It does what it says on the tin, stops back draughts through a bathroom extract fan and also fits perfectly within the 200mm diameter extract pipe.
Cheaply made wouldn’t purchase again
Not convinced I really needed this part but it fitted with ease and along with replacement fan works a treat
Simple product did the job
Not 100mm they are 97mm fitted inside ducting. Had to bond them in to stop the fan blowing them along the ducting. One spring Inadequate, not strong enough to keep the flaps closed.