It is important to make sure that when appliances or fans are turned off, that air is not leaking back into the home through the duct system. Range Hoods, Bathroom Fans, Clothes Driers, Water Heaters and Furnaces, Vent Fans of all types should be exhausted directly to the outside of you home. As part of the installation, a back draft damper needs to be installed in the duct work to prevent air from coming back in.
The Cape Backdraft Damper is a great product that will perform to your expectations by preventing air from coming in when the appliance is off. And it is designed so that it will not diminish the performance of your appliance as it offers no resistance to out-flow.