Two Fans One Duct

2 bathroooms lead to the fan then exit through one pipe out the roof.

Two fans one duct.

This setup requires an in line centrifugal fan mounted in the attic drawing air simultaneously from both bathrooms see photo. Rather than two cheap axial fans into one duct outlet how about two ducts into a more powerful centrifugal fan which is a bit more expensive but has a lot more oomph. I think it may be possible to arrange for one fan to suck air equally from two inlets. You should not put more than one fan into a common duct each fan should have its own duct run to atmosphere.

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. There may also be bypass ductwork around the second fan if only one fan is run for a period of time. Total pressures are used instead of static pressure because the fans can actually be different sizes and a change in fan or connecting duct areas has an influence upon static pressure values. It would then clear both.

Note that the backdraft dampers will add to the resistance of the duct work. You d often blow air from one bathroom into the other and local building inspectors wouldn t approve it. 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. You may wish to move up to the next size of duct.