The duct, or air distribution, system used in cooling and heating your home is a collection of tubes that distributes the heated or cooled air to the different rooms. This branching network of round or rectangular tubes usually constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite is found within your home. The duct system is designed to supply rooms with air that is “conditioned” that is, heated or cooled by the heating, ventilation, and air conditioning (HVAC) equipment and to circulate or return the same volume of air back to the HVAC equipment.