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Old 05-21-2020, 07:16 PM   #1
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Bath fan exhaust line length?

Hello all,

I have a set of three bath fans to vent outside the house. Two are easy. I can get out into the eaves in a relatively short run. The third one is on the second floor in a dormer.

The house is a steep-pitch cape with front and rear dormers. The bathroom dormer is a shed dormer. The shed dormer pitch is fairly flat.

The dormer has a flattish roof and the plumber suggested not venting through the roof because of ice buildup from the exhaust.

He suggested going out the side of the dormer with a dryer vent. It could work, but the only place it could fit would vent just above the main roofline and could plug with snow. (It's at the top of the shed, almost where the rooflines merge.) It would also build ice. And dormer valleys are already prone to ice damming and leakage. Putting a warm moisture source there in winter seems like a bad idea.

I'm thinking about running it down the wall and out the first floor eave. But it's a long run. How long of a run is okay for code (or practicality)? And can the exhaust run down or should it always run up?

Finally, can multiple bath fans share a common vent, or should they each have their own?


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Old 05-21-2020, 07:42 PM   #2
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Re: Bath fan exhaust line length?

I'm smiling as I read your post. I'm sitting in a cape where I had battled for years with ice dams. My bath vent goes out the back 2' under the overhang.

With the roof being flattish I'm guessing those soffits are closer to the top venting than to other soffits providing low venting, mine is. With that soffit positioned high there is the possibility it is acting as an exhaust vent rather than the intended intake function, again mine was. That problem directs warm attic air out under that low slope and contributes to the snow melt and ice problems. More details if needed.

Now, as for venting bath exhausts out into soffits, not good as that exhaust can be drawn in by the natural ventilation.

Can you post some pictures of those soffit areas?

You mention snow, how cold is your climate?

I volunteer my help and opinions, but you are responsible for what YOU choose to do with that information.
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Old 05-21-2020, 10:31 PM   #3
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Re: Bath fan exhaust line length?

Moved thread to HVAC and deleted duplicate post
When posting in forums, letting us know your location will help others give better feedback/advice/solutions to your questions
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