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.