Short definition
A smoke test pushes visible smoke into a sealed-off DWV (drain, waste, vent) system or sewer line. Visible smoke escaping at any joint, fixture, or fitting indicates a leak. WA-adopted UPC accepts smoke test as one of three valid DWV verification methods. SPU and Tacoma sewer crews also use smoke testing on public mains to find illicit connections and inflow/infiltration.
What it is
The procedure is straightforward: seal the lower end of the DWV system or sewer section being tested, plug any open connections, and pump pressurized smoke into the line through a roof vent or accessible opening. Smoke is generated either from a smoke machine or, traditionally, from burning oil-soaked waste pellets. The smoke fills the system under low pressure and any leak — at a joint, broken vent, dry trap, or unsealed cleanout — produces visible smoke escape that’s easy to spot.
Two distinct contexts where smoke testing happens:
Private (homeowner-commissioned). A plumber smoke-tests a residential DWV system to find a sewer-gas leak indoors, locate a broken vent inside a wall, or verify the integrity of a system before drywall closure. Used as an alternative to or alongside a hydrostatic test.
Public (utility-commissioned). SPU in Seattle and Tacoma’s sewer division periodically smoke-test public sewer mains, side sewers, and combined-sewer basins. The goal: find illicit connections (storm drains tied into sanitary sewer, missing manhole traps, broken cleanouts) that contribute to inflow and infiltration (I/I) — wet-weather flow that overwhelms treatment plants. Notice is typically posted to homeowners 24–48 hours in advance.
Why it matters to a homeowner
Two scenarios.
Sewer-gas smell indoors. When a homeowner reports a persistent sewer-gas smell that source-tracing hasn’t pinned down (dry P-trap, broken seal at toilet base, failed AAV), a plumber may smoke-test the DWV to find the leak source. The test reveals every leak in the system at once — a much faster diagnostic than chasing one symptom at a time.
Public utility smoke test in your neighborhood. When SPU or Tacoma posts notice of a smoke testing program, you’ll see (harmless, theatrical-grade) smoke rising from manhole covers and sometimes from your roof vent. Smoke from a roof vent is normal — it’s just confirming your vent is open. Smoke from inside your basement, a drain, or any indoor location is abnormal and indicates a DWV leak, broken vent, dry trap, or trap failure that you should fix.
If you see smoke indoors during a public test, photograph the source location and call a plumber. The smoke itself is non-toxic, but the leak it reveals can let real sewer gas into your home year-round.
When you’ll encounter this term
- A persistent sewer-gas smell that source-tracing hasn’t located.
- An SPU or Tacoma sewer notification about scheduled smoke testing in your neighborhood.
- A pre-purchase sewer scope finding suspicious results that warrant follow-up.
- A new construction’s DWV final test if hydrostatic isn’t practical.
Common variants and disambiguation
- Smoke test vs. hydrostatic test. Smoke is faster and finds odor leaks (broken vent, unsealed cleanout, dry trap). Hydrostatic finds water-loss leaks. UPC accepts either for DWV.
- Public smoke test vs. private smoke test. Public utilities smoke-test mains for inflow/infiltration. Private homeowners commission smoke tests during sewer scope or sewer-gas investigation.
- Smoke test vs. sewer scope (camera). Smoke test finds leaks; camera finds physical defects (cracks, root intrusion, sags). Often used together on diagnostic projects.
Washington note
WA-adopted UPC §712 lists smoke test as one of three acceptable DWV verification methods, alongside the water test (10-foot head) and the air test (5 psi for 15 minutes). Inspectors generally accept smoke test on rough-in inspection if the contractor proposes it, though water and air tests are more common.
For public-utility smoke testing in WA:
- Seattle Public Utilities (SPU) runs an inflow & infiltration program that smoke-tests combined-sewer basins on a rotating schedule. Smoke testing is one of SPU’s standard tools for identifying illicit storm-to-sanitary connections that contribute to combined sewer overflows during heavy rain.
- Tacoma’s wastewater division uses smoke testing in Hilltop, North End, and other combined-sewer neighborhoods on a similar program.
Notice to affected homeowners is typically posted 24–48 hours in advance, often by door-hangers and neighborhood-website notifications. The smoke is non-toxic, theatrical-grade, and dissipates quickly. If you see smoke indoors during a posted test, document and call a plumber for DWV diagnosis.