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.
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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