A smoke test finds air leaks in drain and vent pipes, a camera shows the inside of a pipe, and a peppermint test finds only odor leaks you can smell. For a sewer smell, smoke test first if traps are full. Use a camera for suspected cracks, roots or blockages.
What each one does
| Method | What it shows | Best for | Misses |
|---|---|---|---|
| Smoke test | Where air and gas leak out of drain and vent pipes | Hidden leaks in walls, ceilings, vents, cleanouts, dry traps | Fully blocked lines, very wet ground, water leaks |
| Camera inspection | The inside of a pipe | Cracks, roots, offsets, buildup, collapses, blockages | Air leaks at vents and joints above the floor |
| Peppermint test | Odor escaping from the system | A cheap check for gross leaks and dry traps | Small or hidden leaks, and anything you cannot smell |
Smoke test
Smoke is visible, so you see the path. Per the Wikipedia entry on the method, "with properly installed plumbing, the traps will prevent the smoke from entering the house and redirect it out the plumbing vents," while "defective plumbing systems or dry traps will allow smoke to enter the inside of the house" (Smoke testing)). Start here when the smell is in the building and the traps are full. This is the test HQ runs with liquid-smoke equipment that forces air into the vent system. If smoke appears inside, here is what it means. The city's version of the test is a different job; see who does which.
Camera inspection
A sewer camera goes down the line and helps locate "debris buildups, collapsed pipe sections, root infestations, and pipe offsets," as well as cracks and corrosion (CCTV drain camera)). It needs a way in, such as a cleanout, and it only sees what the cable reaches. Use it when the smell may be a broken pipe under the slab, or when drains back up.
Peppermint test
The test puts concentrated peppermint oil and hot water down the roof vent and seals the vent, per one plumbing guide. The 2024 IPC's drainage and vent final test section describes a smoke test and does not describe a peppermint test (IPC 312.5). It depends on someone smelling peppermint where they should not. One guide points out its limits: hidden defects may not be found, and if the smell continues after a negative test, the source is still unknown (Engineer Fix). More in does the peppermint oil test find a sewer leak.
What it means for you
Fill every trap first, since that is free. If the smell stays, smoke test the building's drain and vent system. If smoke shows nothing and drains are slow, camera the line. See when a smoke test is the right tool for the decision.
For a site-specific test plan, see HQ's plumbing smoke-testing service.