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Summary: I have a sewer smell in my condo unit (top floor). Recently a smoke test was done and smoke was coming out, from what appears to be the division wall between my unit and the unit next to me (which also connects to the units below us because it is very close from where the electrical main cables run).

The plomber said it was the wax seal for the toilet that is on top of that area. That wax seal was replaced with no change in the smell.

However, since we made a hole below the toilet I feel a strong air current coming out the division wall. I believe this is a "chimney effect" - basically air from the basement is pushed up to the top of the building (my unit). I am pretty confident of that.

Today I decided to remove a tile that was on the floor in the basement (in the electrical room) and I found a backwater valve access sleeve. That access is maybe 15 inches away from the wall in which all electrical cables run.

That cap is basically completely loose, not only that, the edge of the pipe is all broken/cracked (see photos). For sure that wasn't sealed - if that releases smell, for sure it was leaking.

I just applied like a lot of silicone to it to seal it at least temporarily to see if that's the source of the smell.

My questions:

  1. Is the backwater valve access cap supposed to be tight sealed? Can it release sewer smell?
  2. Is there a risk because I glued (silicone) it to the pipe so it is sealed?

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A sewer back water valve should seal tight but needs to be accessible incase they plug up. Silicone should be ok if a bead around the cap, if this valve is close to the plumbing riser it could very well be the source of your bad smell , I would have also sealed nobody wants sewer gas coming in their home. I think you may have found the root cause I was skeptical in the electrical room but not with this find. Good job at tracking it down.

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  • I agree. I am somewhat concerned about why the valve is leaking sewer gas. Seeing the top of the cover a little torn up, I wonder if the valve isn't damaged or mis-installed/mis-aligned, etc. – Greg Nickoloff Oct 16 '19 at 18:58

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