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minimum Bath fan duct size

Posted: Tue Jul 10, 2018 9:36 pm
by greg1
What is the minimum pipe size for bathroom exhaust fan? Im tight on space lol

Re: Bath fan

Posted: Tue Jul 10, 2018 10:28 pm
by Shannon
Most require a 4” duct but there are the odd 3” ones out there. Shop for fan first to find the one that works for you.

Re: Bath fan

Posted: Tue Jul 10, 2018 10:49 pm
by greg1
I think I found some that are 3 inches and some that are 4 inches also have 3 inch adapter.

Does it have to be connected using metal duct or can it be one of those flexible ones?

Re: Bath fan

Posted: Wed Jul 11, 2018 12:02 am
by A. Spruce
They come in 3" and 4", and you can use an adapter to go from one to the other. Going smaller to larger won't affect efficiency, going from larger to smaller will reduce the capacity of the fan.

You can use a flex duct, however, it will reduce air flow and create more surface area for moisture to condense and drop out of the air stream, which can cause water to run back down into the fan. Your climate will dictate, cold, don't use it, hot, it doesn't matter.

It is a good idea to make sure that there is a horizontal section of pipe that slopes away from the fan to prevent condensation from running back to the fan. Insulating the duct will help it move the warm, moist air further before condensation occurs, meaning you'll exhaust more of the moisture out before it has a chance to become a problem.

Tape your joints with metal tape, keeps them from leaking both air and moisture. Venting through the roof is the better choice, but gable walls are acceptable too. If you vent to a soffit, know that all that warm moist air is going to make its way right back into the attic through the soffit vents, keep this in mind when locating your exhaust point.

Re: Bath fan

Posted: Wed Jul 11, 2018 8:30 am
by greg1
Wow! thank you Spruce! thank you Shannon.
Lots of good info.