Intermittent Sewer Gas Smell


Old 03-24-16, 07:02 PM
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Intermittent Sewer Gas Smell

Thanks in advance for reading the novel below; I will try to make this as quick as possible. I have an old and air-leaky house from the 1920s. In times of warm weather, there is an intermittent (but not constant) sewer gas smell. It is coming from a hall closet upstairs, next to the bathroom. It is never too much to handle but often noticeable. This closet has a false floor, and underneath it the bathroom drain line connects to the main standpipe. The standpipe jogs about 12" horizontally via a couple 90° elbows right at this location as well. There are no signs of moisture, mold, or leaks. I am wondering, if the stand pipe threads on the elbows may allow minor air flow into the house causing the smell? And if so, what's the best solution? Is there a mastic or sealant I could simply wipe on the pipe joints to resolve this issue? Tearing out original pipes and plaster/lathe walls seems like too big a task for a minor odor now and then. Ideas? Thanks!
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Old 03-24-16, 07:45 PM
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Are there any fixtures draining into the stack above this point or is it just vent stack above the bathroom drain connection? If just vent, it's certainly possible you could have a leak without seeing any moisture since there wouldn't be any water flowing in it.

I don't know that I would trust any kind of mastic as a long term fix, even on a vent.

There are ways to pressure test the entire drain system by plugging the main line with a balloon and using a test plug on the roof to slightly pressurize the pipes (the water in the traps will hold back a few PSI). There are also smoke tests that can be done. Probably means calling a plumber with the right equipment. Plus a pro might be able to spot a funky joint based on experience.
Old 03-24-16, 07:49 PM
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I doubt that a little leak (that's small enough to not actually leak water) would be causing the smell.

I wonder if your main stack has a larger hole further down that you can't see, which is wafting upwards through the wall cavity into your closet.

Next question, does the stack vent out the roof? Can you trace this stack in the attic or in the basement/crawlspace?

I'll also ask if the bathroom is used at least weekly. Traps of unused bathrooms tend to dry out and can cause the smell that travels.

Just some things to consider, I'm sure others will be around soon with suggestions.
Old 03-25-16, 07:16 AM
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I will try to answer all questions below:

There are no fixtures above these two 90° elbows in the stack. This house has a couple additions over the decades are the wall cavity here is larger than normal, with a space several cubic feet in volume below it. So if there were a leak in the 5' vertical length below this point, it would likely waft out the same location. But that would be noticeable somewhere, yes?

The stack is visible in the basement - I haven't noticed any smells down there or any other rooms except this one.

The bathroom is used daily - no concern about traps going dry.

The stack is traceable to the basement floor and through the attic through the roof.

I am not been able to get on the roof and look down the stack, if that would be the next step. Not sure how one inspects a stack anyways?

Because of the larger wall cavity (and my home air leaks like a sieve), I wonder if it could be a musty smell? It doesn't smell like "basement," and is intermittent as if certain air pressures are pulling air out of the wall cavity into the room. But can a sewer gas smell be intermittent? Wouldn't a hole in the stack always be pushing more air into the room? And wouldn't it create a visible leak?

Part of me wonders if I should simply air seal off this cavity from the room, and be done with it. I am looking to avoid replacing the stack, as that would be thousands of dollars.
Old 10-14-16, 12:54 PM
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I'll spare you all the details, but the vertical vent stack was cast iron, but ALSO teed off to serve the toilet (still cast iron). This horizontal section had collapsed on top, allowing the gas smell without any water leaks or damage. Replaced horizontal pipe and stack with new PVC. Thanks for the help.

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