I have a new bathroom fan that I'm installing that is 70 CFM. It will only be in the water closet part of the bathroom as the bathroom already has a larger 150 CFM fan in it.

The 70 CFM fan has a 3 in. duct outlet. I was wanting to use insulated ducting to run to the outside of my house but can't find any locally and am not finding many options online either.

Am I ok in using a 3 inch to 4 inch duct increaser to attach a 4 inch duct to my fan or would I be causing some issues with the air movement?

The duct will be about 12 feet in length and will have a pretty strait shot venting out through an exterior wall.

2 Answers 2


There's no problem doing so. You'll have a lower air velocity at the outlet, but that shouldn't be an issue. It certainly won't result in lower overall exhaust efficiency.


The bigger the better for exhaust. As stated previously it will reduce velocity which can cause dripping if the air is really humid like above a shower.

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