Ventilation Dampers sales@airsync.eu View more products Back Draught Shutter PVA Request more information Used for preventing the reverse of air-flow in circular duct systems. The pressure relief valve is used for preventing the reverse of air-flow in circular duct systems. The casing of PVA is made from galvanized steel,the blades are made of aluminium. With a inner foam seal for better tightness and noise reduction. The valve opens when overpressure occurs and closes by means of spring when the pressure drops. It is recommended to install the shutter into horizontal ducting. Ventilation Damper TYPE B Request more information Ventilation Damper TYPE C Request more information Iris Damper IRIS Request more information New type offer a better air-tightness sealing and easier control in size.