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Two of my bathrooms are on the main floor, adjacent to one another (they share a wall). One is the ensuite, the other is the main bath. We recently noticed that scents were traveling from one bathroom to the other when one fan was running but the other wasn't.

I went up into the attic and realized that the hose from the main bath was joined to the ensuite fan's exhaust hose with a wye before going to a single roof vent. The fans themselves are less than 10' apart in the attic. Figured the simplest and quickest fix was to put a damper flap in one of the hoses. Which seemed to do the trick as far as stopping the scents from the ensuite going into the other bathroom. But. Always a but.

The main bath is now steaming up noticeably more than it was before, which makes it less comfortable and increases mold risk. I did the tissue test and the fan won't even slightly hold it against the grille, so it obviously isn't moving enough air. I checked to make sure I didn't install the damper backwards, and it doesn't seem like it's binding or stuck. I think the fan just doesn't have enough oomph.

Other than just pulling out the damper, I think I've got two options: try installing a more powerful fan (the current one is several years old and looks like a 50cfm contractor model), or cut a hole in the roof and vent it separately.

Could a higher CFM fan push enough to overcome the damper? I've had to replace bathroom fans before, so I don't have much worry about tackling it myself.

I assume a separate vent is the ideal solution, but I'd like to avoid it if I reasonably can. Aside from from just not wanting to put another hole in the roof, there's already snow on the roof here, which makes it a little trickier for at least the next 4-5 months. If I do decide to install another vent, is there a minimum distance requirement?

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  • How much hose do you have from each fan to the wye, and how much beyond the join to the vent?
    – Chris H
    Commented Nov 7, 2023 at 16:09
  • Each hose is maybe 6-8'. The wye is immediately below the vent, maybe a foot of hose from the wye to the vent.
    – SpycheDad
    Commented Nov 7, 2023 at 17:01
  • That's a shame, I was going to wonder if keeping the runs as independent as possible would be enough
    – Chris H
    Commented Nov 7, 2023 at 17:36

2 Answers 2

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A higher cfm fan would just push more air the wrong way. The ideal solution is to separate the vents. You have obviously found the join, so separate them.

You dont have to take the new one through the roof. If you can access the soffit, you can put the exhaust there.

You have to ask your local municipal body if there is a regulation. Where I am, there isn't one for it. Some of my vents have 9 inches of separation and they work just fine.

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    +1 to checking local regulations. Around here, if soffits have openings into the attic, you have to be very careful about where you vent out. There are also limits on how close you can get to an operating window. Any chance your roof jack could accommodate two custom bent demi-lune pipes? Commented Nov 7, 2023 at 13:54
  • Just curious, @AloysiusDefenestrate, regulations about a bathroom vent near open windows? Combustion venting, yes 100%, but humidity & smells?
    – FreeMan
    Commented Nov 7, 2023 at 14:43
  • My soffits are vented, so I think the exhausted air would just go right back into the attic.
    – SpycheDad
    Commented Nov 7, 2023 at 18:38
  • @freeman -- codes and amendments vary, but this link illustrates where I got my crazy ideas: thebuildingcodeforum.com/forum/threads/… Commented Nov 7, 2023 at 19:13
  • Ah. Makes sense.
    – FreeMan
    Commented Nov 7, 2023 at 19:19
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You could solve this with a bigger fan.

The trick is where you put the bigger fan.

Which is "in the attic, after the Wye" and wire it so that either bathroom can turn the fan on, and it will always vent both bathrooms, regardless of which called for fan.

So gut or replace the bathroom fans to just be intake grilles, and put an inline fan in the attic blowing out the single roof vent, sucking in from both bath vents. Attempt to arrange it so both bath vents are roughly the same amount of duct to the Wye point.

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  • Even a weak inline fan, used additionally, would eliminate the backflow into the other room
    – Chris H
    Commented Nov 9, 2023 at 15:52

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