Caryaire model AG 663 damper is non return gravity operated damper. It is specially designed for use in air conditioning, ventilation and for air intake and exhaust. It has horizontally mounted blades free to rotate on horizontal axis. Normally damper is closed; Air pressure or air velocity opens & closes the damper. It is manufactured either in aluminium or galvanized steel sheet. Back draft dampers are used in HVAC systems to allow flow of air in one direction and prevent airflow in the opposite direction. Pressure at which Pressure relief damper starts to open is adjustable.