during the 1920s. Domestic use came later. By 1914, the first air conditioner was installed residentially. Yet widespread use in American houses came much later, and central air did not become common in American houses until the 1960s and 1970s.

The term "air conditioning" also took shape in this period. It is often credited to the textile engineer Stuart W. Cramer, who used it in a 1906 patent application, although the sources consulted for this section did not confirm that attribution. By the 1920s the phrase described the whole field of controlling indoor air, and Carrier's definition, which combined cleaning, temperature, humidity and circulation, helped fix its meaning.

Mid-Twentieth-Century Expansion

By the 1920s, cooling had moved beyond the factory floor into places where the public gathered. Public places such as movie theaters, department stores, and restaurants were not air-conditioned in any significant number until that decade. Office buildings and homes took even longer to receive mechanical cooling. Cinemas were the most important early customers. The motion-picture business gave comfort cooling its biggest boost. The first theatre systems were crude. They were essentially heating plants with refrigeration added, and floor-level outlets left upper seating muggy while lower rows turned chilly. Carrier's engineers improved the design in 1922 at Grauman's Metropolitan Theater in Los Angeles, where a top-down arrangement delivered cool air from higher vents. Dating is disputed. Carrier's own account places the public debut of the centrifugal chiller at New York's Rivoli Theater in May 1922, while a widely cited timeline dates the Rivoli installation to 1925.

Department stores followed. Detroit's J.L. Hudson's, the city's largest store, equipped itself in 1924 with three centrifugal units rated at 195 tons each. A later account places the store's first cooled summer in 1926, and its advertising claimed that the basement ran 8 to 12 degrees colder than the street. Retailers in other cities soon adopted the same approach, and summer sales rose quickly after installation. Through the 1920s and 1930s, the systems spread into offices and even railway carriages, where cooling was credited with helping staff remain productive in summer. Cool interiors turned cinemas and shops into summer destinations, changing how people used city centres.

The room air conditioner came later and was aimed at a different market. A room unit was not invented until the 1930s, and the residential sector did not adopt such units on a large scale until the 1950s. The first window-mounted design was developed in 1931 by two engineers, H.H. Schultz and J.Q. Sherman. The main obstacle was the compressor. Piston-driven machines were the norm, which made equipment bulky and made air conditioners and refrigerators unaffordable to most homeowners. In 1941, Westinghouse sold what were described as the first hermetically sealed, compact, self-contained window air conditioners. A collector's site describes the Servel "Supermetic" line, made soon after the Second World War, as the first hermetic compressor design mass-manufactured for the window-unit market. Sealed compressors of this kind were cheaper to build and easier to package, which is what allowed home use to become affordable.

The postwar boom rested on this manufacturing shift. By the 1950s, the industry had found ways to apply rotary compressor technology in hermetic, mass-produced equipment. Manufacturers advertised beyond trade journals, in home magazines aimed at postwar consumers. A 1953 trade report said room units were being bought by nearly all income groups, yet output outran demand: more than a million units were made that year, while roughly 800,000 were sold. Suburban house design also changed. Mechanical cooling made deep porches, wide eaves and high ceilings less necessary, and these features became rarer in new construction. Car air conditioning, which spread in the same period, is discussed in the transport section of this entry.

Migration patterns shifted with the technology. The Sun Belt, which stretches across the southern and southwestern portions of the country from Florida to California, was the main beneficiary. From about 1945 through 1975, an average of 30 million individuals each year changed residence, usually moving to one of the Sunbelt states. During this time, the Sunbelt states nearly doubled their population, while the industrial Northeast shrank. Air conditioning made summers more bearable and allowed more manufacturing and industry to locate in the Sun Belt. Cities such as Houston, Phoenix, Las Vegas and Miami grew rapidly after the war, which made air conditioning central to the region's urbanization and to the growth of its metropolitan areas. Historians do not credit cooling alone. The shift also drew on cheap air travel, automobiles, the interstate system, and the growth of air conditioning, and milder winters are commonly cited as well.

Types and System Configurations

Air-conditioning equipment is usually classified by how many rooms it serves and how it moves refrigerant and air. The simplest form is the window or through-wall unit, in which the compressor, coils and fans share one cabinet that fits into a window opening or a wall sleeve. Installation is quick and cheap, but the unit blocks an opening, transmits noise into the room it cools, and is generally less efficient than split equipment. Portable units place the same kind of cabinet on casters, so they must vent hot exhaust through a hose, and they draw warm air from the room they are cooling, which further reduces their performance.

Split systems separate the outdoor condenser from the indoor evaporator. It consists of two main components: an indoor unit that delivers cool air and an outdoor unit that expels heat. The two halves are joined by refrigerant lines and control cabling, which requires skilled installation, but the compressor's noise stays outside. Multi-split or ductless (mini-split) systems extend this arrangement. Several indoor units can be connected to a single outdoor unit, which removes the need for ductwork in many homes. The wiring between them determines flexibility: in a two-pipe multi-split, all indoor units must operate in the same mode, whereas a three-pipe arrangement lets one room cool while another heats. Hence the trade-off between cost and independent control.

Central ducted systems place a fan and evaporator coil, the air handler, in a closet, basement or attic, and send conditioned air through a network of ducts to registers in each room, while the compressor and condenser sit outdoors. Packaged rooftop units take the opposite approach for commercial buildings: all major components sit in one weatherproof cabinet on the roof, which supplies conditioned air through ductwork and avoids a separate mechanical room. In the United States, two-thirds of households use central AC or a central heat pump as their main AC equipment.

Large buildings usually rely on chillers rather than dozens of local units. A chiller cools water, which is pumped through a closed loop to coils in air handlers or fan-coil units on each floor. Air-cooled chillers release heat directly to the ambient air, whereas water-cooled chillers reject it through a cooling tower. Centralising the refrigeration plant keeps compressors and noise away from occupied space. Variable refrigerant flow (VRF) systems offer a middle path for mid-size buildings. A single outdoor unit with a variable-speed compressor serves several indoor units, and can adjust the amount of refrigerant flowing to each indoor unit based on the cooling or heating needs of each space. Piping length is limited, and some heat-recovery designs can heat some zones while cooling others.

Reversible heat pumps are the fastest-growing category. A reversing valve lets the same refrigerant loop either cool a space or heat it, so the same unit both heats and cools a space. Heat pumps first became popular in mild climates such as the US South, and in recent years their cold-weather performance has improved, and they've caught on in more frigid regions, too. In the United States, heat pumps' monthly share of residential cooling equipment sales rose from 33% to 47% over the past decade, though these counts exclude window units, a point the same data source notes.

Components and Materials

Compressors

The compressor circulates refrigerant around the loop. It draws refrigerant in as a gas from the evaporator and delivers it at higher pressure to the condenser. Compressor designs differ mainly in how they compress the gas. Reciprocating compressors use a back-and-forth motion, while rotary compressors, including rotary vane, scroll, screw and centrifugal types, use rotation.

Reciprocating compressors work like an engine: a piston moves inside a cylinder. They have the longest service history and remain common in refrigeration equipment. Scroll compressors use two spiral-shaped scrolls: one is fixed, and the other orbits around it. Because there are no pistons or valves, they run more quietly and have fewer parts that can wear, which is why they are favored in premium residential and light commercial systems. Rotary units are generally quieter and more efficient than reciprocating ones, and they are better suited to smaller capacities. Screw compressors rely on two large helical rotors (screws) that mesh together and are used in large commercial buildings. At the top end, centrifugal compressors are used in the largest HVAC systems, employing centrifugal force and an impeller to compress refrigerant. These units are powerful and well-suited to extensive cooling demands.

Sizing matters as much as type. A system that's too small won't meet your needs, while one that's too large wastes energy. Note that manufacturers' classifications vary; some catalogues list rotary vane separately, and others group it under rotary.

Heat Exchangers, Expansion Devices and Fans

The evaporator and condenser are the two heat-exchange coils at the boundaries of the refrigeration loop. In the evaporator, the working fluid receives heat from airflow and produces a conditioned airflow that is delivered by a duct subsystem to the conditioned space. Heat moves from room air into the refrigerant, which boils (see evaporation). In the condenser, the process reverses: as the hot, high-pressure refrigerant flows through the condenser coil, it releases heat to the outside air, causing the refrigerant to cool and condense into a liquid. Fans are essential to both coils. A condenser fan may blow outside air through the condenser to help cool the working fluid flowing there through. In many compact window units, a single motor drives both fans from opposite ends of a single drive shaft.

Between the two coils sits the metering device. The expansion device is typically a capillary tube or fixed orifice in small capacity or low-cost air conditioning systems, and a thermal expansion valve (TXV or TEV) or electronic expansion valve (EXV) in larger units. A capillary tube is simple but offers no adjustment, so it depends on the correct refrigerant charge, whereas a thermal expansion valve allows for a greater variation in refrigerant charge without impacting performance. A TXV uses a sensing bulb on the suction line, and it uses that temperature information along with the pressure of the refrigerant in the evaporator to modulate (open and close) the valve. Electronic valves replace the mechanical bulb with an actuator driven by control signals. Metering the flow correctly is important because too little refrigerant reduces cooling, while too much can flood the compressor, potentially leading to damage.

Filters, Dehumidification and Humidity Control

Filters are the main barrier against particles. Their performance is commonly rated by MERV (Minimum Efficiency Reporting Value) numbers; one manufacturer suggests MERV 8 for basics, MERV 11 for most homes, or MERV 13 for allergy and respiratory concerns. Filters also need regular replacement, and one manufacturer's guidance is every 3-6 months. A clogged filter restricts airflow and can reduce both comfort and filtration performance.

Dehumidification is a by-product of cooling. When air passes over a coil colder than its dew point, moisture condenses on the surface and drains away, which is why a cooling system also lowers indoor humidity (see latent heat). Humidity is not a single quantity, and controls must account for this. Relative humidity is not a measure of the absolute amount of water vapor in the air. The same relative humidity can correspond to very different moisture loads at different temperatures, so controls based only on relative humidity can overshoot or fluctuate. Where cooling alone cannot hold moisture low, systems may use a dedicated dehumidifier, which can be installed as a standalone unit or connected to your existing ductwork to use with your HVAC system fan. Some designs reheat air after cooling so that it is dried without overcooling the room.

Fresh air is controlled with dampers. Mixing outdoor air into the return flow improves indoor air quality by diluting polluted and/or stale indoor air, but it adds load to the system and must be metered by a damper, which is part of the controls discussed below.

Controls and Sensors

The simplest control is an on-off thermostat. The room temperature is sensed, and the compressor is switched on when cooling is needed. It starts working when the unit needs to provide cooling, and is usually activated via temperature control systems. Such on-off cycling wastes energy and produces uneven temperatures, because the compressor runs at full output whenever it operates.

Variable-speed or inverter drives address this. By changing the frequency supplied to an electric motor driving the compressor, the system matches its output to the instantaneous load, reducing start-stop losses and holding temperature more steadily. Larger buildings extend this logic through building automation systems, in which sensors measure temperature, humidity, carbon dioxide and pressure; a controller compares these readings to setpoints and adjusts fan speed, damper position, valve opening and compressor staging. This is a classic feedback arrangement studied in control theory, and advanced automation and machine-learning methods are increasingly used to tune setpoints and schedule operation against occupancy and electricity prices.

Refrigerants and Their Evolution

Early working fluids

Mechanical refrigeration in the nineteenth century used whatever fluids performed well thermodynamically, with little regard for safety. Between the late 1800's and 1929, mechanical refrigeration used natural refrigerants such as methyl chloride, ammonia and sulfur dioxide. The first working refrigerator was built by American inventor Jacob Perkins, who used ether as the refrigerant. Ammonia remained the workhorse of large industrial plants because of its efficiency, but it is toxic and can be corrosive at leak concentrations. Sulfur dioxide is a severe respiratory irritant, and methyl chloride is both flammable and toxic. A methyl chloride leak in a hospital refrigerator in Cleveland in 1929 killed more than 100 people, and it directly motivated the search for a safer alternative.

Chlorofluorocarbons

The search was organized at General Motors in the late 1920s, when a research team was formed by Charles Franklin Kettering at General Motors to find a replacement for the dangerous refrigerants then in use, such as ammonia. The team was headed by Thomas Midgley Jr. The target properties were clear: nonflammable, non-toxic, stable and with a boiling point between 0 and -40C. The answer was dichlorodifluoromethane, sold as Freon-12. It was odorless, non-toxic when inhaled in moderate amounts, non-flammable, and possessed a boiling point of -29.8°C. Midgley publicly demonstrated its safety in 1930 by inhaling the vapour and using it to blow out a candle. GM and DuPont appointed Midgley vice president of Kinetic Chemicals Inc., their joint venture to market CFCs. Adoption was rapid: by 1978 17.6 million American homeowners had central air conditioning, and 25.1 million owned room units, nearly all of which used CFCs. Because CFCs are chemically stable, they remain intact long enough to reach the stratosphere, which is why their later effects on the ozone layer were so serious.

HCFCs and HFCs

Once CFCs were linked to ozone depletion, the industry moved first to hydrochlorofluorocarbons (HCFCs) such as R-22. HCFCs contain hydrogen, so they break down more readily in the lower atmosphere, but they still contain chlorine and retain a smaller ozone-depleting effect. R-22 has a global warming potential (GWP) of about 1,810. Hydrofluorocarbons (HFCs) contain no chlorine, so they do not deplete ozone. They are nonflammable and of low toxicity, and their thermodynamic behaviour is close enough to earlier fluids that they could be used in similar equipment. Their chief drawback is climate impact: R-410A (Puron) has a GWP of 2,088, while R-32 has a GWP of 675. These figures are why HFCs later came under the same international regulatory pressure as earlier substances, which is covered in the section on regulation.

Natural refrigerants and low-GWP blends

Natural refrigerants have returned to favour because of their low climate impact. CO2 (or R-744) has a GWP of 1 and an ozone depletion potential (ODP) of 0. R-744 makes an excellent replacement for HFCs due to its non-flammable and non-explosive qualities, but it works at high pressure. CO2 must be used in the transcritical state on the high-pressure side of the system to allow heat rejection to ambient air. Pressures are often in excess of 100 bar, again resulting in an energy penalty and also a significantly higher capital cost. Propane (R-290) has a GWP of 3, but hydrocarbons are highly flammable. Safe use therefore often requires small refrigerant charges or a secondary circuit, which reduces energy efficiency and increases costs, or severely limits the maximum cooling duty. Ammonia has no GWP but remains toxic. Its supporters argue that it is energy-efficient; one industry body claims that ammonia is 3–10% more efficient than competitive refrigerants.

Between the natural fluids and the older HFCs lie newer low-GWP blends based on hydrofluoroolefins (HFOs). R-454B, for example, has a GWP of about 466 and is an HFO blend. These fluids are mildly flammable, and their mild flammability has slowed their adoption, as they have not yet been fully approved in building code regulations in most states. The choice among fluids therefore balances flammability, toxicity, operating pressure, efficiency and climate impact, and no single option is optimal for every application.

Ozone Protection and Global Refrigerant Regulation

From atmospheric chemistry to the 1987 treaty

Refrigerant regulation began as a question in atmospheric chemistry. In 1974, Sherwood Rowland and Mario Molina argued that chlorofluorocarbons (CFCs) could persist long enough to reach the stratosphere, where they would release chlorine that attacks ozone (Rowland and Molina proposed that chlorofluorocarbons (CFCs) were stable enough to reach the stratosphere). The idea met resistance; industry representatives at the time described the hypothesis as far-fetched. Confidence grew after the British Antarctic Survey reported a "hole" in the ozone shield over Antarctica in 1985, and the two chemists later shared the 1995 Nobel Prize in Chemistry with Paul Crutzen (Along with Dutch chemist Paul Crutzen, Rowland and Molina were awarded the 1995 Nobel Prize for Chemistry).

The Montreal Protocol was concluded in Montreal in September 1987 and went into effect on New Year's Day 1989. Its first commitments were modest: parties were to cut their CFC use by 50 percent by 1996, measured against 1986 national baselines. Later revisions tightened the treaty toward a full phase-out of production. The protocol was strengthened to require an eventual worldwide phase-out of CFCs and other ozone-depleting chemicals. Support for developing countries was built in from the start, and the agreement provided money to developing countries to help them phase out these substances.

The Kigali Amendment and HFCs

Hydrofluorocarbons (HFCs) became the main substitutes for CFCs because they leave the ozone layer intact, but they are strong greenhouse gases. Their warming potential can be many times that of carbon dioxide (they are extremely potent greenhouse gases with global warming potentials that can be many times higher than carbon dioxide). Negotiators therefore brought them under the treaty. Parties met in Kigali, Rwanda, in October 2016, and 197 countries adopted an amendment to phase down HFCs under the Montreal Protocol. The target is ambitious: countries committed to cut the production and consumption of HFCs by more than 80 percent over the next 30 years. The amendment entered into force on 1 January 2019, after the threshold was met on 17 November 2017, when it was ratified by 20 parties.

The schedules differ by country group. For developed (non-Article 5) parties, the baseline is average HFC consumption in 2011–2013, plus a fraction of their HCFC baseline. Their cuts began in 2019, reaching 40 percent by 2024 and 85 percent by 2036 (This group began its phase-down in 2019. They must achieve a 40% reduction by 2024 and an 85% reduction by 2036.). The largest developing group uses a 2020–2022 baseline, must freeze HFC consumption by 2024 and achieve an 80% reduction by 2045. A smaller second developing group, which includes Bahrain, India, Iran, Iraq, Kuwait, Oman, Pakistan, Qatar, Saudi Arabia, and the United Arab Emirates, has a later baseline and must freeze consumption in 2028 and reach an 85% reduction target by 2047.

National implementation and its effects

Each country carries the amendment into domestic law. All countries must establish a system for licensing the import and export of controlled HFC substances. Because group baselines differ, national authorities translate the totals into permitted volumes, which industry guidance describes as differentiated baselines and phase-down schedules. For manufacturers, capped volumes push product lines toward lower–global-warming-potential refrigerants. For technicians, servicing and certification requirements change with the refrigerants they handle. For consumers, equipment and service prices may shift. The sources I consulted did not quantify these effects for particular markets, so national rules should be checked directly.

Debates over pace, cost and benefit

Pace is contested mainly along the developing-country divide. India's government said the agreement recognises the development imperatives of high-growth economies, and under the schedule Indian reductions start only after developed countries have cut their consumption by 70 percent (This means India will start reducing its HFC consumption when the developed countries would have reduced their consumption by 70 per cent.). Critics argue for faster action. One analysis notes that evidence suggests that the Amendment has contributed to reducing HFC emissions by 20% relative to projections, but more action is needed.

The sources I consulted did not give a sourced estimate of retrofit or replacement costs, so that part of the debate cannot be assessed from them. Climate benefits are better documented. Full implementation of the Kigali Amendment could avoid up to 0.4°C of warming by 2100. Modelling indicates that without any controls, HFC emissions were projected to contribute 0.28-0.44 °C to global surface warming by 2100, compared to a contribution of about 0.04 °C by 2100 with Kigali Amendment controls. Estimates vary with implementation speed, and some analyses suggest faster implementation that leapfrogs over HFCs during the ongoing phaseout of HCFCs can sweep up the additional 0.1oC to achieve the full 0.5oC. Meanwhile, global HFC emissions have continued to grow rapidly, and researchers note that leaking existing equipment remains a separate source that the amendment does not fully address (tackling ODS and HFC emissions from leaking banks of equipment and products is identified as a further step).

Energy Efficiency and Performance Standards

Performance metrics

Air conditioner efficiency is expressed as a ratio of useful cooling to electrical input. The energy efficiency ratio (EER) is a single-point measure. In the United States, it is calculated by dividing the cooling capacity in BTUs per hour by the power input in watts, so US figures carry the unit BTU/Wh. EER measures efficiency at a single operating temperature, specifically when the outdoor temperature is 95 degrees F. Where both quantities are in watts, as in IEA and most international tables, the same ratio is dimensionless:

The seasonal energy efficiency ratio (SEER) addresses the fact that a single temperature says little about annual energy use. The SEER rating of a device is the cooling output during a typical cooling season divided by the total electric energy input during the same period. The SEER rating is tested and calculated at a variety of outside temperatures ranging from 65 to 104 degrees Fahrenheit. Part-load operation is also averaged in, which is why SEER is designed to provide a measure of efficiency specific to climates in the middle of the United States.

Test procedures have changed. Under the revised US procedure, SEER2, ratings include external static pressure in testing, and SEER2 ratings on units manufactured after 2023 reflect real-world efficiency conditions and run about 0.5 points lower than older SEER ratings on similar systems. Other jurisdictions use their own seasonal indicators. Malaysia, for example, rates split units with a cooling seasonal performance factor in Wh per Wh, and the MEPS requirement is equivalent to its 2-star labelling programme, set at 3.10 for units below 4.5 kW and 2.90 for units below 7.1 kW. The European Union and Japan likewise use seasonal methods of their own, so figures from different markets are not directly comparable.

Minimum standards and labelling

Governments set floors through minimum energy performance standards (MEPS), usually paired with labels that grade each model. In the United States, legislation passed in 1987 and taking effect in 1992 required a minimum SEER rating of 10. Current rules are far stricter in hot regions: air conditioners sold in the Southeastern U.S. must be rated at 14.3 SEER2 or higher, with high-efficiency variable-speed central systems reaching 21 SEER2 or more.

Thresholds differ between countries because climate, electricity prices and product mix differ. One widely cited guideline puts an efficient room unit in milder climates at an EER of about 9, while for hotter climates this should be over 10. The IEA also reports that ACs sold in Japan and the European Union are typically 25% more efficient than those sold in the United States and China. Nonetheless, policy coverage has grown rapidly. The IEA counts more than 90 countries with MEPS for air conditioners and more than 95 with labelling policies, and MEPS and labelling now cover more than 86% of global space cooling energy consumption in the residential sector.

The efficiency gap

The efficiency gap is the distance between what the best available equipment can achieve and what is actually sold. The efficiency of today's best available technologies for air conditioners is considerably higher than average installed efficiencies. The gap is widest in equipment bought on price. Commercial packaged rooftop units illustrate the problem: they have experienced little to no efficiency improvement since 1992, when the Energy Policy Act of 1992 imposed federal minimum standards. The majority of units sold have energy efficiency ratios at or just above the current national minimum. A 2015 shipment analysis found that small air-cooled commercial packaged air conditioners rated 14 IEER and higher represented just 3% of the market. Regional sampling shows the same pattern: only 0.15% of rooftop units sold in the Northeast and Mid-Atlantic regions met high-performance efficiency levels.

Proposed remedies combine stricter MEPS with labelling, consumer information and financial incentives. Through stringent minimum energy performance standards and other measures such as labelling, the IEA estimates that the average energy efficiency of the stock of ACs worldwide could more than double between now and 2050. The same analysis suggests that efficiency improvements could cut the energy growth from AC demand in half through mandatory energy performance standards. Individual governments are acting on this. China's 2022 update to its room air conditioner MEPS allows for a transition from fixed-speed ACs to the more efficient variable-speed models.

Energy use per unit of cooling

The energy cost of cooling depends heavily on the equipment. Packaged rooftop units alone have been estimated to use on the order of 76 billion kWh annually in the US, at a cost of roughly $5.6 billion. Gains since the 1990s have come mainly from better compressors, better heat exchangers and better controls. Variable-speed compressors are central to this shift. Unlike single-speed designs, they can modulate their output from 20% to 100% capacity, matching cooling demand precisely. Running closer to the load, rather than cycling fully on and off, reduces wasted work. Larger or better-designed evaporator and condenser coils lower the temperature lift the compressor must overcome, and electronic controls and building automation align output with occupancy and outdoor conditions. One manufacturer's comparison claims that newer compressors can be twice as efficient as units from just 12–15 years ago. Such figures come from industry sources and depend on the models compared, so they should be read as indicative rather than as independent measurements.

Electricity Demand, Grids and Peak Load

Cooling has become one of the fastest-growing uses of electricity, although published shares depend on the year and on what is counted. The IEA's figure for air conditioners and electric fans is that they draw about a fifth of the total electricity in buildings around the world – or 10% of all global electricity consumption. A later estimate, drawing on IEA data, puts space cooling at around 2,100 terawatt-hours (TWh) of power in 2022, against around 29,000 TWh of global electricity use that year, or roughly 7%. Local shares are far higher. In a number of cities in both rich and poor countries, cooling can make up over 50% of peak electricity consumption.

Uneven Demand and Peak Load

Demand is concentrated in the hottest hours. Load peaks in the late afternoon and early evening, when units work hardest. Ownership is also very uneven. The 2.8 billion people living in the world's hottest regions have a penetration rate of only about 8%, far below the near-universal household ownership found in parts of North America and East Asia. A peak is therefore shaped less by average consumption than by how many units run at the same moment during a heat wave.

Heat waves also weaken the supply side. The heaviest air conditioning use with the greatest risk for overloading the power grid comes during heat waves, and electricity generation tends to be below peak during heat waves as well. Equipment ages under the same stress: aging transformers, substations, and transmission assets might be less capable of operating under sustained high-temperature conditions.

The consequences can be severe. On 6 September 2022, California's grid operator recorded an all-time high of 51,426 megawatts (MW), and shortly after peak load it issued an emergency alert because it had forecast energy deficiencies later in the day, one step short of rolling blackouts. Transmission is part of the response: a network that can handle peak periods can mitigate blackouts risks during heat waves, mainly by moving power from elsewhere into overloaded areas.

Flattening the Peak

Since a peak lasts only a few hours, meeting it with new plants means building capacity that sits idle for most of the year. Demand response is a cheaper alternative. In the United States, the actual peak demand reduction in 2020 was approximately 10,387 MW by demand response, whereas the potential peak demand reduction was 29,470 MW, which suggests considerable untapped flexibility. Such programmes are typically price-based or incentive-based.

Thermal storage shifts cooling in time. An ice-based system can charge the ice storage during off-peak hours at night and provide cooling during peak hours during the day. By raising the system's load factor and reducing ramping, storage can reduce steady-state and ramping costs. Storage does not eliminate energy use, however: one study found that the use of TES implies increased energy demand, while costs can decrease when tariffs differ strongly between peak and off-peak hours. Storage therefore moves load rather than removing it. Efficient equipment reduces the underlying demand. The IEA's Efficient Cooling Scenario finds that through stringent minimum energy performance standards and other measures such as labelling, the average energy efficiency of the stock of ACs worldwide could more than double between now and 2050, which would greatly reduce the need to build new electricity infrastructure.

Growth in Emerging Economies and Decarbonization

Growth is expected to come mainly from hotter, lower-income regions. As incomes and living standards improve in many developing countries, the growth in AC demand in hotter regions is set to soar. Without efficiency action, energy demand for space cooling will more than triple by 2050 – consuming as much electricity as all of China and India today. The IEA also notes that AC use is expected to be the second-largest source of global electricity demand growth after the industry sector.

This growth interacts with power-sector decarbonization. Where the grid still depends on fossil fuels, cooling carries emissions: the IEA estimated that space cooling caused around 1 billion tonnes of CO2 from electricity use in 2022. Added load therefore has to be met mainly via low-carbon energy sources if it is not to deepen warming. Timing matters here too. Because cooling demand and sunlight both peak in the daytime, solar generation can meet a meaningful share of cooling load directly, and storage can carry it into the evening. The climate sensitivity of demand is also significant: a U.S. study projected that summer air-conditioning demand could rise 8% at the lower and 13% at the higher threshold under warming of 1.5 and 2.0 degrees Celsius, with a risk of prolonged blackouts during peak summer heat if states do not expand capacity or improve efficiency.

Climate Change, Urban Heat and Feedback Effects

An air conditioner does not remove heat from a building so much as move it. The indoor heat absorbed by the evaporator is released outdoors at the condenser, so the outdoor environment receives the heat plus the electrical work that drove the cycle. Studies of cities show that this waste heat has measurable local effects. A Los Angeles simulation found that heat discharged from buildings rose by as much as 20 percent during a heat wave, and that most of it came from air conditioning. Urban heat island effects are most visible at night. In Phoenix, researchers found that daytime waste heat had little effect on near-surface temperatures, whereas at night it raised air temperatures by roughly 1 °C. A Tokyo modelling study likewise attributed a rise of 1–2 °C or more on weekdays in office districts to air-conditioning waste heat.

Cooling demand itself responds to temperature, which creates a feedback loop. The IEA estimates that each degree of global warming by mid-century would raise cooling degree days by roughly a quarter. Local data show the same sensitivity: in Texas, each 1 °C rise in average daily temperature above 24 °C has been associated with about a 4 percent increase in electricity demand. The loop runs as follows. Higher temperatures raise demand for cooling; cooling consumes electricity, which in most grids still comes largely from fossil fuels, and releases waste heat into the street; both effects add to greenhouse gas concentrations and to local warming, which then increases demand again.

Emissions from Electricity and Refrigerants

Cooling produces greenhouse gases through two channels. The first is electricity use. The IEA states that cooling accounts for around 10 percent of global electricity consumption and about a fifth of electricity used in buildings. The emissions attached to this electricity depend on the grid's energy mix, so the same air conditioner can have very different climate impacts in a region dominated by coal and one supplied mainly by renewables.

The second channel is refrigerant leakage. Most refrigerants used in modern equipment are hydrofluorocarbons (HFCs), which were adopted to replace ozone-depleting substances but have very high global warming potential. Leakage occurs throughout the equipment's life: one estimate cited by the NRDC puts annual leak rates at about 10 percent for home air conditioners, and industry estimates suggest that close to 90 percent of refrigerant in end-of-life equipment is vented in the United States, with the share close to 100 percent in the developing world. Modelling by one research group projects that venting from air conditioners alone could release about 33 billion tonnes of CO₂-equivalent between now and 2050. Vehicle air conditioning is a related case: IEA analysis attributes roughly 30 percent of its greenhouse gas emissions to refrigerant leakage rather than fuel use.

Scenarios for Future Demand

Projections differ widely, mainly according to assumptions about income growth, efficiency policy and grid decarbonization. In the IEA's baseline, cooling-related energy demand roughly triples by 2050, whereas its efficient-cooling scenario lowers that figure by about 45 percent, equivalent to the electricity the European Union used in 2016. The UNEP reports that, under business-as-usual conditions, cooling-sector emissions could more than double by 2050 and account for over 10 percent of global emissions. It estimates that a combination of passive cooling, higher efficiency standards and a faster HFC phase-down could cut those projected emissions by 60 to 96 percent, with the upper figure requiring rapid decarbonization of power systems. The COP28 Global Cooling Pledge sets a target of a 68 percent reduction in cooling-related emissions by 2050.

Refrigerant policy is a major variable within these scenarios. An analysis of HFC emissions published in 2022 found that existing national policies could reduce projected 2050 emissions substantially, and that the additional provisions of the Kigali Amendment would reduce them further. Uncertainty remains large, however: a separate study of residential cooling found that its projections of household air-conditioner uptake were more conservative than the IEA's, and that the energy outcome depends heavily on whether the power sector is deeply decarbonized.

Building Design, Passive Cooling and Indoor Air Quality

Reducing the cooling load before equipment

The cheapest cooling is the heat that never enters a building. Heat reaches interior spaces through conduction across walls, roofs and glazing, through solar radiation passing through windows, and through air leakage or deliberate ventilation with warm outdoor air. Insulation slows conduction, and continuous air barriers limit uncontrolled infiltration. Because glass transmits sunlight and conducts heat much faster than opaque walls, window area, glazing type and shading usually decide how large the cooling load will be. External shading such as overhangs, louvres and blinds intercepts radiation before it reaches the glass, and it matters most on east- and west-facing openings, where low sun angles in the morning and evening are hard to block with interior curtains. Orientation and layout control how much facade faces the sun at the hours when outdoor temperatures peak, and a compact plan reduces exposed envelope area.

Thermal mass, meaning dense materials such as concrete, brick, stone and rammed earth, absorbs heat during the day and releases it later. Its value depends on the daily temperature swing. In a hot climate with cool nights, the stored heat can be flushed out by night ventilation so that the mass starts the next day cooler. A full night-flush strategy, in the sense used by American low-energy design guidance, needs substantial internal mass, high airflow (often estimated at roughly 12 to 23 air changes per hour), and a fairly dry climate with cool nights. Partial night flushing is possible almost anywhere. Mass that is exposed to summer sun but not shaded can heat up and radiate into the evening, so it must be protected. Humid climates are a different case: thermal mass combined with night ventilation is generally regarded as a weaker strategy there, because nights stay warm and moist, so designers rely more on shading, cross-ventilation and reflective roofs. Pairing a cool roof with night ventilation has been shown in simulation studies to reduce annual cooling energy substantially compared with a dark roof without night ventilation.

Natural and mechanical ventilation

Natural ventilation uses wind pressure and the buoyancy of warm air. Cross-ventilation through openings on opposite sides of a room and stack ventilation through high outlets both move large volumes of air at almost no energy cost. Its limits are practical as well as physical: airflow depends on wind and temperature differences, so it is unreliable on still days, it admits noise, dust and pollution with the air, and it offers little control over humidity. In humid regions, opening a building to outdoor air can add moisture that then has to be removed by other means.

Mechanical ventilation uses fans and ducts to supply air at a controlled rate, and it can filter the incoming air. Its main energy penalty is the heat carried by the outdoor air that must be conditioned. Heat recovery ventilators (HRVs) transfer heat between the outgoing and incoming airstreams. Energy recovery ventilators (ERVs) also transfer some moisture, which helps in climates where either humid summers or very dry winters are a problem. The design task is to balance air quality against energy: enough outdoor air to dilute occupants' emissions and moisture, but not so much that the cooling load becomes dominated by the ventilation itself.

Indoor air quality, humidity and standards

Indoor air quality depends on three things: how well incoming air is filtered, how much outdoor air is supplied, and how humidity is controlled. Filters remove particles, including pollen and fine dust, and they need regular replacement to keep working. Outdoor air supply dilutes carbon dioxide, odours, volatile compounds and biological contaminants generated indoors. Humidity control matters because relative humidity that stays high encourages mould and dust mites, while very dry air causes discomfort. Both occupants and ventilation air add moisture, so dehumidification is often needed even where temperature is already acceptable. An airtight building can trap pollutants unless it has a mechanical supply system, which is why air-tightness and ventilation are designed together.

In the United States, ASHRAE Standard 62.1 is the main reference for commercial ventilation. It was first published in 1973 as Standard 62 and specifies minimum ventilation rates and other measures to keep indoor air acceptable with minimal health effects. Its ventilation rate procedure sets outdoor air quantities per person and per unit floor area for each space type, and it covers outdoor air quality, moisture and biological growth. The standard is revised continuously through public review, and many jurisdictions have incorporated it into building codes. Other countries apply their own codes and standards with similar logic, although the required rates and enforcement vary.

From glass towers to passive and net-zero design

Early glass-walled office towers showed how completely air conditioning could reshape architecture. Large sealed facades admitted heavy solar gain and could not be opened, so the building depended on mechanical systems for comfort. Later design reversed this logic. The Passivhaus standard, developed in Germany from the late 1980s, sets performance limits that make cooling and heating demand very small: annual heating and cooling demand of no more than 15 kWh per square metre, peak load of no more than 10 W per square metre, airtightness of no more than 0.6 air changes per hour at 50 Pa, heat recovery efficiency of at least 75 percent, and no more than 10 percent of hours above 25 °C. Its designers originally targeted temperate central Europe, and its early versions were criticised for southern climates, which led to adaptations that treat cooling more explicitly.

Other projects show how passive methods can be combined with minimal equipment. The BRE Environmental Building in the United Kingdom was designed to remain comfortable without air conditioning, relying on passive ventilation stacks, solar shading and hollow concrete floor slabs cooled by night ventilation, with a target of reducing energy use and CO₂ emissions by 30 percent relative to best-practice guidelines. Net-zero and net-positive buildings extend the same approach by supplying their remaining energy from on-site or nearby renewable sources. The practical lesson for designers is that the most efficient cooling system is usually the one with the smallest load to meet, and that comfort, air quality and energy use must be designed together rather than treated as separate problems.

Health, Productivity and Social Effects

Heat, mortality and protective indoor environments

Extreme heat is a serious health risk for older people, and the risk rises with age. In a epidemiological case-crossover study of 73,578 nursing-home deaths from 2010 to 2023, the odds of death on extreme-heat days were about 11% higher in facilities without air conditioning, while no significant increase appeared in facilities with it. The effects of heat lingered for roughly three days after exposure. Heat-related deaths in the United States rose by more than 100% between 1999 and 2023, and adults aged 65 and over accounted for about 39% of them. Heat-related mortality is driven by the body's weakening thermoregulatory capacity, by medications, by chronic disease, and by reduced mobility and social isolation.

Much of this mortality happens inside homes rather than outdoors. During the 2021 heat dome in British Columbia, 98% of heat deaths occurred in indoor environments, which means whether an older person can cool their home largely determines how safe they are during a heatwave. Indoor exposure is not fully solved by owning equipment, however. A Houston study linked short-term rises in indoor heat to excess cause-specific mortality and emergency admissions among residents aged 65 and over, with stronger associations among African American residents. A Phoenix and Houston analysis found that energy poverty or faulty systems could leave a considerable share of older residents who owned air conditioners exposed to excess heat for more than half of the summer.

Cognition, work and sleep

Laboratory and field studies suggest that heat impairs mental performance, but the size and generality of the effect remain contested. Experiments on hyperthermic participants show reduced working-memory performance over time, while response speed and attention are less affected. Complex tasks tend to suffer more than simple ones, and the sensitivity of individuals varies. One review concluded that ambient temperatures above about 32 °C were associated with a significant decline in cognitive function. A 2018 Harvard field study found that students in dormitories without air conditioning performed worse on cognitive tests during a heatwave than students in cooled dormitories. An econometric study of mental-arithmetic performance estimated a drop of about 0.13% for each additional degree above roughly 16.5 °C. Researchers differ on how far such laboratory and short-term results can be extrapolated to real workplaces, and on how large the productivity losses are in aggregate. Heat exposure is also linked to sleep disruption: a systematic review of heat-exposed workers reported associations with insomnia, shorter sleep and poorer sleep efficiency, which the authors connected to impaired cognition and accident risk.

Unequal access to cooling

Access to cooling is sharply stratified by income and region. About 3.5 billion people live in regions with high temperatures, yet only about 15% of them own an air conditioner. In East Asia and the Pacific, roughly a quarter of low-income households own a unit, compared with over three-quarters of the richest households. In sub-Saharan Africa, ownership outside the wealthiest quintile is extremely limited, partly because of unreliable electricity access. Europe has lower overall ownership, around 20%, but a more even distribution across income groups. Globally, about a quarter of households own air conditioning, and projections suggest this could reach 41% by 2050, while up to 4 billion people may still lack it. Cross-country analysis shows that in cool areas adoption is close to zero at every income level, whereas in warmer areas adoption rises steeply once household income passes roughly USD 10,000 a year. Renters and marginalized groups also show lower ownership rates in many studies.

Cooling access and urban vulnerability

Ownership alone does not determine who is protected. A Southern California study found that lower-income households ran air conditioners for fewer night-time hours, which the authors attributed likely to cost concerns, and it called for equity-oriented adaptation strategies that reduce the cost burden of cooling. Because the most vulnerable residents are often those with the least capacity to pay, lack of cooling in low-income neighbourhoods compounds heat-related risk, especially for older residents living alone and for outdoor workers. These differences make cooling access a question of public health and urban equity as well as of household finance.

Transport, Data Centres and Industrial Uses

Air conditioning spread into automobiles from the mid-twentieth century onward, and it is now standard in most new cars. In a conventional car its load is carried by the engine's fuel, but in an electric vehicle it draws on the traction battery, so it cuts directly into driving range. One estimate holds that air-conditioning can reduce an EV's charge depletion range by up to 35%. Studies of high-temperature operation report larger losses: thermal regulation demands for battery packs and cabin compartments reduce driving range by 35-50% under high-temperature operating conditions. The compressor dominates this penalty, with compressor power consumption constituting 60-80% of the total HVAC energy expenditure in some empirical work. Designers therefore pursue load reduction and timing strategies. In one simulation, the recirculation of cabin air leads to a power reduction of 48.8–60.8%, while another study found that cabin preconditioning with the help of grid power increased the range by 9.86%.

Data centres present the opposite problem: the goal is not comfort but keeping equipment within tolerances. Almost all the electrical power a server draws is released as heat, so the temperature in a data center will naturally rise due to the electrical power dissipating heat into the air as a byproduct of the computing process. Unless the heat is removed, the ambient temperature will rise, resulting in electronic equipment malfunction. ASHRAE guidance places the recommended range at 18° to 27°C (64° to 81°F) with an acceptable relative humidity range of 20 and 80%. Precision systems control the air entering the equipment rather than the room as a whole, since air intake to IT equipment is the important specification to meet. Humidity matters in both directions, as too much humidity may lead to condensation on internal components. Later ASHRAE editions have also widened the permitted envelope, partly to reduce cooling energy: those revisions result from more relaxed manufacturer guidelines for IT equipment and from a desire to reduce data centre energy consumption. Telecommunications sites follow similar logic, with continuous operation and tight limits on heat and moisture.

Controlled environments are also central to cold chains, cleanrooms, laboratories and museums, although these settings are less documented in the sources consulted here. Refrigerated warehouses and transport containers hold food and many vaccines within narrow temperature bands, and pharmaceutical storage usually requires continuous monitoring. Semiconductor fabrication depends on cleanrooms in which filtered, positively pressurised air is held at stable temperature and humidity, because particles and electrostatic discharge can ruin wafers. Museums and archives control humidity closely to slow the deterioration of artworks and documents, and laboratories with fume hoods often require high air-change rates that make their ventilation energy-intensive.

Underground mining imposes a different kind of demand. Deep mining operations are increasingly affected by severe thermal hazards as mining depths exceed 1000 m, resulting in high-temperature and high-humidity underground environments. Simply moving more air becomes uneconomic with depth: with increasing mining depth, larger quantities of air will be required to remove heat from a mine to maintain a comfortable working temperature. Additional cooling will be required to compensate for high ambient surface air temperature, greatly increasing both capital and operating costs. Regulators commonly target mine cooling systems... to keep WBT in underground working areas below the required thresholds (27 °C), a wet-bulb limit that reflects the role of humidity. Common approaches include the refrigeration unit positioned underground that directly produces chilled water on-site, which is transported via pipelines to the working face air cooler, and ice-based systems that utilize the latent heat of ice melting to cool the mine airflow. Because heat removed underground must still exit through ventilation air, the positioning of refrigeration plants underground offers only a limited solution since the extracted heat has to be removed by the ventilation air exhausted to surface. Equipment has to tolerate harsh conditions: the equipment must be waterproof and explosion-proof. Hybrid dehumidification and sensible-heat considerations add further complexity, since standard refrigeration cycles waste energy when the required ratio of sensible to latent cooling varies widely.

Technological Directions and Open Questions

Heat pumps and electrification

The vapour-compression hardware that cools a room can run in reverse to deliver heat, so the heat pump has become the central technology for buildings that need both cooling and heating. Its market is uneven. Global heat pump sales fell by about 2% in 2025. Sales in China and Japan were largely flat, while Europe saw a strong return of demand. In the United States, heat pump sales fell by around 13% in 2025. In China, reversible air conditioners (used as primary heating equipment) make up about half the Chinese heat pump market, which shows how closely the cooling and heating markets are now tied.

Heat pumps matter for decarbonisation mainly because they replace on-site fuel combustion with electricity. Their climate benefit therefore depends on the emissions intensity of the power grid they draw from. The IEA reports that electrification increased to the highest-ever level even though global heat pump sales were broadly flat, which suggests that policy and investment are shifting toward electric building services more broadly.

Smarter compressors, controls and data

Inverter-driven compressors vary their speed to match the actual cooling load, instead of switching fully on and off, which reduces part-load losses and temperature swings. Building controls are moving in the same direction. Research on ice storage notes that recent progress in model predictive control, AI-assisted forecasting, and hybrid optimization has improved scheduling, renewable utilization, and operational robustness, while PV coupling increases on-site self-consumption and reduces evening grid imports. Applications of machine learning and automation in building management, and the use of control theory to pre-cool buildings when renewable output is high, remain active areas of development. The aim is demand-flexible operation that follows the supply of cheap clean power rather than simply following outdoor temperature.

Thermal storage, phase-change materials and radiant cooling

Thermal storage is one of the most mature ways to flatten the cooling peak. Ice storage uses the high heat of fusion of water: as a form of latent heat thermal energy storage (LHTES), ITES utilizes the high heat of fusion of water to store cooling energy in the form of ice. Because the chiller can run at night, replacing existing air conditioning systems with ice storage has the advantage of shifting the load from on-peak times to off-peak times that often have excess generation. A review of the field concludes that economic viability depends on tariff structure, capital recovery, and utilization rather than operating savings alone.

Phase-change materials offer a related route. Packed into walls, ceilings or water tanks, they absorb heat while melting near a comfortable temperature and release it when they solidify, which can delay indoor warming during the afternoon. Their practical limits lie in cost, cycling durability and the narrow temperature window in which each material performs well.

Radiant cooling circulates chilled water through ceiling or floor panels and removes heat mainly by radiation rather than by moving large volumes of air. Because the water can be kept only a few degrees below room temperature, such systems can use less energy than conventional air-based cooling, but they must be paired with a separate ventilation system to control humidity and condensation. Passive cooling research, including radiative sky cooling surfaces and improved night ventilation, aims to reduce peak loads before any equipment runs, though most of these approaches are still at the demonstration or early commercial stage.

Rebound effects

Efficiency gains lower the running cost of cooling, and households and firms may respond by using more of it. A 2012 review by the American Council for an Energy-Efficient Economy found that available evidence indicates that direct rebound effects will generally be 10% or less. The air-conditioning case is less settled. A Singapore study estimated a rebound or take-back of 38% relative to expected savings from new units. A Japanese household survey found that energy efficiency improvements reduce the probability of the behavior of setting air conditioner temperatures at 28 °C or more by approximately 25–45% during summer, meaning that cheaper cooling leads people to lower their thermostat setting.

The evidence is also disputed in its interpretation. One widely cited argument notes that U.S. air conditioners became about 28% more efficient between 1993 and 2005 while the energy used for cooling in air-conditioned homes rose by 37%. Critics argued that rising incomes and larger homes explain much of this increase, and that efficiency gains are not "lost" when households spend them on comfort. A 2022 study of Taiwanese households found that oversized AC was the main driving force of rebound effects.

Unresolved questions

Several questions remain open. The first is how to reconcile access with climate goals. Cooling demand is rising fastest in hot, lower-income regions, and affordable cooling there may increase total electricity use even as each unit becomes more efficient. Policies must therefore protect vulnerable households from heat without locking in high-emission growth.

The second is how to design low-carbon refrigerants that are both safe and efficient. Alternatives with low global warming potential often trade higher flammability or toxicity, or lower efficiency, against climate performance, and retrofitting existing equipment adds cost. The third is how to limit rebound effects, so that efficiency gains translate into lower total energy use rather than being absorbed by larger units, longer operation and more comfort-seeking behaviour. Addressing these issues will require combining better equipment with pricing, standards and building designs that reduce the need for cooling in the first place.

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