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I have these three units within 5 feet of eachother. The place where the bathroom vents to is the only place within reach to have a penetration to outside.

My options are to stack these three exhausts, or up the size, add some backflow preventers, combine them, or something else that you guys might know.

Also, I'm concerned the dryer vent may form a vapor trap or some sort, but I don't know enough about it to figure that out.

Diagram

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Is there any reason not to run all three outside, side-by-side? –  wallyk Jan 24 at 7:32
    
There's a lot of plumbing there, it's where all the drain pipes from the house converge, so where the bathroom vent goes is basically the only clear spot. But also, that exterior door opens up to under the deck. There's a deck about 10 inches to the left of that existing hole, and concrete 8 inches to the right –  kavisiegel Jan 24 at 16:47
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If you have a gas dryer, that would be a no for the dryer, no question. For electric dryer and the other devices, it feels to be against code to me, though I can't find a specific citation on a quick look. In any case, there is a risk for one device to back feed another device unless all devices have effective backdraft dampers on the device itself. I doubt dryers and vacs have such devices, and for the ones I've seen on bathroom fans, I would not deem them "effective".

It's not a good idea. Is there any way to get enough room by transitioning into equivalent rectangular ducts ganged together? It may take a custom built fitting, but you need to exhaust through a combined equivalent area of all 3 ducts added together whether combined or not. So if there's not enough room for this, it's not going to work anyway.

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