HVAC (pronounced either "H-V-A-C" or, occasionally, "aitch-vak") is an initialism/acronym that stands for "Heating, Ventilating, and Air Conditioning".  HVAC is sometimes referred to as "climate control" and is particularly important in the design of medium to large industrial and office buildings such as sky scrapers and in marine environments such as aquariums, where humidity and temperature must all be closely regulated whilst maintaining safe and healthy conditions within. In certain regions (e.g., UK) the term "Building Services" is also used, but may also include plumbing and electrical systems. Refrigeration is sometimes added to the field's abbreviation as HVAC&R or HVACR, or ventilating is dropped as HACR (such as the designation of HACR-rated circuit breakers).

Heating, ventilating, and air conditioning is based on the basic principles of thermodynamics, fluid mechanics, and heat transfer, and to inventions and discoveries made by Michael Faraday, Willis Carrier, Reuben Trane, James Joule, William Rankine, Sadi Carnot, and many others. The invention of the components of HVAC systems goes hand-in-hand with the industrial revolution, and new methods of modernization, higher efficiency, and system control are constantly introduced by companies and inventors all over the world.

The three functions of heating, ventilating, and air-conditioning are closely interrelated. All seek to provide thermal comfort, acceptable indoor air quality, and reasonable installation, operation, and maintenance costs.  HVAC systems can provide ventilation, reduce air infiltration, and maintain pressure relationships between spaces.  How air is delivered to, and removed from spaces is known as room air distribution.

In modern buildings the design, installation, and control systems of these functions are integrated into one or more HVAC systems.  For very small buildings, contractors normally "size" and select HVAC systems and equipment.  For larger buildings where required by law, "building services" designers and engineers, such as mechanical, architectural, or building services engineers analyze, design, and specify the HVAC systems, and specialty mechanical contractors build and commission them. In all buildings, building permits for, and code-compliance inspections of the installations are the norm.

The HVAC industry is a worldwide enterprise, with career opportunities including operation and maintenance, system design and construction, equipment manufacturing and sales, and in education and research.  The HVAC industry had been historically regulated by the manufacturers of HVAC equipment, but Regulating and Standards industries such as ASHRAE, SMACNA, ACCA, and AMCA, have been established to support the industry and encourage high standards and achievement.  Most recently, the ICC has been established to create international standards that many countries, including the US, Canada, the UK, Australia and many others have been adopting.


Heating, ventilating and air-conditioning (HVAC) systems can play several roles to reduce the environmental impact of buildings. The primary function of HVAC systems is to provide healthy and comfortable interior conditions for occupants.  Well-designed, efficient systems do this with minimal non-renewable energy and air and water pollutant emissions. Cooling equipment that avoids chlorofluorocarbons and hydrochlorofluoro carbons (CFCs and HCFCs) may eliminate a major cause of damage to the ozone layer.

However, even the best HVAC equipment and systems cannot compensate for a building design with inherently high cooling and heating needs.  The greatest opportunities to conserve non-renewable energy are through architectural design that controls solar gain, while taking advantage of passive heating, day-lighting, natural ventilation and cooling opportunities.  The critical factors in mechanical systems' energy consumption - and capital cost - are reducing the cooling and heating loads they must handle. 


Heating systems may be classified as central or local. Central heating is often used in cold climates to heat private houses and public buildings.  Such a system contains a boiler, furnace, or heat pump to heat water, steam, or air, all in a central location such as a furnace room in a home or a mechanical room in a large building.  The system also contains piping or duct work to distribute the heated fluid, and radiators to transfer this heat to the air.  The term radiator in this context is misleading since most heat transfer from the heat exchanger is by convection, not radiation.  The radiators may be mounted on walls or buried in the floor to give under-floor heat.

In boiler-fed or radiant-heating systems, all but the simplest systems have a pump to circulate the water and ensure an equal supply of heat to all the radiators. The heated water can also be fed through another heat exchanger inside a storage cylinder to provide hot running water.

Forced air systems send heated air through duct work.  During warm weather the same duct work can be reused for air conditioning.  The forced air can also be filtered or put through air cleaners. Most ducts cannot fit a human being (as they do in many films) since this would require a greater duct-structural integrity and create a potential security liability.

Heating can also be provided from electric, or resistance heating using a filament that glows hot when you cause electricity to pass through it.  This type of heat can be found in electric baseboard heaters, portable electric heaters, and as backup or supplemental heating for heat pump (or reverse heating) system.

The heating elements (radiators or vents) should be located in the coldest part of the room and typically next to the windows to minimize condensation. Popular retail devices that direct vents away from windows to prevent "wasted" heat defeat this design parameter.  Drafts contribute more to the subjective feeling of coldness than actual room temperature.  Therefore, rather than improving the heating of a room/building, it is often more important to control the air leaks.

The invention of central heating is often credited to the ancient Romans, who installed a system of air ducts called "hypocaust" in the walls and floors of public baths and private villas.  The ducts were fed with hot air from a central fire. Generally, these heated by radiation; a better physiologic approach to heating than conventional forced air convective heating. 


Ventilating is the process of "changing" or replacing of air in any space to remove moisture, odors, smoke, heat, dust and airborne bacteria.  Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types.  Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, and to keep interior building air circulating, to prevent stagnation of the interior air.


"Mechanical" or "forced" ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. But in humid climates, much energy is required to remove excess moisture from ventilation air.

Kitchens and bathrooms typically have mechanical exhaust to control odors and sometimes humidity.  Factors in the design of such systems include the flow rate (which is a function of the fan speed and exhaust vent size) and noise level.  If the ducting for the fans traverse unheated space (e.g., an attic), the ducting should be insulated as well to prevent condensation on the ducting.  Direct drive fans are available for many applications, and can reduce maintenance needs.

Heat recovery ventilation systems employ heat exchangers to recover some heat from exhausted air, to preheat the incoming outside air.

Ceiling fans and table/floor fans are very effective in circulating air within a room.  Counter-intuitively, because hot air rises, ceiling fans may be used to keep a room warmer. Ceiling fans do not provide ventilation, however. 


Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with operable windows when the spaces to ventilate are small and the architecture permits. In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside (stack effect) thus forcing cool outside air to be drawn into the building naturally through openings in the lower areas.  These systems use very little energy but care must be taken to ensure the occupants' comfort. In warm or humid months, in many climates, maintaining thermal comfort via solely natural ventilation may not be possible so conventional air conditioning systems are used as backups.  Air-side economizers perform the same function as natural ventilation, but use mechanical systems' fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate. 


Air Conditioning and refrigeration are provided through the removal of heat.  The definition of cold is the absence of heat and all air conditioning systems work on this basic principle.  Heat can be removed through the process of radiation, convection, and conduction using mediums such as water, air, ice, and chemicals referred to as refrigerants. In order to remove heat from something, you simply need to provide a medium that is colder -- this is how all air conditioning and refrigeration systems work.

An air conditioning system, or a standalone air conditioner, provides cooling, ventilation, and humidity control for all or part of a house or building.  The Freon or other refrigerant provides cooling through a process called the refrigeration cycle.  The refrigeration cycle consists of four essential elements to create a cooling effect.  A compressor provides compression for the system.  This compression causes the cooling vapor to heat up. The compressed vapor is then cooled by heat exchange with the outside air, so that the vapor condenses to a fluid, in the condenser.  The fluid is then pumped to the inside of the building, where it enters an evaporator.  In this evaporator, small spray nozzles spray the cooling fluid into a chamber, where the pressure drops and the fluid evaporates.  Since the evaporation absorbs heat form the surroundings, the surroundings cool off, and thus the evaporator absorbs or adds heat to the system.  The vapor is then returned to the compressor.  A metering device acts as a restriction in the system at the evaporator to ensure that the heat being absorbed by the system is absorbed at the proper rate.

Central, 'all-air' air conditioning systems are often installed in modern residences, offices, and public buildings, but are difficult to retrofit (install in a building that was not designed to receive it) because of the bulky air ducts required.  A duct system must be carefully maintained to prevent the growth of pathogenic bacteria in the ducts.  An alternative to large ducts to carry the needed air to heat or cool an area is the use of remote fan coils or split systems.  These systems, although most often seen in residential applications, are gaining popularity in small commercial buildings. The remote coil is connected to a remote condenser unit using piping instead of ducts.

Dehumidification in an air conditioning system is provided by the evaporator.  Since the evaporator operates at a temperature below dew point, moisture is collected at the evaporator.  This moisture is collected at the bottom of the evaporator in a condensate pan and removed by piping it to a central drain or onto the ground outside.  A dehumidifier is an air-conditioner-like device that controls the humidity of a room or building.  They are often employed in basements which have a higher relative humidity because of their lower temperature (and propensity for damp floors and walls).  In food retailing establishments, large open chiller cabinets are highly effective at dehumidifying the internal air. Conversely, a humidifier increases the humidity of a building.

Air-conditioned buildings often have sealed windows, because open windows would disrupt the attempts of the HVAC system to maintain constant indoor air conditions.



Water heating is more efficient for heating buildings and was the standard many years ago.  Today forced air systems can double for air conditioning and are more popular. The most efficient central heating method is geothermal heating.

Energy efficiency can be improved even more in central heating systems by introducing zoned heating.  This allows a more granular application of heat, similar to non-central heating systems.  Zones are controlled by multiple thermostats. In water heating systems the thermostats control zone valves, and in forced air systems they control zone dampers inside the vents which selectively block the flow of air.


The performance of vapor compression refrigeration cycles is limited by thermodynamics.  These AC and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe the performance of these devices.  The Coefficient-of-Performance (COP) measures performance, but this dimensionless measure has not been adopted, but rather the Energy Efficiency Ratio (EER).  To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER is used, and is the Seasonal Energy Efficiency Ratio (SEER).  The SEER article describes it further, and presents some economic comparisons using this useful performance measure.



A central-heating system provides warmth to the whole interior of a building (or portion of a building) from one point to multiple rooms.

When combined with other systems in order to control the building climate, the whole system may comprise a HVAC (Heating, Ventilation and Air-Conditioning).

Central heating differs from local heating in that the heat generation occurs in one place, such as a furnace room in a house or a mechanical room in a large building (though not necessarily at the "central" geometric point).  The most common method of heat-generation involves the combustion of fossil fuel in a furnace or boiler.  The resultant heat then gets distributed: typically by forced air through duct work, by water circulating through pipes, or by steam fed through pipes.  Increasingly, buildings utilize solar-powered heat sources, in which case the distribution-system normally uses water-circulation.

In much of northern Europe and in urban portions of Russia, where people seldom require air-conditioning in homes due to the temperate climate, most new housing comes with central heating installed. Such areas normally use gas-fired heating, district heating, or oil-fired systems.  In the western and southern United States natural-gas-fired central forced-air systems occur most commonly; these systems and central-boiler systems both occur in the far northern regions of the USA.  Steam-heating systems, fired by coal, oil or gas, feature in the USA, Russia and Europe: primarily for larger buildings. Electrical heating systems, far less energy-efficient, occur less commonly.


Cities in the northern Roman Empire used central heating systems circa 100AD, conducting air heated by furnaces through Empty spaces under the floors and out of pipes in the walls — the system known as a hypocaust.  The Cistercian monks revived central heating using river-diversions combined with indoor wood-fired furnaces.  The well-preserved Royal Monastery of Our Lady of the Wheel (founded 1202) on the Ebro River in the Aragon region of Spain provides an excellent example of such an application.

By about 1700 Russian engineers had started designing hydrologically-based systems for central heating.  The Summer Palace (1710 - 1714) of Peter the Great in Saint Petersburg provides the best extant example. Slightly later, in the year 1716, came the first use of water in Sweden to distribute heat in buildings.  Martin Triewald, a Swedish engineer, used this method for a greenhouse at Newcastle upon Tyne.  Jeane Simon Bonnemain (1743-1830), a French architect, introduced the technique to industry on a cooperative, at Château du Pêcq, near Paris.

Angier March Perkins developed and installed some of the earliest steam-heating systems in the 1830s.  The first was installed in the home of Governor of the Bank of England, John Horley Palmer so that he could grow grapes in England's cold climate.


Common components of a central-heating system using water-circulation include:

           Gas supply lines (sometimes including a propane tank), oil tank and supply lines or district heating supply lines

           Boiler (or a heat exchanger for district heating) — heats water in a closed-water system

           Pump — circulates the water in the closed system

          •  Radiators — wall-mounted panels through which the heated water passes in order to release heat into rooms

Engineers in the United Kingdom and in other parts of Europe commonly combine the needs of room heating with hot-water heating and storage.  These systems occur less commonly in the USA. In this case, the heated water in a sealed system flows through a heat exchanger in a hot-water tank or hot-water cylinder where it heats water from the normal water supply before that water gets fed to hot-water outlets in the house.  These outlets may service hot-water taps or appliances such as washing machines or dishwashers.


A sealed system provides a form of central heating in which the water used for heating usually circulates independently of the building's normal water-supply.  A pressure vessel contains compressed gas, separated from the sealed-system water by a diaphragm.  This allows for normal variations of pressure in the system.  A safety valve allows water to escape from the system when pressure becomes too high, and a valve can open to replenish water from the normal water supply if the pressure drops too low.  Sealed systems offer an alternative to open-vent systems, in which steam can escape from the system, and gets replaced from the building's water supply via a feed and central storage system.


Electric heating or resistance heating converts electricity directly to heat.  Electric heat is often more expensive than heat produced by combustion appliances like natural gas, propane, and oil.  Electric resistance heat can be provided by baseboard heaters, space heaters, radiant heaters, furnaces, wall heaters, or thermal storage systems.

Electric heaters are usually part of a fan coil which is part of a central air conditioner.  They circulate heat by blowing air across the heating element which is supplied to the furnace through return air ducts. Blowers in electric furnaces move air over one to five resistance coils or elements which are usually rated at five kilowatts.  The heating elements activate one at a time to avoid overloading the electrical system.  Overheating is prevented by a safety switch called a limit controller or limit switch.  This limit controller may shut the furnace off if the blower fails or if something is blocking the air flow.  The heated air is then sent back through the home through supply ducts.

In larger commercial applications, central heating is provided through an air handler which incorporates similar components as a furnace but on a larger scale.


Hydronic heating systems are systems that circulate a medium for heating. Hydronic radiant floor heating systems use a boiler or district heating to heat up hot water and a pump to circulate the hot water in plastic pipes installed in a concrete slab.  The pipes, embedded in the floor, carry heated water that conducts warmth to the surface of the floor where it broadcasts energy to the room.

Hydronic systems circulate hot water for heating. Steam heating systems are similar to heating water systems, except steam is used as the heating medium instead of water.

Hydronic heating systems generally consist of a boiler or district heating heat exchanger, hot water circulating pumps, distribution piping, and a fan coil unit or a radiator located in the room or space.  Steam heating systems are similar except no circulating pumps are required.

Hydronic systems are closed loop: the same fluid is heated and then reheated. Hydronic heating systems are also used with antifreeze solutions in ice and snow melt systems for walkways, parking lots and streets.  They are more commonly used in commercial and whole house radiant floor heat projects, while electric radiant heat systems are more commonly used in smaller "spot warming" applications.


In mild climates a heat pump can be used to air-condition the building during hot weather, and to warm the building using heat extracted from outdoor air in cold weather.  Air-source heat pumps are generally uneconomic for outdoor temperatures much below freezing. In colder climates, geothermal heat pumps can be used to extract heat from the ground.  For economy, these systems are designed for average low winter temperatures and use supplemental heating for extreme low temperature conditions.  The advantage of the heat pump is that it reduces the purchased energy required for building heating;  often geothermal source systems also supply domestic hot water.  Even in places where fossil fuels provide most electricity, a geothermal system may offset greenhouse gas production since most of the energy furnished for heating is supplied from the environment, with only 15-30% purchased.


From an energy-efficiency standpoint considerable heat gets lost or goes to waste if only a single room needs heating, since central heating has distribution losses and (in the case of forced-air systems particularly) may heat some unoccupied rooms without need.  In such buildings which require isolated heating, one may wish to consider non-central systems such as individual room heaters, fireplaces or other devices.  Alternatively, architects can design new buildings to use low-energy building techniques which can virtually eliminate the need for heating, such as those built to the Passive House standard.

However, if a building does need fully heating, combustion central heating offers a more environmentally friendly solution than electric-air central heating or than other direct electric heating devices.  This stems from the fact that most electricity originates remotely using fossil fuels, with up to two-thirds of the energy in the fuel lost (unless utilized for district heating) at the power station and in transmission losses. In Sweden proposals exist to phase out direct electric heating for this reason — see oil phase-out in Sweden. Nuclear and hydroelectric sources reduce this factor.

In contrast, hot-water central-heating systems can use water heated in or close to the building using high-efficiency condensing boilers, biofuels, or district heating. Wet underfloor heating has proven ideal.  This offers the option of relatively easy conversion in the future to use developing technologies such as heat pumps and solar combisystems, thereby also providing future-proofing.


Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature.  The term cooling refers generally to any natural or artificial process by which heat is dissipated.  The process of artificially producing extreme cold temperatures is referred to as cryogenics.

Cold is the absence of heat, hence in order to decrease a temperature, one "removes heat", rather than "adding cold." In order to satisfy the Second Law of Thermodynamics, some form of work must be performed to accomplish this.  This work is traditionally done by mechanical work but can also be done by magnetism, laser or other means. However, all refrigeration uses the three basic methods of heat transfer: convection, conduction, or radiation.


The first known method of artificial refrigeration was demonstrated by William Cullen at the University of Glasgow in Scotland in 1748. Cullen used a pump to create a partial vacuum over a container of diethyl ether, which then boiled , absorbing heat from the surrounding air. The experiment even created a small amount of ice, but had no practical application at that time.

In 1805, American inventor Oliver Evans designed but never built a refrigeration system based on the vapor-compression refrigeration cycle rather than chemical solutions or volatile liquids such as ethyl ether.

In 1820, the British scientist Michael Faraday liquefied ammonia and other gases by using high pressures and low temperatures.
An American living in Great Britain, Jacob Perkins, obtained the first patent for a vapor-compression refrigeration system in 1834.  Perkins built a prototype system and it actually worked, although it did not succeed commercially.

In 1842, an American physician, John Gorrie, designed the first system for refrigerating water to produce ice.  He also conceived the idea of using his refrigeration system to cool the air for comfort in homes and hospitals (i.e., air-conditioning).  His system compressed air, then partially cooled the hot compressed air with water before allowing it to expand while doing part of the work required to drive the air compressor.  That isentropic expansion cooled the air to a temperature low enough to freeze water and produce ice, or to flow "through a pipe for effecting refrigeration otherwise" as stated in his patent granted by the U.S. Patent Office in 1851.  Gorrie built a working prototype, but his system was a commercial failure.

Alexander Twining began experimenting with vapor-compression refrigeration in 1848 and obtained patents in 1850 and 1853.  He is credited with having initiated commercial refrigeration in the United States by 1856.

Meanwhile, James Harrison who was born in Scotland and subsequently emigrated to Australia, begun operation of a mechanical ice-making machine in 1851 on the banks of the Barwon River at Rocky Point in Geelong.  His first commercial ice-making machine followed in 1854 and his patent for an ether liquid-vapour compression refrigeration system was granted in 1855.  Harrison introduced commercial vapor-compression refrigeration to breweries and meat packing houses and by 1861, a dozen of his systems were in operation.

Australian, Argentinean and American concerns experimented with refrigerated shipping in the mid 1870s, the first commercial success coming when William Soltau Davidson fitted a compression refrigeration unit to the New Zealand vessel Dunedin in 1882, leading to a meat and dairy boom in Australasia and South America.

The first gas absorption refrigeration system using gaseous ammonia dissolved in water (referred to as "aqua ammonia") was developed by Ferdinand Carré of France in 1859 and patented in 1860.  Due to the toxicity of ammonia, such systems were not developed for use in homes, but were used to manufacture ice for sale. In the United States, the consumer public at that time still used the ice box with ice brought in from commercial suppliers, many of whom were still harvesting ice and storing it in an icehouse.

Thaddeus Lowe, an American balloonist from the Civil War, had experimented over the years with the properties of gases.  One of his mainstay enterprises was the high-volume production of hydrogen gas.  He also held several patents on ice making machines. His "Compression Ice Machine" would revolutionize the cold storage industry.  In 1869 he and other investors purchased an old steamship onto which they loaded one of Lowe’s refrigeration units and began shipping fresh fruit from New York to the Gulf Coast area, and fresh meat from Galveston, Texas back to New York. Because of Lowe’s lack of knowledge about shipping, the business was a costly failure, and it was difficult for the public to get used to the idea of being able to consume meat that had been so long out of the packing house.

Domestic mechanical refrigerators became available in the United States around 1911


Probably the most widely-used current applications of refrigeration are for the air-conditioning of private homes and public buildings, and the refrigeration of foodstuffs in homes, restaurants and large storage warehouses.  The use of refrigerators in our kitchens for the storage of fruits and vegetables has allowed us to add fresh salads to our diets year round, and to store fish and meats safely for long periods.

In commerce and manufacturing, there are many uses for refrigeration.  Refrigeration is used to liquify gases like oxygen, nitrogen, propane and methane for example.  In compressed air purification, it is used to condense water vapor from compressed air to reduce its moisture content.  In oil refineries, chemical plants, and petrochemical plants, refrigeration is used to maintain certain processes at their required low temperatures (for example, in the alkylation of butenes and butane to produce a high- octane gasoline component).  Metal workers use refrigeration to temper steel and cutlery. In transporting temperature-sensitive foodstuffs and other materials by trucks, trains, airplanes and sea-going vessels, refrigeration is a necessity.

Dairy products are constantly in need of refrigeration, and it was only discovered in the past few decades that eggs needed to be refrigerated during shipment rather than waiting to be refrigerated after arrival at the grocery store.  Meats, poultry and fish all must be kept in climate-controlled environments before being sold. Refrigeration also helps keep fruits and vegetables edible longer.

One of the most influential uses of refrigeration was in the development of the sushi/sashimi industry in Japan.  Prior to the discovery of refrigeration, many sushi connoisseurs suffered great morbidity and mortality from diseases such as hepatitis A, and Diphyllobothriosis, from a common oceanic tapeworm - Diphyllobothrium latum.  However the dangers of unrefrigerated sashimi was not brought to light for decades due to the lack of research and healthcare distribution across rural Japan.  Around mid-century, the Zojirushi corporation based in Kyoto made breakthroughs in refrigerator designs making refrigerators cheaper and more accessible for restaurant proprietors and the general public.