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HVAC is an initialism that stands for "heating, ventilation, and air-conditioning".

These three functions are closely interrelated, as they all change the temperature, pressure and humidity of the air within the building. In modern building designs, the design, installation and control systems of these functions are integrated into a single "HVAC" system.

Table of contents
1 Heating
2 Ventilation
3 Air-Conditioning
4 Thermostats
5 External links


Heating systems may be classified as central or local.

Central Heating

Central heating is often used in cold climates to heat private houses and public buildings. Such a system contains a central boiler or furnace to heat water, pipework to distribute the heated water, and heat exchangers or radiators to conduct 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 heat exchangers may be mounted on walls, or buried in the floor to give under-floor heating. When so mounted it is often referred to as "radiant heating".

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 is often fed through another heat exchanger inside a storage cylinder to provide hot running water.

Forced air systems send air through ductwork. The ductwork can be reused for air conditioning and the air can be filtered or put through air cleaners.

The heating elements (radiators or vents) should be located in the coldest part of the room, typically next to the windows. 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. Thus 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 in walls and floors of public baths and private villas. The ducts were fed with hot air from a central fire. Perhaps there are examples from other early civilisations waiting to be unearthed.

Local Heating

Local heating devices are self-contained heaters that are usually controlled manually. Such devices include:

While central systems are more efficient, local systems offer greater flexibility. In sparsely occupied building, the unused rooms can be left unheated in local heating systems.


Ventilation in includes both exchange of air to the outside as well as circulation of air within the building. Methods for ventilating a building may be divided into natural and forced types.

Natural Ventilation

In cold climates natural ventilation is often just a matter of opening a window, but in hot climates it is an important consideration in the design of buildings.

Forced Ventilation

Forced ventilation may be used to control
humidity or odours. Kitchens and bathrooms typically have mechanical ventilation to control both. 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 vans traverse unheated space (e.g. an attic), the ducting should be insulated as well to prevent condensation on the ducting.

Heat recovery ventilation systems employ heat exchangers to bring the fresh air temperature to room temperature.

Ceiling fans and table/floor fans are very effective in circulating the air in the room. Paradoxically, because heat rises ceiling fans may be used to keep a room warmer.

See also punkah.


An air-conditioning system provides heating, cooling, ventilation and humidity control for a building. It is often installed in modern offices and public buildings, but is difficult to retrofit (install in a building that was not designed to receive it) because of the bulky air ducts required. The system must be carefully maintained to prevent the growth of pathogenic bacteria in the ducts.

A dehumidifier is an air-conditioning-like device that controls the humiditity of a room or building. They are deployed in basements, which because of their lower temperature have a higher relative humidity. (Conversely humidifiers increase the humidity of buildings.)

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


Thermostats control the operation of HVAC systems, turning on heating or cooling to bring the building to the set temperature. Typically the heating and cooling systems have separate control systems (even though they may share a thermostat) so that the temperature is only controlled "one-way". That is, in winter, a building that is too hot will not be cooled by the thermostat.

See also: HVAC control systems, refrigeration

External links