Air-conditioning systems serve to maintain indoor air within the two-dimensional comfort zone defined by a temperature and relative humidity region. As such they are part of space-conditioning systems for heating or cooling indoor space, although typically viewed as cooling systems that function when the outdoor conditions are outside the comfort zone. This comfort zone has been defined by teams collaborating e.g. under the ASHRAE (Am. Soc. for Heating, Refrigeration and Air-Conditioning Engineers) umbrella. Additional factors contributing to comfort are: air velocity, radiant temperature, human activity and type of clothing.

Air-condioning (AC or cooling) systems my be grouped into several design, size and application categories:

• By Size, for Residences: Window ACs, central ACs, or AC plants for apartment buildings
• By application, for comfort control in stationary space; in automotive, aircraft or marine cabins, or for temperature control in industrial processes