An air conditioning system, or a stand-alone air conditioner, provides heating,
cooling, ventilation, and humidity control for all or part of a building.
'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.

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.