Short definition
Induced siphonage (sometimes called aspiration) is trap-seal loss caused by another fixture’s discharge — not the trap’s own. When water moves through a poorly-vented branch or stack, it creates negative pressure that pulls the seal out of a nearby trap that wasn’t being used. The classic symptom: you flush a toilet and an adjacent shower or sink gurgles or starts smelling like sewer.
What it is
Every time a fixture drains, water moving down the pipe creates pressure differences. With proper venting, those differences equalize through air entering at the vent terminal. Without enough venting, the pressure equalizes the only other way it can — by pulling water out of the next trap seal upstream. Because the affected trap belongs to a fixture that wasn’t running, the failure is “induced” by the neighbor.
The same physics produce two related but distinct mechanisms:
- Self-siphonage — the same fixture’s discharge siphons its own trap.
- Induced (aspiration) siphonage — a different fixture’s discharge siphons this trap.
A wet vent at capacity often does it. So does a long unvented branch carrying multiple fixtures, or a stack base where lower-floor fixtures take the back-pressure pulse from upstairs flush events.
Why it matters to a homeowner
The diagnostic clue is the timing. If a shower trap dries out only when an upstairs toilet is flushed, the toilet didn’t draw water from the shower’s drain — but its discharge induced the siphon. That’s a venting problem, not a trap problem, and pouring more water into the shower drain is at best a temporary fix.
When a remodel adds a fixture to a long branch without recalculating the vent layout, induced siphonage often shows up days or weeks later as randomly appearing sewer smells. The fix is re-venting at the rough-in stage, which is much cheaper if caught before sheetrock goes back up.
Common variants / not the same as
- Aspiration / induced vs. self-siphonage. Induced = neighbor fixture. Self = same fixture’s drain pulls its own trap.
- Aspiration vs. evaporation. Aspiration is dynamic (during flow). Evaporation is static (over weeks of disuse).
- Aspiration vs. capillary action. Capillary is hair or string draped over the trap weir wicking water out. Aspiration is hydraulic.
- Aspiration vs. compression / back-pressure. Aspiration pulls trap water away. Compression pushes it through, often blowing the seal up into the fixture.