In the Field · Part 4
Sewer gas: trap seals, trap primers, vents, and the sequence that finds the smell
A sewer smell is a trap that has lost its seal, a vent that is open where it should not be or blocked where it should be open, a drain fitting that is not gastight, or a fixture that was piped without a trap at all. The customer cannot tell which and often blames the wrong room. This lesson is the sequence: rule out natural gas first, then find the dry trap (the floor drain, the guest shower, the washer standpipe), then the trap that siphons because its arm is too long, then the vent that terminates by a window or has a cracked stack in a wall, then the fitting or the wax ring that leaks gas without leaking water. Each finding has its code section and its rung on the ladder, and the trap primer is the one most plumbers skip.
What the full lesson covers
- First: is it natural gas? A rotten-egg or sulfur smell near a gas appliance is lesson 4.21's protocol, not this lesson's. Sewer gas is the drain smell, strongest at a fixture or a floor drain, worse when the house is closed up.
- A trap seal is 2 to 4 inches of water (CPC 1005.1). A dry trap in a rarely used floor drain, shower, or standpipe is the most common cause; the code's answer is a trap primer (1007.1), not a promise to run the water monthly.
- A trap that siphons dry every time the fixture drains has an over-length trap arm or no vent (Table 1002.2); the fix is a vent, not a deeper trap.
- A vent terminating within 10 feet of a window, or below 6 inches above the roof, or a stack cracked inside a wall, puts sewer gas where people breathe (906.1, 906.2); smoke testing finds the crack.
- Toilets leak gas before they leak water: a failed wax ring, a cracked flange, or a toilet set on shims with a gap is a smell with no puddle.
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More lessons in The Field: Method, Code, and Cases
- 1The diagnostic method: verify, isolate, cheapest test first, cause not symptom, the options ladder, document
- 2Permits: when one is required, what is exempt, pulling it online, the inspection sequence, what inspectors look for, and handling corrections
- 3Daily-use code: drainage slope, venting, water sizing, cleanouts, trap arms, and fixture units, with the section for each
- 4Gas piping in practice: sizing, CSST bonding, the pressure test, appliance connectors and shutoffs, sediment traps, and seismic shutoff valves
- 5Water heaters I: tank. Seismic strapping, the T&P discharge line, expansion tanks, sediment traps, pans, venting, and the garage rules
- 6Water heaters II: tankless and heat pump. Gas sizing, venting, condensate, the 2025 Energy Code direction, and where the gas-ban ordinances stand
- 7Sewer and drain I: the camera, the locator, hydro-jetting vs. cabling, and when each is the right tool
- 8Sewer and drain II: trenchless vs. open trench, whose lateral it is (it varies by city), backwater valves, and point-of-sale lateral ordinances
- 9Case: the hissing line. The meter test, the supply leak that fed the roots, the lateral to the city tap, and the contract and permit trail
- 10Leak detection: slab leaks, acoustic and pressure methods, thermal imaging, and repipe vs. reroute
- 11Low pressure: the PRV, galvanized pipe, and the sequence that finds which one it is
- 12Sewer gas: trap seals, trap primers, vents, and the sequence that finds the smellYou are here
- 13Water treatment: where softeners are restricted in California, filtration, pressure regulators, and thermal expansion
- 14Fixtures and conservation: CALGreen flow rates, SB 407 retrofit-on-alteration and disclosure-on-sale, 1.28 gpf, and the ADA basics a plumber needs
- 15Materials and methods: PEX vs. copper vs. CPVC, ABS vs. PVC (it is regional), cast iron, and the transitions that fail
- 16Commercial basics: grease interceptors, the backflow program, and the tester certification that opens a second line of work
- 17Cal/OSHA for a small shop: the IIPP every employer must have, heat illness prevention, trenching and the 5-foot permit, confined space, silica, and ladders
- 18Environmental rules: EPA lead-safe certification for any pre-1978 home, asbestos and transite pipe, and stormwater
- 19Job-site craft: sequencing with the other trades, protecting the home, and the punch list that ends the job
- 20Case: the water heater that failed inspection twice
- 21Case: the gas leak call. The protocol, and the walk-away
- 22Case: the "customer doesn't want a permit" job
- 23Template: the job-site checklist, from the truck to the walk-through
- 24Template: the pre-inspection checklist by stage. Underground, rough, water heater final, gas, and trim
- 25Template: the Injury and Illness Prevention Program starter for a plumbing shop of one to five
- 26Template: the Heat Illness Prevention Plan for a plumbing shop, outdoor and indoor
- 27Template: the customer options sheet (good / better / best) with the cost band, the permit path, and the code trail on each rung