A central air conditioner uses the furnace's blower to draw room air into the unit through return-air ductwork and a filter that removes dust, hair and lint. The room air moves past a chilled indoor coil-the evaporator-to release its heat. The resulting cold air travels on to the plenum, usually a large metal box at the top of the furnace, where it's channeled to air supply ductwork and returned to rooms.
--Don Vandervort
