Strange smell downstairs bathroom, strange draining toilet upstairs...

[bmbouter]

I live in a 2-story townhouse, and here's what I'm experiencing.

# For a several years now
The upstairs toilet slow-drains to the point where after being out of the house with no flushes for a day or two, you come home and there is no water in the bowl. Because of that it smells. Meanwhile the downstairs toilet works like a champ.

# Two weeks ago
We start an upstairs bathroom remodel. The plumbers are told about the issue. They pull the upstairs toilet look at it, and they can't see any toilet paper (or dental floss or anything) that would be wicking water out of the toilet. They cap the upstairs sewer line with a screw cap.

Roughly that same day, the downstairs toilet starts to mysteriously smell like sewer. It's very noticable but doesn't seem to come from any particular place. No other place in the house smells. There is no visible leak. The downstairs bathroom sink does not smell, nor does the p-trap in the vanity. It continues to hold water like normal and flush like a champ which is great seeing as it's our only toilet at this point.

# My take
I believe these two issues are somehow related. It's too coincidental that the downstairs would start at the same time. It's also too strange that the upstairs toilet was pulled, and yet it looks in great condition and the seal and flange all looked good when it was pulled.

What are possible root causes that you can think of? The plumbers are coming back in a week to finish their part of the job. I could have them pull the downstairs toilet but similar to the upstairs I suspect it will "look fine". Help!

 

[Aquarius]

Are there any air admittance valves on these soil pipes (sewer lines)?

 

[bmbouter]

I had not heard of an air admittance valve so I read up on them. I looked around the under sink areas and I don't see any. If there is, I can't locate it.

The sewer pipe does run up to the attic and I believe out through the top of the house. In my air admittance valve reading I see it's an option only if there isn't a pressure relief in the system, which I imagine would be at the top of this pipe on my roof. If my observation that there are no air admittance values, what would you expect the top of this pipe to have on it and do you expect it to go out through the roof?

 

[Aquarius]

E Yes pipe out through the top of the roof, or an air admittance valve fitted at the end, so could be in attic? I have seen it where nothing was at the top and all smells were in the attic, and smells were coming from there.