The term HVAC refers to the three disciplines of Heating,
Ventilating, and Air-Conditioning. A fourth discipline,
Controls, pervades the entire HVAC field. Controls determine
how HVAC systems operate to meet the design goals of comfort,
safety, and cost-effective operation.
* Heating can be accomplished by heating the air within
a space (e.g. supply air systems, perimeter fin-tube "radiators"),
or by heating the occupants directly by radiation (e.g.
floor/ceiling/wall radiation or radiant panels).
* Ventilating maintains an adequate mixture of gases in
the air we breath (e.g. not too much CO2), controls odors,
and removes contaminants from occupied spaces. "Clean"
air helps keep occupants healthy and productive. Ventilation
can be accomplished passively through natural ventilation,
or actively through mechanical distribution systems powered
* Air-conditioning refers to the sensible and latent cooling
of air. Sensible cooling involves the control of air temperature
while latent cooling involves the control of air humidity.
Room air is cooled by transferring heat between spaces,
such as with a water loop heat pump system, or by rejecting
it to the outside air via air-cooled or water-cooled equipment.
Heat can also be rejected to the ground using geothermal
exchange. Cool air is not comfortable if it is too humid.
Air is dehumidified by condensing its moisture on a cold
surface, such as part of mechanical cooling), by removing
the moisture through absorption (desiccant dehumidification).
In dry climates, humidification may be required for comfort
instead of dehumidification. Evaporative humidification
also cools the air. Further, in such climates it is possible
to use radiant cooling systems, similar to the radiant heating
systems mentioned above.
* Controls ensure occupant comfort, provide safe operation
of the equipment, and in a modern HVAC control system enable
judicious use of energy resources. HVAC systems are sized
to meet heating and cooling loads that historically occur
only 1% to 2.5% of the time. It is the function of the controls
to ensure that the HVAC systems perform properly, reliably,
and efficiently during those conditions that occur 97.5%
to 99% of the time.