Forced-air central heating uses air to transfer heat. Air distribution is handled by vents, plenums, and ductwork, independent of the HVAC system. Air is returned to a central air handler through the return plenum from several large return grills (vents). Supply plenums direct air from the heating unit to the rooms to be heated. Each air handler contains an air filter, blower, heat exchanger/element/coil, and controls. Thermostats control forced air heating systems like any other central heating system.