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Reduce bathroom fan vent from 4" to 3"?

Posted by anniel89 (My Page) on
Sun, Jul 20, 14 at 10:28

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.


Follow-Up Postings:

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RE: Reduce bathroom fan vent from 4" to 3"?

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


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RE: Reduce bathroom fan vent from 4" to 3"?

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


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RE: Reduce bathroom fan vent from 4" to 3"?

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.


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RE: Reduce bathroom fan vent from 4" to 3"?

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.


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RE: Reduce bathroom fan vent from 4" to 3"?

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


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