The majority of home and smaller commercial air conditioning systems circulate a compressed gas refrigerant in a closed “split” system to cool and condition inside air. The refrigerant has to be re-cooled and condensed, and outside air is the medium most often used to accomplish this. The term “split” simply means that components are divided into inside and outside portions as opposed to being located together in a “package” unit.

The refrigerants, widely recognized by the trademark “freon” (which is a registered trademark of the DuPont company for refrigerants), helps cool and dehumidify the inside air. In a “forced air” system, an internal blower circulates the conditioned air through ducts to the rooms where the cooler air is needed. The air ducts generally run either below the ceiling and inside the rooms (conditioned air) or in the attic (unconditioned air). An outside fan pulls air across the external parts of the system to cool and condense the refrigerant.