Description: 【BACKDRAFT DAMPER】: Square air duct connector with Check valve, made of durable material, resistant to oil and heat. The backdraft damper keeps odors and exhaust gases out of the house, preventing backflow. 【WORKING PRINCIPLE】 : 1. When the exhaust starts, the air pressure pushes the butterfly valve to open, and odor and exhaust is quickly discharged. 2. After the exhaust is completed, the pipeline butterfly valve will automatically close under the action of gravity. The check valve will always remain closed to prevent the exhaust, dust and odor from entering the room. 【VALVE OPENS AND CLOSES AUTOMATICALLY】: The Duct adaptor with butterfly Check valve design facilitates smooth opening of the valve when driven by during exhaust. After exhaust, the butterfly valve will automatically close under the action of gravity. 【EASY TO INSTALL】 : Wall mount air duct check valve, with duct adapter rings, can be perfectly match with soft and hard duct. 【WITH SEALING STRIP】: Backdraft damper for range hood with sealing strips reducing the wall, can be closely attached to the wall, enhance the sealing performance, no leakage, and ensure good sealing performance. Size Chart: S Size: 20.5cmx20.5cm/8.07inchx8.07inch,suit for 150~180mm diameter ducting hose. M Size: 21cmx21cm/8.27inchx8.27inch, suit for 160~180mm diameter ducting hose. L Size: 22cmx22cm/8.66inchx8.66inch, suit for 160~200mm diameter ducting hose. Package Includes: 1 Backdraft Damper Note: Please allow slightly errors due to manual measurement and different monitors.