Two Fans Into One Duct

Plus each fan has its own damper built into the housing.

Two fans into one duct.

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. The run is only perhaps 6ft total and i ran two 4 into a 6. It would then clear both. This lead to fan malfunctioning as i think it overheated.

You should not put more than one fan into a common duct each fan should have its own duct run to atmosphere. 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. Because the 800 cfm fan won t fit in the duct i would have to build a larger frame for it then connect it with some sort of funnel to the duct. Same as above only with two 800 cfm fans.

The fan is to take air from in the house run it thru the heater then back into the house. So i have an input and an output duct. I felt air coming from bathroom 2 s fan down through the metal duct pipe for venting bathroom 1. When both fans are running at the same time both taking a shower the smaller fan gets totally overwhelmed and cannot move enough air.

The 2 fans have their own ducts but just before they vent out the roof their duct pipes are attached to a common duct upside down y which means they vent out the same hole in. I used the same dampers shown above the cloth ones. Buy one 650 cfm fan and one 800 cfm fan and install the 650 blowing into the duct and the 800 sucking out of the duct or maybe the other way around. You d often blow air from one bathroom into the other and local building inspectors wouldn t approve it.