Guided evidence worksheet
Drain pressure: normal venting versus trap disturbance
Record which fixture is used, where gurgling occurs, whether flow is slow, and whether trap water or odor changes afterward.
Reasoning guardrail
Separate the routes before choosing a repair.
A gurgle at one drain does not prove that the blockage is directly beneath that fixture.
- Trigger
- Which fixture activates the sound and whether several fixtures are affected are decisive clues.
- Area
- Plumbing
- Provenance
- Original explanatory diagram, not a site photograph.
1. Freeze the evidence
What happened first?
2. Competing hypotheses
Record evidence for and against each route.
Partial branch or main drain restriction
Wastewater displaces air and creates pressure because the drainage path is narrowed.
Evidence that raises fit
Evidence that lowers fit
Notes
Vent obstruction or inadequate venting
The drain cannot admit or release air through the intended vent path, so traps respond.
Evidence that raises fit
Evidence that lowers fit
Notes
Dry, siphoned or leaking trap seal
The local water seal is absent and odor enters even without a major restriction.
Evidence that raises fit
Evidence that lowers fit
Notes
3. Decision points
Change one condition at a time.
Are several fixtures slow or interacting during a large discharge?
Yes: Move a shared restriction ahead of a local trap issue.
No: Check venting and the affected trap seal.
Observed answer and evidence
Does adding water to an unused trap temporarily stop the odor?
Yes: Document why the trap is drying or losing its seal.
No: Continue with pressure interaction and restriction checks.
Observed answer and evidence
Safety boundary
Stop rather than create a better clue.
- Do not mix drain chemicals or add a product after another chemical has been used.
- Stop and call a professional for sewage backup, repeated overflow or inaccessible vent work.
5. Current conclusion
