The Master Flow 4 in. Starting Collar is suitable for most HVAC applications. It provides a transition from a flat surface like a plenum made from metal, ductboard or insulated to round pipe or insulated flexible duct. It can be used with ductboard, an internally lined or unlined plenum. This galvanized steel starting collar features extra-long tabs to fit most duct board thicknesses.Connects round pipe or insulated flexible duct to a flat surface to start a duct runInstall with sheet metal screws not includedFits most HVAC types for universal applicationCan be used with an internally lined or unlined plenum