Two Fans One Duct

You d often blow air from one bathroom into the other and local building inspectors wouldn t approve it.

Two fans one duct.

But while you can t have two fans with one vent you can make one fan and one vent serve two bathrooms. 2 backdraft dampers one on each leg at the y to stop the backflow of noxious gases from one fan on to the other off. This setup requires an in line centrifugal fan mounted in the attic drawing air simultaneously from both bathrooms see photo. Note that the backdraft dampers will add to the resistance of the duct work.

You may wish to move up to the next size of duct. It would then clear both. There may also be bypass ductwork around the second fan if only one fan is run for a period of time. 2 bathroooms lead to the fan then exit through one pipe out the roof.

The problem as i understand it is that if both fans are using the same duct and are both running at the same time the fans will be less efficient and more noisy because it will be like installing a fan with a smaller duct that it was rated for. I think it may be possible to arrange for one fan to suck air equally from two inlets. Of course there is some issue with the volume of air from 2 ducts going into 1 duct but i m sure some engineer has figured that part out. You should not put more than one fan into a common duct each fan should have its own duct run to atmosphere.