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?