Short definition
A sediment trap (drip leg) is a short vertical capped nipple in the gas line just upstream of every gas appliance’s control valve. Gas turns 90° at the junction; sediment, scale, water, and rust fall straight down into the trap rather than continuing into the gas valve. WA-amended IFGC requires one at every gas appliance.
What it is
The fitting is straightforward: a tee in the main gas line, with the run going to the appliance shut-off and the branch going down at least 3 inches into a capped nipple. As gas turns the corner, anything denser than gas falls into the trap by gravity.
Standard sizing: 3-inch minimum trap length, 1/2-inch or 3/4-inch pipe matching the gas supply line, capped on the bottom with a regular threaded pipe cap.
A code-compliant gas water heater install in WA combines:
- A listed gas shut-off valve close to the appliance
- A sediment trap (drip leg) just upstream of the shut-off (or sometimes between shut-off and connector — varies by AHJ)
- A flexible gas connector from the shut-off to the appliance inlet
NFPA 54 (the National Fuel Gas Code) and IFGC §408.4 both require the trap. WA-amended IFGC enforces it rigorously — missing sediment traps are one of the most common WA gas-appliance permit failures.
Why it matters to a homeowner
When your installer’s quote on a new water heater lists “sediment trap” as a line item, that’s a code-required fitting, not an upsell. A skipped trap fails inspection, and the inspector will require it before passing the job.
The trap also has a real engineering job: it keeps debris out of the gas valve, which extends valve life by years. On a 1990s gas tank that’s still working, you can sometimes find a thimble of rust dust at the bottom of the trap when you open the line for service — that’s the trap doing exactly what it should.
The most common DIY mistake on gas-appliance installs is omitting the trap, or installing it horizontally so the cap points sideways. Either fails inspection. Either also lets rust into the gas valve.
When you’ll encounter this term
- A water-heater or furnace replacement quote lists the sediment trap as a fitting.
- A pre-purchase inspection or permit inspection flags a missing or wrong-orientation trap.
- A gas-dryer install (less common but same rule) requires a trap upstream.
- A 1990s WA install passes inspection, in part because the original trap is doing its job.
Common variants and what a sediment trap is not
- Sediment trap vs. shut-off valve. The shut-off is the appliance gas valve; the sediment trap is the tee+nipple+cap between the supply and the shut-off (or between shut-off and connector, depending on AHJ).
- Sediment trap vs. flex connector. The connector goes from the shut-off to the appliance. The trap is upstream of both.
- Sediment trap vs. pressure regulator. A regulator drops pressure (when needed). A trap catches debris. Different functions.
Common failure modes (install errors)
- Missing trap. Permit fails. Gas valve fails prematurely from rust ingress.
- Trap installed horizontally. Tee pointing sideways, not down — doesn’t catch sediment. Effectively missing.
- Trap downstream of valve. Useless. Must be upstream of the appliance shut-off.
- Cracked cap. Gas leak. Replace immediately.