Postings of random sewer odors pop up frequently. Sometimes the cause is found. Often it goes unresolved. So my question.
The black tank flush anti siphon valve has an atmospheric vent to allow air to enter the pipes to prevent siphoning. The atmosphere it vents to, of course, is the inside of the RV. That vent closes while flushing, and then opens when not flushing to allow air to enter the pipe so the water will drain into the black tank. Seems not many people are aware of that atmospheric venting function. So much for the operation.
Now if that anti siphon vent is open, and all the water drains out of the pipe to the black tank, and the water in the black tank is below the spray head, then there is an open air path from the black tank thru the pipe to the anti siphon vent. That means that any positive pressure in the black tank could push odors up to that vent. Positive pressure could be due to flushing with a blocked or partially blocked black tank roof vent, or strong wind blowing just right to try to push air down that roof vent, or …. Another possible cause would be running a ceiling vent fan with all windows and doors closed. Air going out the vent has to be replaced by new air coming in thru any crack or crevice open to outside air. And that could also potentially pull air thru the anti siphon valve vent.
Have never heard of anyone tracing the odor to the anti siphon valve. But then has anyone ever thought of that possibility. Never heard anyone mention it. Curious minds wonder.
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Bill & Patricia
Riley, our Golden
2007 3075RL (recently sold, currently without)
I guess that's possible. I have read of people having a smell under the kitchen (island) sink. They covered the "Air Admittance valve" under there with a baggie and taped it off. The smell went away. Bad Air Admittance valve.
I thought was an excellent illustration and hypothesis and most likely the absolute truth. As with so many great bits of information gained from this forum - I hope my brain can remember this when I need it! Thanks BB!
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MikenDebbie Aggie ‘77 in the sticks near Austin TX
2019 Chevy 3500 High Country DRW
2018 Montana 3921FB
Aussie Gus + Texas Heeler Jimmy
Hmmm. This is certainly a plausible explanation. I know we've experienced worse smells when the air vent is open, fan is off and a very light wind outside blows air from the black tank vent pipe toward the open air vent.
Air vent open, fan on and no other windows cracked in the RV can really cause a smell to occur - and your diagram shows a very good reason why.
Ironically, the toilet paper holder is mounted on the inside of the cabinet door EXACTLY where the anti-siphon valve is located. Can't take care of business without accessing that area!!
Like Mike said, I hope I remember to check this on our next trip.
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Clark & Peggy
2022 Montana 3791RD, 800i solar, 2x100ah Battleborn
2017 Ford F350, CC, LB, 6.7, 4x4
It will be interesting to see if anyone can actually trace the sewer odor to the anti siphon valve. Many of us removed the anti siphon valve, connected the two pipes together, and installed an external vacuum breaker valve due to having had the anti siphon valve leaking (as mine did), or in some cases as a proactive measure to eliminate the possibility of a leak. Of course that also unknowingly removed the possibility of odor coming from that potential source.
Simply the musings of an idle mind.
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Bill & Patricia
Riley, our Golden
2007 3075RL (recently sold, currently without)
I think I finally got so far off in left field no one dared venture a comment. 
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