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

  • 3
    I had that, and the toilet needed to be reseated. Toilets have the p-trap build in, but. they can leak/fumes at the wax seal.
    – Traveler
    Aug 28, 2022 at 18:19
  • Before you freak out about replacing the pipes, Do you see any leak or dripping, or wet pipes anywhere ?
    – Traveler
    Aug 29, 2022 at 1:01
  • @Ruskes no drips/leaks anywhere that I can see -- but if there's a break somewhere in the vent stack, there wouldn't be any water to detect.
    – LShaver
    Aug 29, 2022 at 1:07
  • correct about vent stack, The exhaust fan is in the upstairs bathroom, window there is the smell, mostly in the morning, after the fan run all night ? It might be drying up P-trap
    – Traveler
    Aug 29, 2022 at 1:35
  • @Ruskes well it wasn't the wax ring, I replaced it yesterday and the smell was back tonight. It's worst in the morning, but present at night too, so I don't think it's just a dry p-trap.
    – LShaver
    Aug 30, 2022 at 2:12

2 Answers 2


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.

  • That sounds like a fun problem to fix! The house was built in 1919 so it's possible. If this were the issue, what's the first place to check? Top or bottom?
    – LShaver
    Aug 28, 2022 at 19:44
  • Often the problem is not with the pipes themselves but rather the joints, especially with old cast iron pipes.
    – Hot Licks
    Aug 28, 2022 at 21:40
  • 1
    Try narrowing the problem, close all of the rooms off and later check where the strongest smell is. Yes keep the ventilation fans off.
    – Gil
    Aug 28, 2022 at 23:43
  • @Gil we're headed out of town for a bit, I'll give that a shot.
    – LShaver
    Aug 29, 2022 at 0:53
  • Good luck I hope it is an easy fix!
    – Gil
    Aug 29, 2022 at 15:06

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.

  • Question did sound like a joint leak. This with location and type of pipe should help. Did not know about smoke for leaking joints.
    – crip659
    Aug 28, 2022 at 21:18
  • 1
    I'm not sure I follow... where is the smoke machine in this scenario? And if the failed joints are in the wall, how does this help me find them?
    – LShaver
    Aug 29, 2022 at 0:54
  • @LShaver "In the basement where the plumbing chase is exposed . . . ." You can stand in the basement under the chase and look up with a flashlight to see the pipe and all the pipe joints straight up to the attic, right? That's how it was in a 1950 house where I lived. If you can't, or you can't see the same from the attic then there's no point in using smoke. The smoke machine can be either on the roof attached to the vent or in the house feeding any sink drain whose P trap has been cleared of standing water, with a loose cap covering the roof vent.
    – MTA
    Aug 29, 2022 at 1:53
  • Ah OK. Well the chase is a stud bay in a 2x6 wall, and there's water supply pipes and some electric conduit too, so I can't see very far up it.
    – LShaver
    Aug 30, 2022 at 2:16
  • This sounds like an excellent suggestion, even if a couple of exploratory holes must be opened in the walls to look for smoke. Consider removing switches/outlets and their boxes if you happen to have any in the stud bay/chase - they are ready made holes that are easy to "patch" by putting everything back.
    – FreeMan
    Aug 30, 2022 at 14:25

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