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Hello I have an off grid cabin with one bathroom and the toilet room smells of sewer pretty bad at times, esp with rain and colder weather. I looked under the crawl space for venting and there is a vent line installed but it appears to vent into the soffit and not through the roof. I’m sure this probably isn’t the proper way but could I just cut that vent line off in the crawl space and add an AAV right there and eliminate the venting up through the walls and into the soffit ? If not is there another solution here?

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  • If it is vented(even badly though soffits) should not have a smell in the room(unless vented right by the window). In the room I would check for loose P trap joints/leaking wax seal on the toilet.
    – crip659
    Oct 15, 2023 at 21:01
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    Municipal/city sewer connection, or septic tank? Oct 15, 2023 at 21:16
  • It’s a septic tank
    – Matt Brown
    Oct 16, 2023 at 1:32
  • And just recently put to use so it’s basically a new system
    – Matt Brown
    Oct 16, 2023 at 1:49

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Definitely replace that vent with an AAV. The sewer gases could be wafting the sewer gases down through a ceiling fan or some hole in the wall.

Also check the P-traps. They could dry out, or hair could drain them. If you have a floor drain, check that too. There are ways to keep traps full by maintaining a film of mineral oil on the water, for example.

You mentioned rain and colder weather. It seems like a temperature change affects the air currents in some way to bring the sewer gases to your nose. I'd expect it to create a "stack effect" where warm indoor air goes up and cold air goes in at the bottom of the cabin (if there are gaps or cracks). This brings up the possibility that plumbing cracks below your floor could be leaking sewer gas.

The toilet flange is at the floor so it's worth looking at as well. Even if the toilet-floor boundary is sealed with silicone, the sewer gases could leak down into the crawl space, which through the stack effect could make it into the bathroom.

Good luck.

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    OP doesn't indicate if there are any code authorities to deal with, but AAVs are often disallowed by code and when not explicitly disallowed are only allowed in very specific instances, like when there's a sink in an island. AAVs have a tendency to fail and need replacement far more often than an outdoor vent. Since the venting was 90% completed, it seems to make more sense to finish that work than abort it and replace it with a maintenance headache.
    – FreeMan
    Oct 16, 2023 at 16:50

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