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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.

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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.

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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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