Use a smoke test when the traps are full and the smell keeps coming back, when the source is hidden in walls or vents, or when the building was remodeled. Skip it for a smell that vanishes after water goes in the drains, a strongly backed-up line, or a plain blockage.
Try these first
- 1Fill every trap. Pour water into every floor drain, mop sink, tub and seldom used sink. In Phoenix, a trap can dry out fast. See why your building may smell like sewer gas.
- 2Check the obvious spots. A wax ring, a missing cleanout cap, a grease trap. Look at why a restaurant smells like sewer.
- 3Wait a day and see if it returns.
Smoke test makes sense when
- The smell returns within days even with full traps.
- It is in a wall, a ceiling or a room with no drain nearby.
- The building was remodeled and vents may have been cut or capped, as after a tenant improvement.
- The smell moves around or only shows up at certain times.
- More than one tenant complains and you need to find which line.
- An inspector or lease dispute needs evidence.
Smoke test is the wrong tool when
- The smell stops after water goes in a drain. That was the fix.
- A drain is backing up or gurgling. A blockage or a vent problem may be involved; the plumber chooses drainage, vent and camera checks to suit the symptoms. Wikipedia notes that completely blocking the line can prevent smoke from escaping through defects (Smoke testing)), so a blocked line needs clearing first.
- You only want a cheap odor check. The peppermint test is an option, with limits.
- The smell is more like natural gas than sewer. Leave, and call the gas utility. See what to do about a house that smells like gas or rotten eggs.
- The smell is a water smell coming from the tap, not a drain smell.
Weather and ground
Smoke testing "may not be done after rain or when ground water is unusually high," per Wikipedia). Indoors, that matters less than it does for a city main, but a yard or parking lot check will be better on a dry day.
If you got a city notice about testing, that is a separate job; see building smoke test vs city sewer main test.
What it means for you
Do the free steps, log what you did, and call a plumber for the test if the smell is still there. HQ does smoke testing with professional liquid-smoke equipment that forces air into the vent system, and we are licensed and insured, give free estimates and upfront pricing, and take emergency calls 24/7. Compare the tools in smoke test vs camera vs peppermint.
For a site-specific test plan, see HQ's plumbing smoke-testing service.