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Over the last several weeks we've noticed a sewer gas smell in our upstairs bathroom at night when we run a whole house exhaust fan in the bathroom window. I found that in the basement where the plumbing chase is exposed, there is a slight draft where the smell is strongest.

The drain stack is the original cast iron, though over the years portions have been replaced with PVC, and most of the lateral lines have been replaced with copper, PVC, or steel.

Here's what I've tried so far:

  • Running drano through all the drains and re-filling p-traps with water
  • Confirming that all drains are properly draining
  • Checking the roof vents for obstructions
  • Replacing the wax ring on the upstairs toilet (downstairs toilet was installed less than two years ago)

What should I check next? Before opening walls and looking for loose plumbing connections, what else should I try?

Below is a diagram of the entire drain and vent system.

Plumbing drain and vent diagram

LShaver
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2 Answers2

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You might be seeing the end-of-life of the (I'm guessing) cast iron pipes in the plumbing chase. They last a long time, but past 100 years or so they can get chancy for corroding through.

To me, a plumbing chase typically means plumbing was added to the building after it was built, and that might well have been before 1922, so it begins to be thinkable.

Ecnerwal
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If you feel that the pipes are still in decent shape but an oakum / lead joint in the main stack may be leaking air -- possibly due to shaking in the 2008 quake in your area -- then you may be able to find the air leak with a smoke machine.

I've never done this myself, but I've heard of it being done. If you stand below the chase with a flashlight and/or a laser pointer, you may localize escaping smoke to a particular area and eliminate much exploratory hacking of walls.

MTA
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