Types Of Sheet Metal Duct Ducts are integral parts of ventilation systems, including heating and air conditioning. They are the pathways for airflow and are important for indoor air quality. Ducts are usually not visible, being inside walls and ceilings, connected to furnaces and central air conditioning units. Sheet metal ducts are made from aluminum sheet metal or galvanized steel sheet metal. There are many kinds, sizes and shapes of sheet metal ducts, but some common duct pieces are plenums, dampers, duct sleeves, roof curbs, and squares to rounds. Plenums A plenum is a sheet metal box that attaches to the furnace or air unit and other duct work in the airflow system. In an HVAC system, there is a plenum for the supply air and one for the return air. The plenum has flanges or flashing used to attach it to the furnace, eliminate gaps and create a good barrier for the air flow without leaks. Dampers A damper is a sheet metal part--either round or square depending on the shape of the duct work it will be used with--that is used to control drafts or surges in the air flow through ducts. It opens when the system is on and pushing air through the vents, and it closes when there's no active air flow to keep out drafts or chilled or hot air. Dampers are used at central locations in the intake and outflow of the air system. They can be a flat metal part on a swivel or a levered series of flat metal pieces that open and close together. Duct Sleeves Duct sleeves are the long rectangular metal ducts through which air flows in a heating and air system. They don't attach directly to the furnace or air unit; instead, they attach to other duct sleeves and other duct work pieces like plenums and corner connectors. They make up the main pathway for air flow. Roof Curbs A roof curb is a sheet metal duct box that connects a rooftop heating or air unit to the duct work system inside the building. It has flashing around the bottom edge and the connecting edge to seal any air leaks and is made to fit the unit it will connect to. Squares to Rounds Squares to rounds are sheet metal duct connector pieces designed to connect square duct work to round duct work. One end is square shaped. The sides are formed and attached to a round collar the right size to connect with the round duct work. These are transition pieces for systems that have different shapes of duct work