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.
  • 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
    Commented May 17, 2013 at 21:39
  • 1
    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
    Commented Jan 5, 2016 at 2:58

5 Answers 5


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.


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

1 Y-Connector at the outside vent.

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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).

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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.

  • 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
    Commented May 17, 2013 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. Commented May 17, 2013 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.

  • The problem with this, would be finding a vent cap.
    – Tester101
    Commented May 17, 2013 at 14:43

None of the above solutions going to work.

That being said, the only plausible workaround is to install no fans in the washrooms. But run only the ductwork with a Y adapter. The ducting for each leg of the Y adapter to each washroom must be nearly identical. You can replace the fan opening with a grill for each washroom. Then add an 'inline fan' to the remaining leg of the Y adapter and run ducting to vent outside.

The above scenario will work provided the following: 1. When inline fan is in operation, air from both washrooms will vent out. 2. Both washrooms must have nearly identical square-footage. 3. Both washrooms doors are expected to be in closed position when inline fan is in operation. 4. Duct diameter must be identical for each leg of the Y adapter.

Folks, this is THE ONLY WAY. No other solutions exist nor work.


My suggestion is to use a mechanical damper such as ALDES. These dampers open with a very low pressure drop. I have always achieved good success using two to three Fantech fans installed in the attic (very quiet, and the ceiling plenum box operates under a slight vacuum) with the ALDES dampers vented out a 6" roof vent. They fit snugly inside the galvanized duct just after the fan. They are truly Cadillac dampers. The fabric dampers have not held up for me. The plastic dampers that come with ceiling mounted bath fans or those on Fantech fans just do not prevent air backflow well enough to tie multiple fans together.

Base on my air flow measurements the ALDES dampers has reduced the airflow only about 10%.

One last recommendation is DO NOT vent bathroom vents through a soffit. And do not buy a home that has bath vents in the soffits.

  • The problem here is that there can be a differential between the two exhaust sources that is a low pressure drop, meaning one exhaust path will be closed while its fan is running. It's not a good idea.
    – isherwood
    Commented Mar 19 at 13:12

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