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I have two bathroom exhaust fans that need to be vented out of the house, but I'd like to do it using a single wall penetration. Is there a code compliant, safe way to do this?

  • Both bathrooms are on the same level of the house.
  • There is no guarantee that both fans will be the same type, size, power, etc.
  • Each fan runs independently of the other.
  • Pipe runs will be different lengths from fan to exit point.
  • Fans are on separate electrical circuits.
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Nothing in the IRC actually touches on this. I have though had inspectors tell me they wouldn't pass if I did it and I have seen exhausts vent to the soffit vents pass... I did have a friend who did this and he said that every once in a while you would get a whiff of a dump from the other bathroom. Personally I would rather vent one to the attic then tie them together. – DMoore May 17 '13 at 21:39
How about this...use a non-motorized ceiling vent and tie it's 4" duct into the existing fan's duct with that Y adapter. Would the motorized fan create enough venturi to draw air through the non-motorized vent? – user47355 Jan 5 at 2:58

I don't know about code compliance. But, this might work for you:

1 Y-Connector at the outside vent.

enter image description here

2 Backdraft Dampers (one on each leg) at the Y to stop the backflow of noxious gases from one fan(on) to the other (off).

enter image description here

Note that the backdraft dampers will add to the resistance of the duct work. You may wish to move up to the next size of duct.

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What if both fans are on at the same time, do I have to double the size of the outlet on the wye to compensate? – Tester101 May 17 '13 at 14:34
I don't think so. Pressure and velocity should increase to compensate somewhat. 5" duct is 1.5 times bigger than a 4" (and a 6" is 1.5 times bigger than a 5") so that would probably mitigate. But we should build a mockup, test it, and write a blog post about the experiment. – Chris Cudmore May 17 '13 at 14:43

You could run both vent ducts to the same area and then install two vent caps side-by side. Your wall opening would have to be twice as big, but at least you would only have to do one cut, flashing, etc.

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The problem with this, would be finding a vent cap. – Tester101 May 17 '13 at 14:43

I did this setup and would NOT recommend it. I have an 80cfm panasonic in a small bathroom and a 110cfm older fan in my main bathroom. I used the same dampers shown above, the cloth ones. Plus each fan has its own damper built into the housing. The run is only perhaps 6ft total and I ran two 4" into a 6". When both fans are running at the same time (both taking a shower) the smaller fan gets totally overwhelmed and cannot move enough air. I'm thinking about changing the setup so both run out the side with their own wall vent.

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