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After several hours and no use, the bathroom accumulates some noticeable "sewer gas" smell, with kind of garlic. It is gone after a few minutes when the ventilating fan is turned on. The bathroom and house are new (circa 1990). There is a normal wood-frame townhouse apartment attached to an old brick house. There is no basement in the apartment. The floors start from ground level.

I think there could be a tiny air leak from the sewer tubes through poorly applied PVC cement. But I don't know if it is a common and easy to fix defect. I even tried to seal a few small holes in the drywall with paper and glue where the sewer tubes from the sink are going through the wall but it did not help.

What could be the cause of this smell?

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up vote 11 down vote accepted

The two most common sources I've seen are dried out traps and a failing wax seal under the toilet. A failing wax seal may also result in water damage appearing from beneath the toilet. The dried out traps are simple to fix, just run a little water into each drain every few months.

For a, more rare, problem at a PVC joint, you'll need to find the offending joint first. Look for signs of water damage, or you may get lucky putting some soapy water over each joint and flushing a toilet.

Edit: here's a good description of how to replace the wax seal under the toilet: How to fix a leaky toilet (that only leaks from underneath)?

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Magic! Thank you. I remember now, when inspector asked to tighten down the toilet. It was on list of defects and was fixed. Probably not well fixed after all. Sounds like inexpensive fix. – user2817 Jun 2 '11 at 17:09
Yeah, all you need is the wax seal and some muscle to move the John around. Definitely drain and unhook it first. – KeithS Jun 22 '11 at 19:16

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