Air conditioning is the treatment of air to control simultaneously its temperature, humidity, cleanliness, and distribution to meet the requirements of a conditioned space. Common use of the term "air conditioning" applies it to the cooling of air; however, true air conditioning treats all aspects of indoor atmospheric comfort.

An air conditioning system uses an assembly of equipment to treat air. Normally the assembly includes a heating system for modifying winter indoor temperature and humidity; a refrigeration system for modifying summer temperature and humidity, a means to maintain indoor air quality (i.e., air filters and fresh air intake); a method of distribution of conditioned air; and a control system, such as a thermostat, to maintain desired comfort conditions.

Air conditioning systems fall into two broad classes. Comfort air conditioning, which accounts for most applications, is used to modify and maintain the indoor environment for habitation...