A plumbing smoke test introduces purpose-made visible smoke into drain and vent pipes to look for escaping air paths. Unexpected smoke can reveal dry traps, open fittings or defects that need confirmation. It does not diagnose supply-water leaks, and hidden or obstructed defects may produce no visible smoke.
What a smoke test finds
Smoke follows the path of least resistance, so it goes straight to the weak spots. Utility crews use the same idea on city sewers, where smoke "identifies private property connected downspouts and other inflow sources such as vents and cleanouts," according to Erie County's sewer division. Inside a building, the usual finds are:
- A dry trap, a floor drain or fixture trap that has lost its water seal. See what a P-trap is.
- A cracked or broken drain or vent pipe inside a wall or ceiling.
- A cleanout with a missing or loose cap.
- A failed joint at a toilet flange or a floor drain.
- A vent that was cut, capped or disconnected during a remodel.
What it does not find
A smoke test finds air leaks. It does not find a water leak, a slow clog, or a supply line problem. It also cannot push smoke through a line that is fully blocked, and results get weaker when the ground is very wet, which is why smoke testing "may not be done after rain or when ground water is unusually high," as Wikipedia's plumbing entry) puts it.
Why plumbers and inspectors use it
The model plumbing code names the test outright. Section 312.5 of the 2024 International Plumbing Code says that "where a smoke test is utilized, it shall be made by filling all traps with water and then introducing into the entire system a pungent, thick smoke produced by one or more smoke machines" (2024 IPC 312.5). Several Valley cities, including Phoenix, Mesa, Chandler, Glendale, Surprise and Tempe, build on the 2024 IPC, and Phoenix adopts both the 2024 IPC and the 2024 UPC and lets the designer choose. We have not confirmed the smoke test wording in every city's adopted and amended code or in the UPC, so ask your local building department whether a test is required on your job.
The reason it earns its keep is simple. A sewer smell has a dozen possible causes, and a smoke test turns a guess into something you can see. For the step-by-step version, read how a sewer smoke test works. Testing the city main is a different job, covered in building smoke test vs city sewer main test. Smoke is not the only method either, so see the peppermint oil test and what it misses. If smoke shows up in the wrong place, read what smoke indoors means. To decide whether you need one at all, see when a smoke test is the right tool for a sewer smell.
What it means for you
If your building smells like sewer and the drains are full of water, a smoke test is one of the few methods that can show you the leak instead of hinting at it. A licensed plumber runs it. You do not need to buy smoke bombs or improvise, since the equipment and the smoke fluid are made for this job. HQ Plumbing and Air 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.
For a site-specific test plan, see HQ's plumbing smoke-testing service.