Air conditioning

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From Wikipedia, the free encyclopedia
This article is about cooling of air. For the Curved Air album, see Air
Conditioning (album). For a similar device capable of both cooling and
heating, see heat pump.
"a/c" redirects here. For the abbreviation used in banking and book-keeping,
see Account (disambiguation). For other uses, see AC.

There are various types of air conditioners. Popular examples include: Window-
mounted air conditioner (Suriname, 1955); Ceiling-mounted cassette air conditioner
(China, 2023); Wall-mounted air conditioner (Japan, 2020); and ceiling-mounted
console (Also called ceiling suspended) air conditioner (China, 2023).

Air conditioning, often abbreviated as A/C (US) or air con (UK),[1] is the
process of removing heat from an enclosed space to achieve a more
comfortable interior temperature (sometimes referred to as 'comfort cooling')
and in some cases also strictly controlling the humidity of internal air. Air
conditioning can be achieved using a mechanical 'air conditioner' or by other
methods, including passive cooling and ventilative cooling.[2][3] Air conditioning
is a member of a family of systems and techniques that provide heating,
ventilation, and air conditioning (HVAC).[4] Heat pumps are similar in many
ways to air conditioners, but use a reversing valve to allow them both to heat
and to cool an enclosed space.[5]
Air conditioners, which typically use vapor-compression refrigeration, range in
size from small units used in vehicles or single rooms to massive units that
can cool large buildings.[6] Air source heat pumps, which can be used for
heating as well as cooling, are becoming increasingly common in cooler
climates.

Air conditioners can reduce mortality rates due to higher temperature.

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

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

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

An array of air conditioner condenser units outside

a commercial office building
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, but have
now come under criticism for contributing significantly to climate-change due
to their high electricity consumption and the warming of their immediate
surroundings in large cities.[14][15]

Development
[edit]
Preceding discoveries
[edit]
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.[16][17][18] 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.[19] 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." [16]

In 1758, Benjamin Franklin and John Hadley, a chemistry professor at

the University of Cambridge, conducted experiments applying the principle of
evaporation as a means to cool an object rapidly. Franklin and Hadley
confirmed that the evaporation of highly volatile liquids (such
as alcohol and ether) could be used to drive down the temperature of an
object past the freezing point of water. They experimented with the bulb of
a mercury-in-glass thermometer as their object. They used a bellows to speed
up the evaporation. They lowered the temperature of the thermometer bulb
down to −14 °C (7 °F) while the ambient temperature was 18 °C (64 °F).
Franklin noted that soon after they passed the freezing point of water 0 °C
(32 °F), a thin film of ice formed on the surface of the thermometer's bulb and
that the ice mass was about 6 mm (1⁄4 in) thick when they stopped the
experiment upon reaching −14 °C (7 °F). Franklin concluded: "From this
experiment, one may see the possibility of freezing a man to death on a warm
summer's day."[20]

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.[21] 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.[21][22] He
envisioned centralized air conditioning that could cool entire cities. Gorrie was
granted a patent in 1851,[23] but following the death of his main backer, he was
not able to realize his invention.[24] 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.[25] 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.[25]
First devices
[edit]

Willis Carrier, 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.[26][27][28][29] In 1902, he installed his first air-
conditioning system, in the Sackett-Wilhelms Lithographing & Publishing
Company in Brooklyn, New York.[30] 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.[31][32]

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.[33] 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.[34]

Domestic air conditioning soon took off. In 1914, the first domestic air
conditioning was installed in Minneapolis in the home of Charles Gilbert
Gates. It is, however, possible that the considerable device (c. 2.1 m × 1.8 m
× 6.1 m; 7 ft × 6 ft × 20 ft) was never used, as the house remained
uninhabited[21](Gates had already died in October 1913).

In 1931, H.H. Schultz and J.Q. Sherman developed what would become the
most common type of individual room air conditioner: one designed to sit on a
window ledge. The units went on sale in 1932 at US$10,000 to $50,000 (the
equivalent of $200,000 to $1,100,000 in 2023.)[21] A year later, the first air
conditioning systems for cars were offered for sale.[35] Chrysler
Motors introduced the first practical semi-portable air conditioning unit in 1935,
[36]
and Packard became the first automobile manufacturer to offer an air
conditioning unit in its cars in 1939.[37]
Further development
[edit]
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.[38] The first inverter air conditioners were
released in 1980–1981.[39][40]

Air conditioner adoption tends to increase above around $10,000 annual

household income in warmer areas.[41] Global GDP growth explains around
85% of increased air condition adoption by 2050, while the remaining 15%
can be explained by climate change.[41]

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 11675
gigawatts.[8][42] The International Energy Agency predicted in 2018 that the
number of air conditioning units will grow to around 4 billion units by 2050 and
the total cooling capacity will grow to around 23000 GW, with the biggest
increases in India and China.[8] Between 1995 and 2004, the proportion of
urban households in China with air conditioners increased from 8% to 70%.
[43]
As of 2015, nearly 100 million homes, or about 87% of US households, had
air conditioning systems.[44] 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).[45][46]

Operation
[edit]
Operating principles
[edit]
Main article: Vapor-compression refrigeration

A simple stylized diagram of the refrigeration

cycle: 1) condensing coil, 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.[47][48] 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.[49]

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.[50]

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[51] and dehumidified before passing over the condenser coil,
where it is warmed again before it is released back into the room.[citation needed]
Free cooling can sometimes be selected when the external air is cooler than
the internal air. 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.[52]
Heating
[edit]
Main article: 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.[53] This is partly because ice forms on the outdoor
unit's heat exchanger coil, which blocks air flow over the coil. To compensate
for this, the heat pump system must temporarily switch back into the regular
air conditioning mode to switch the outdoor evaporator coil back to the
condenser coil, to heat up and defrost. Therefore, some heat pump systems
will have electric resistance heating in the indoor air path that is activated only
in this mode to compensate for the temporary indoor air cooling, which would
otherwise be uncomfortable in the winter.

Newer models have improved cold-weather performance, with efficient

heating capacity down to −14 °F (−26 °C).[54][53][55] 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.

Performance
[edit]
Main articles: coefficient of performance, Seasonal energy efficiency ratio,
and 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.[56][57] 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.[58] Air conditioner equipment
power in the U.S. is often described in terms of "tons of refrigeration", with
each approximately equal to the cooling power of one short ton (2,000 pounds
(910 kg) of ice melting in a 24-hour period. The value is equal to
12,000 BTUIT per hour, or 3,517 watts.[59] Residential central air systems are
usually from 1 to 5 tons (3.5 to 18 kW) in capacity.[citation needed]

The efficiency of air conditioners is often rated by the seasonal energy

efficiency ratio (SEER), which is defined by the Air Conditioning, Heating and
Refrigeration Institute in its 2008 standard AHRI 210/240, Performance Rating
of Unitary Air-Conditioning and Air-Source Heat Pump Equipment.[60] A similar
standard is the European seasonal energy efficiency ratio (ESEER).[citation needed]

Efficiency is strongly affected by the humidity of the air to be

cooled. Dehumidifying the air before attempting to cool it can reduce
subsequent cooling costs by as much as 90 percent. Thus, reducing
dehumidifying costs can materially affect overall air conditioning costs.[61]

Control system
[edit]
Wireless remote control
[edit]
Main articles: Remote control and Infrared blaster
A wireless remote controller

The infrared transmitting LED on the remote

The infrared receiver on the A/C

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 controller 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.

Wired controller
[edit]
Main article: Thermostat
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.

Types
[edit]

Typical Air Typical

Types Mounting
Capacity* supply application

Mini-split small – large Direct Wall Residential

very small –
Window Direct Window Residential
small

very small – Direct / Residential,

Portable Floor
small Ducted remote areas
 Typical Air Typical
Types Mounting
Capacity* supply application

small – very Residential,

Ducted (individual) Ducted Ceiling
large commercial

medium – very Residential,

Ducted (central) Ducted Ceiling
large commercial

Ceiling suspended medium – large Direct Ceiling Commercial

Direct /
Cassette medium – large Ceiling Commercial
Ducted

Direct /
Floor standing medium – large Floor Commercial
Ducted

Direct /
Packaged very large Floor Commercial
Ducted

Packaged RTU
very large Ducted Rooftop Commercial
(Rooftop Unit)

* 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
Mini-split and multi-split systems
[edit]
 Evaporator, indoor unit, or terminal, side of a
ductless split-type air conditioner
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.[62] 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.[63][64][65] In 1969, the first
mini-split air conditioner was sold in the US.[66] 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.[67] 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.[68] Variable refrigerant flow indoor units can also
be turned off individually in unused spaces.[citation needed] The lower start-up power
of VRF's DC inverter compressors and their inherent DC power requirements
also allow VRF solar-powered heat pumps to be run using DC-providing solar
panels.

Ducted central systems

[edit]
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.[69]
Central plant cooling
[edit]
See also: Chiller

An industrial air conditioning unit on top of the

shopping mall Passage in Linz, Austria
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.[citation needed]

Portable units
[edit]
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. Such 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.[70]

Window unit and packaged terminal

[edit]
 Through-the-wall PTAC units, University Motor
Inn, Philadelphia
Main article: Packaged terminal air conditioner
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.[71]

Packaged air conditioner

[edit]
Packaged air conditioners (also known as self-contained units)[72][73] 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),[74][75] 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.[69][76][77][78][79][80]

Types of compressors
[edit]
 Typic
Compressor Common al Efficien Durabili Repairabil
types applications capaci cy ty ity
ty

very low
Refrigerator, W (small
alk-in freezer, small – capacity)
Reciprocating very low medium
portable air large medium
conditioners (large
capacity)

Residential mini
Rotary vane small low low easy
splits
Commercial and
Scroll central medium medium medium easy
systems, VRF
Commercial medium
Rotary screw medium medium hard
chiller – large
Commercial very
Centrifugal medium high hard
chiller large
Maglev Centrifu Commercial very
high very high very hard
gal chiller large
Reciprocating
[edit]
Main article: Reciprocating compressor
This compressor consists of a crankcase, crankshaft, piston
rod, piston, piston ring, cylinder head and valves. [citation needed]

Scroll
[edit]
Main article: Scroll compressor
This compressor uses two interleaving scrolls to compress the
refrigerant.[81] 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]
Screw
[edit]
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]

Capacity modulation technologies

[edit]
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]

Hot gas bypass

[edit]
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%.[82]

Manifold configurations
[edit]
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]

Mechanically modulated compressor

[edit]
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]

Variable-speed compressor
[edit]
Main article: 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]

Impact
[edit]
Health effects
[edit]

Rooftop condenser 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 from excessive perspiration fluid and electrolyte
disorders kidney failure,[8] in addition to other problems related
to hyperthermia.[83] Heat waves are the most lethal type of weather
phenomenon in the United States.[84][85] A 2020 study found that areas
with lower use of air conditioning correlated with higher rates of
heat-related mortality and hospitalizations.[86] 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 25 °C (77 °F) per floor during heatwaves.[8]

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.[87][88] 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.[89]

Economic effects
[edit]
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.[90]

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.[91] 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.
[92]
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.[7]

The spread of the use of air conditioning acts as a main driver for
the growth of global demand of electricity.[93] 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).[8] 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.[94]

Environmental effects
[edit]

Air conditioner farm in the facade of a

building in Singapore
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.[8] The report predicts
an increase of electricity usage due to space cooling to around 6200
TWh by 2050,[8][95] 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.[8] 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.[96] 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.[96]

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.[97] CFCs and HCFCs refrigerants such as R-
12 and R-22, respectively, used within air conditioners have caused
damage to the ozone layer,[98] and hydrofluorocarbon refrigerants
such as R-410a and R-404a, which were designed to replace CFCs
and HCFCs, are instead exacerbating climate change.[99] 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.[100]

Hydrofluorocarbons would have raised global temperatures by

around 0.3–0.5 °C (0.5–0.9 °F) 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 0.06 °C (0.1 °F).[101]

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]

Social effects
[edit]
Socioeconomic groups with a household income below around
$10,000 tend to have a low air conditioning adoption,[41] which
worsens heat-related mortality.[7] 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.
[86]
Premature mortality in NYC is projected to grow between 47%
and 95% in 30 years, with lower-income and vulnerable populations
most at risk.[86] Studies on the correlation between heat-related
mortality and hospitalizations and living in low socioeconomic
locations can be traced in Phoenix, Arizona,[102] Hong Kong,[103] China,
[103]
Japan,[104] and Italy.[105][106] 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.[106]

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.[106] 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.[107] Especially in cities,
Redlining creates heat islands, increasing temperatures in certain
parts of the city.[106] This is due to materials heat-absorbing building
materials and pavements and lack of vegetation and shade
coverage.[108] There have been initiatives that provide cooling
solutions to low-income communities, such as public cooling spaces.
[8][108]

Other techniques
[edit]
Buildings designed with passive air conditioning are generally less
expensive to construct and maintain than buildings with
conventional HVAC systems with lower energy demands.[109] 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.[12]

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.[110]

Passive ventilation
[edit]
This section is an excerpt from Passive ventilation.[edit]
 The ventilation system of a

regular earthship Dogtrot houses are

designed to maximise natural ventilation.

A roof turbine ventilator, colloquially

known as a 'Whirly Bird' is an application of wind driven ventilation.
Passive ventilation is the process of supplying air to and removing
air from an indoor space without using mechanical systems. It refers
to the flow of external air to an indoor space as a result
of pressure differences arising from natural forces.

There are two types of natural ventilation occurring in buildings: wind

driven ventilation and buoyancy-driven ventilation. Wind driven
ventilation arises from the different pressures created by wind
around a building or structure, and openings being formed on the
perimeter which then permit flow through the building. Buoyancy-
driven ventilation occurs as a result of the directional buoyancy force
that results from temperature differences between the interior and
exterior.[111]

Since the internal heat gains which create temperature differences

between the interior and exterior are created by natural processes,
including the heat from people, and wind effects are variable,
naturally ventilated buildings are sometimes called "breathing
buildings".
Passive cooling
[edit]
This section is an excerpt from Passive cooling.[edit]

A traditional Iranian solar cooling design

using a wind tower
Passive cooling is a building design approach that focuses on heat
gain control and heat dissipation in a building in order to improve the
indoor thermal comfort with low or no energy consumption.[112][113] This
approach works either by preventing heat from entering the interior
(heat gain prevention) or by removing heat from the building (natural
cooling).[114]

Natural cooling utilizes on-site energy, available from the natural

environment, combined with the architectural design of building
components (e.g. building envelope), rather than mechanical
systems to dissipate heat.[115] Therefore, natural cooling depends not
only on the architectural design of the building but on how the site's
natural resources are used as heat sinks (i.e. everything that
absorbs or dissipates heat). Examples of on-site heat sinks are the
upper atmosphere (night sky), the outdoor air (wind), and the
earth/soil.

Passive cooling is an important tool for design of buildings

for climate change adaptation – reducing dependency on energy-
intensive air conditioning in warming environments.[116][117]
 A pair of
short windcatchers (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 shuksheika (roof lantern vent), used to
shade the qa'a below while allowing hot air rise out of it (stack
effect).[11]
Daytime radiative cooling
[edit]

Passive daytime radiative cooling (PDRC)

surfaces are high in solar reflectance and heat emittance, cooling
with zero energy use or pollution.[118]
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).[119] 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.[118][120]

PDRC applications on building roofs and envelopes have

demonstrated significant decreases in energy consumption and
costs.[120] In suburban single-family residential areas, PDRC
application on roofs can potentially lower energy costs by 26% to
46%.[121] 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.[122][123]

Fans
[edit]
Main article: 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
3 m (10 ft) in diameter and manually powered by prisoners.[124]: 99, 151,
233
In 747, Emperor Xuanzong (r. 712–762) of the Tang
dynasty (618–907) had the Cool Hall (Liang Dian 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.[124]: 134, 151

Thermal buffering
[edit]
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.[13]

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.[13] This
technique is over 3,700 years old in the Middle East.[125] Harvesting
outdoor ice during winter and transporting and storing for use in
summer was practiced by wealthy Europeans in the early 1600s,
[16]
and became popular in Europe and the Americas towards the end
of the 1600s.[126] This practice was replaced by mechanical
compression-cycle icemakers.

Evaporative cooling
[edit]
Main article: Evaporative cooler

An evaporative cooler
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.[11] Evaporative cooling
also makes the air more humid, which can be beneficial in a dry
desert climate.[127]

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.[128]