HVAC

HVAC may also stand for High-voltage alternating current

Software being used to design HVAC systems

HVAC (pronounced either "H-V-A-C" or, occasionally, "H-VAK") is an initialism/acronym that stands for "heating, ventilation and air-conditioning". This is sometimes referred to as climate control.

These three functions are closely interrelated, as they control the temperature and humidity of the air within a building in addition to providing for smoke control, maintaining pressure relationships between spaces, and providing fresh air for occupants. In modern building designs, the design, installation and control systems of these functions are integrated into a single "HVAC" system.

The term air handler can mean a whole unit including the blower, heating and cooling elements, filter racks or chamber and dampers, but not including the ductwork through the building.

Heating

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, furnace or heat pump to heat water, steam, or air; piping or ductwork 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 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. Contrary to fiction, most ducts cannot fit a human being, creating a potential security liability.

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.

Ventilation

Ventilation includes both the 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

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 (chimney effect) thus forcing fresh cool air to be drawn into the building naturally though openings in the lower areas. These systems use very little energy but care must be taken to ensure the occupants' comfort.

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 fans 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.

Air-conditioning

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. A duct system must be carefully maintained to prevent the growth of pathogenic bacteria in the ducts. The alternative to large ducts to carry the needed air to heat or cool an area is the use of remote coils or split systems. These systems are gaining popularity in commercial buildings although are most often seen in residential application. The remote coil is connected to a remote condenser unit using piping instead of ducts. The units usually have a fan to move air across the coil, although recent innovation have seen static units installed in some large office spaces.

A dehumidifier is an air-conditioning-like device that controls the humidity of a room or building. They are deployed in basements, which because of their lower temperature have a higher relative humidity. In food retailing establishments large open chiller cabinets are highly effective at dehumidifing 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 control system to maintain constant air quality.

Thermostats

Thermostats control the operation of HVAC systems, turning on the heating or cooling systems 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. Thermostats may also be incorporated into facility energy management systems in which the power utility customer may control the overall energy expenditure. In addition, a growing number of power utilities have made available a device which, when professionally installed, will control or limit the power to an HVAC system during peak use times in order to avoid necessitating the use of rolling blackouts. The customer is given a credit of some sort in exchange.

See also

• American Society of Heating, Refrigerating and Air-Conditioning Engineers
• Boiler
• Building construction
• Domotics
• HVAC control systems
• Radiator
• Refrigeration
• Solar energy
• LonWorks -- A network protocol used for Building Automation Control
• BACnet -- A competing network protocol used for Building Automation Control

External links

• HVAC Directory - North American Directory for Heating, Ventilation and Air Conditioning Contractors and Consultants.
• Air Conditioning Contractors of America
• Links to HVAC web pages
• Northern Air Conditioning & Heating
• sci.engr.heat-vent-ac newsgroup
• Outdoor Patio Heaters
• Free online HVAC heat load calculator
• Ventilation HowTo's
• The UK Selfbuild FAQ