Reduce bathroom fan vent from 4" to 3"?

anniel89July 20, 2014

I need to install a basement bathroom fan. It will run about 1 1/2 feet and out the wall. I'm pretty sure I have enough space for a 4" vent but I may only have enough for a 3".

I haven't found a fan/light with a 3" vent. Can I reduce from a 4" to a 3"? If so, should I look for a fan with higher cfm (or lower?)? It's not a very long run - any thoughts would be appreciated.

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There are 4" to 3" reducers available at most hdw. stores and for that short a run, you shouldn't affect the efficiency.

    Bookmark   July 20, 2014 at 1:51PM
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You can also transition from 4" round to oval, and then back to round where it connects to the outside cap. The oval shape easily fits inside a wall and you can maintain the same cross-sectional area. Even Amazon carries a variety of oval duct and fittings. Below is one example that is used for a dryer, but you could use it with a fan as well. Or you could just get fittings and make up your own to fit your situation.

Here is a link that might be useful: Round-to-oval-to-round periscope duct

    Bookmark   July 20, 2014 at 2:46PM
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Thanks folks! Mostly I'm worried about the space where it exits the house. I have just about 4" if I'm lucky between the subfloor of the main level and the concrete porch outside. I'm trying to squeeze the vent in that space.

If I can go down to the 3" without damaging the fan it will be no problem.

    Bookmark   July 21, 2014 at 10:26AM
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However, finding a dampered vent cap may be difficult. So you might need to end it at the siding with a flat screen, and no damper.

    Bookmark   July 21, 2014 at 2:34PM
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Amazon has several dampered vent caps...this one for example:

Here is a link that might be useful: Broan, 3

This post was edited by kudzu9 on Wed, Jul 23, 14 at 5:20

    Bookmark   July 23, 2014 at 5:14AM
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