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HELP! Sewer gas mystery

1751 Views 4 Replies 4 Participants Last post by  OpenSights
It all began last summer right after I bought this home. I noticed there was a definite sewer gas smell occasionally coming from one of my two second floor bathrooms. I changed the wax seals on the toilets but it still comes back, but ONLY on days with big temperature swings from cold to hot (15 degrees or more) and it is worse the warmer it is.

The house is a 1910 3 story stucco colonial at the very top of a hill in dense residential area. It was gut renovated last year with fiberglass being added to the walls and a hot roof of closed cell spray foam on the roof. Also added two new HVAC units, one in the 3rd floor knee wall and one in the basement. All new pvc and pex plumbing was installed along with adding a master bathroom on the second floor. The plumbing inspector said he cleared the work with a water fill test (no leaks) and the traps are all installed correctly.

may first I thought the smell was just coming from one bathroom toilet. But then it started coming from the other one too as well as the sinks. I’ve eliminated the cause being: a drying out; a slug of water from one bathroom sucking out the trap in the other; a clogged stack pipe or pipes (there’s two) on the roof; or, the wax seals. I even had the town come and snake them aim from the clean out to the main pipe. There were a couple minor cracks in the clay pipe and tiny root wisps but nothing that could clog. The DPW guy did say it was weird that the airflow direction was coming from the sewer towards the house, which he has never seen.

So I’m stumped. My only two remaining thoughts are that: 1. There is a brad nail in the stack somewhere that opens a crack when the house gets hot and expands (temp swing); 2. Because it is a stucco and spray foam, the house is tight enough to depressurization some how when the a/c is running and the outside atmosphere is warm and high pressure; 3. It has something to do with the house being on the highest point on the hill and the last one on the line before it hits the main on the big road; or, 4. It has something to do with the a/c return and bath fan depressurizing the system (though I don’t think that’s the case).

I’m stumped on where to go from here. If anyone has a suggestion I could very much use some help. Thank you!
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Obviously if you don't clutch to flush it's going to stink. Might want to put fruity pebbles in there too.

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Why ask us why don't you just call a Master Plumber they can answer all your questions,
and stop trying to get something for free from people that work for a living !
Thanks Tango, I’ll install that next.

MacPlumb777: 1. I also work for a living; 2. I intend to call one but I’d like to pull the group mind here to help diagnose the problem; 3. This is a hard one to diagnose so the more input I get the easier it will be for him to fix. Thanks.
I always love it when a homeowner does their own research so they can help me figure out how to do my job.
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