Air duct systems are a vital component of any heating system. Air ducts are used to distribute the heated air throughout the home. They are also an important component of a cooling system, as they help remove excess heat from the inside of the home. Air ducts come in two different styles: supply and return. Supply air ducts carry heated or cooled air from the furnace to various rooms in the home, while return air ducts carry cool or hot air back to the furnace for heating or cooling.