How does a sewer smoke test work, step by step?

1. 1Fill every trap. Water in each P-trap blocks smoke the way it blocks sewer gas. The plumber, or the building staff, runs water in every sink, tub and floor drain. City crews ask residents to do the same, with Erie County's guidance to pour water into seldom-used drains to fill the traps.
2. 2Get access. The crew chooses suitable access and isolation points for the building system and the equipment being used.
3. 3Isolate the section. Technicians may temporarily seal parts of the plumbing so the smoke goes only where they want it.
4. 4Introduce the smoke. A machine pushes the smoke in at low pressure. The 2024 IPC describes the code version of the test: fill all traps with water, introduce "pungent, thick smoke" from smoke machines, and once smoke appears at the roof stack openings, close them and hold a pressure equal to a 1-inch water column for at least 15 minutes (2024 IPC 312.5). A troubleshooting test for an odor can be shorter and looser than that inspection test.
5. 5Watch and mark. One person watches the roof vents, others walk the building, the crawl space or the yard. Unexpected smoke identifies an air path that needs confirmation; it can come from an unfilled trap, open fitting or defect.

With traps filled and test openings properly prepared, unexpected indoor smoke can reveal an air path from the drain or vent system. The crew records the escape point and confirms whether it comes from a dry trap, an open fitting or a defect before deciding on a repair. A negative test does not rule out every hidden defect. The time needed depends on system size, access, isolation, findings and ventilation. Superior Signal advises ventilating when smoke enters a building. Its general public-test guidance is not a duration guarantee for a private building diagnosis.

Dry traps are the classic false alarm. A floor drain that nobody used for a month lets smoke straight up, so fill them first, and note which ones needed water. That is useful information by itself, because a trap that dries out that fast may need a trap primer. Learn more in what a smoke test finds. If smoke appears somewhere odd, this page explains what it means. Detectors can also react, so read whether smoke tests set off fire alarms. The city runs a different kind of test on the public main; see who does what, and for an older odor method see the peppermint test.

Test equipment introduces smoke into the prepared drain or vent section. The crew chooses access, isolation and operating conditions for the equipment and system; the cited manufacturer and municipal guidance describe their particular setups.

Ask the crew how much building access and downtime to plan for, and tell occupants ahead of time. In a shared building, tenants should get notice first, as covered in what tenants need to know before a building smoke test. The smoke itself is made for this use, and its safety is covered here.

For a site-specific test plan, see HQ's plumbing smoke-testing service.