Two Fans One Duct

You should not put more than one fan into a common duct each fan should have its own duct run to atmosphere.

Two fans one duct.

There may also be bypass ductwork around the second fan if only one fan is run for a period of time. 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. 2 bathroooms lead to the fan then exit through one pipe out the roof. 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.

Note that the backdraft dampers will add to the resistance of the duct work. If you try to combine them into a common duct it will bring with it a number of problems including an unbalanced system with positive negative air pressures which play havoc with the fan motors and could cause motor failure. This setup requires an in line centrifugal fan mounted in the attic drawing air simultaneously from both bathrooms see photo. It would then clear both.

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. I think it may be possible to arrange for one fan to suck air equally from two inlets. You d often blow air from one bathroom into the other and local building inspectors wouldn t approve it. 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.