A Forced-air heating system is one which uses air as its heat transfer medium. These systems use ductwork and vents as a means of air distribution. The return plenum carries the air from several large return grills (vents) to a central air handler for re-heating. The supply plenum directs heated air from the central unit to the rooms which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central system, thermostats are used to control forced air heating systems.