The heavy duty dampers are double-flanged, channel-frame style backdraft dampers.

Heavy duty Greenheck tornado dampers are double-flanged, channel-frame style backdraft dampers designed to protect against tornados and rapid pressure changes. Installed in ductwork, or wall mounted, near the exterior of a building, Greenheck tornado dampers allow for air movement (intake or exhaust) during normal weather conditions. If a tornado passes near the building, the negative pressure caused by the tornado will cause the damper to close and prevent the collapse of the ductwork. Once the tornado has passed and the pressure equalizes, the spring-loaded blades will return the damper to its open position and revert back to normal ventilation. Model HTOD-330 will close in the same direction as normal flow and Model HTOD-331 will close in the opposite direction as normal flow.