Can I use flex duct? Andrew March 21, 2022 19:03 Updated Yes you can use flex duct, but it is important to make sure it is pulled tight. Rigid ductwork is preferred/recommended. Related articles QFAS Bath Fan Wiring Diagram What is "Relative Humidity"? Can Air-King bathroom exhaust fans be used with a variable speed control? I require a specific timed fire rating, will the Air King fire rated fan series work for me? Comments 0 comments Please sign in to leave a comment.