Short definition
Trap seal loss is the umbrella term for every mechanism by which the standing water that blocks sewer gas is removed from a trap. Eight mechanisms are documented; in residential settings, the common ones are evaporation (vacation homes), self-siphonage (S-traps and oversize trap arms), induced siphonage (neighbor fixtures), and vent-related pressure events. Each has a distinct fix.
What it is
A trap seal is just water sitting in a U-bend. Anything that removes that water — slowly or in one event — is a trap seal loss. The mechanism matters because it determines what to do about it.
The full taxonomy of mechanisms (per haq Ch.10.11):
| Mechanism | Type | What happens |
|---|---|---|
| Self-siphonage | dynamic | Fixture’s own discharge siphons its own seal |
| Induced (aspiration) siphonage | dynamic | Neighbor’s discharge pulls this fixture’s seal |
| Compression / back-pressure | dynamic | Stack-base hydraulic pulse blows seal out |
| Evaporation | static | Seal slowly dries during disuse |
| Capillary action | static | Hair or string wicks water out |
| Momentum | dynamic | Sudden large discharge dislodges the seal |
| Foaming | dynamic | Detergent foam soaks water out |
| Wavering out | dynamic | Wind across vent terminal oscillates the trap |
A pressure differential beyond about ±400 Pa across a trap is enough to break the seal regardless of mechanism.
Why it matters to a homeowner
Sewer gas in the home is the symptom. Trap seal loss is the cause. Identifying which of the eight mechanisms applies tells you what to fix, and “pour water in the drain” only solves the static failures. The dynamic failures need attention to the trap geometry, the venting, or the upstream source of the pressure event.
Diagnostic by failure pattern
- Vacation cabin smell on return → evaporation. Pour water in every trap and floor drain.
- Whole-house gurgle + sewer smell after a cold morning → vent freeze closure causing induced siphonage when fixtures finally run.
- Sewer smell only when something else flushes → induced siphonage on a long branch.
- Sewer smell that returns within hours of pouring water → self-siphonage; likely an S-trap or an oversize trap arm.
- Detergent froth visible at trap level after laundry → foaming.
- Single fixture dries faster than its neighbors → capillary wicking from hair or string draped over the weir.
Washington note
Three regional patterns dominate trap seal loss in WA:
- Vacation cabins in the Cascades, Olympic Peninsula, and San Juan Islands: evaporation is the most common cause. Standard prep is mineral oil on top of the trap water before leaving for >2 weeks.
- Pre-1970 housing stock in Seattle, Tacoma, Spokane, and Olympia: S-traps and long unvented branches drive self-siphonage and induced siphonage. The fix is a P-trap retrofit and adding a vent during the next remodel.
- Eastern WA in Spokane, Wenatchee, Yakima, and Pullman: vent freeze closure during cold snaps, triggering induced siphonage when fixtures finally run.
Each of these triggers trap seal loss with a distinct mitigation path.