Exhaust fan flapper. You will need to pull down the fan replace the old flapper with this flapper unit. When steam or air forms these flappers fly open and let the air out. Unfortunately the backdraft flapper may get stuck in the closed position. The larger the square footage the more CFMs you need. Make sure you get the appropriate size exhaust fan for your bathroom. Backdraft shutters or Butterfly Dampers are commonly used with inline exhaust fan systems. All fans now are required to be. Many times a little work will save or replace an old fan motor or b.
Backdraft dampers allow air to efficiently flow through exhaust ducts to the outside yet prevent the unwanted flow of air into a house when the exhaust fans are off. But most vents have metal flappers attached to the outside area of the vent. If you have an older fan check to see if it is thermal-overload protected. CFM is cubic feet per minute and is a measurement of airflow. Tape it to the exhaust hood where the flapper hits. Panasonic is one of the few manufacturers that use spring-loaded dampers in their fans. The larger the square footage the more CFMs you need.
When the vent hood fan is shutoff and the airflow diminishes and the flapper begins to close. The flappers are either placed in the fan at the point of attachment of the vent or closet the point where the vent pipe exhausts the air outwards. Bathroom vents usually use flexible 4 tube. The larger the square footage the more CFMs you need. The fan should move one CFM per square foot of space.
Unfortunately the backdraft flapper may get stuck in the closed position. Higher RPM where the flap was open anyway sounds the same. In this way it prevents the exhaust fan from blowing air down. White Gravity Grille Extractor Fan Ducting Wall Flap Ventilation 4 100mm.