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Can sewer gas reenter the house from the stack vent through exhaust fans and dryer vents on the roof

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I am reasonably certain  that sewer gas is exiting the vent pipe and reentering the house through the dryer vent on the roof as well as the exhaust fan in laundry room.

This is especially true when the house is not used for long periods of time.

Vent pipes on roof are below exit points of the exhaust fans, and within 12 to 18 inches of the exhaust fans exit points.

HOA argues that it is not possible for this to happen and that everything was built to code.

What are the codes for this venting and how should it be corrected?
asked in Plumbing by tiffensdad (120 points)
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1 Answer

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Hi Tiffensdad,
Both; the clothes dryer vent and the laundry roof exhaust should be equipped with a damper to prevent back-drafting. Those dampers aren’t air-tight; they are usually very flimsy, but sufficient to prevent infiltration. 
Since you’ve mentioned the sewer gasses odor to be more noticeable after the house hasn’t been occupied for a longer period of time, I would consider another possibility. All the traps under the plumbing fixtures are normally filled with water to prevent sewer gasses from entering the house.
If plumbing fixtures are used on regular basis those traps are being constantly refilled with fresh water. However, in vacant homes the water slowly evaporates from a trap which opens the path for sewer gasses. Maybe this is the problem you’re facing. 
answered by darekrudy (21,730 points)
Sorry, should have mentioned that the drains in the house are all filled with enviormentally safe anti freeze to prevent evaporation.  It is checked / renewed by our parents while we are gone.

A closer look at the roof this morning showed me an approx 10 inch stack right between the two vent openings, which have T caps.  1 is above the stack and 1 below the stack, neither is more than 18 inches from the vent stack.

I think the other post probably answers my question about which code is applicable, unless there is one that better address it that anyone knows of.  I want to have something to quote when I start the expected battle with the HOA and or builder.


I don't believe there's any "better wording" in other code books, as I’ve mentioned in the other Q&A they are all almost identical. The one listed and linked (NSPC) is actually the best in explaining the distances. You can quote my previous post since I'm quoting the code or, even better, follow the link to the code page itself and refer to it when consulting with the developer.