Drain venting problem siphons a trap

Pressure changes in the drainage system pull or push air through fixture traps, causing gurgling and odor risk.

Failure path

• Step 1: Another fixture sends water down the stack.
• Step 2: The vent path cannot balance pressure.
• Step 3: Air moves through a nearby trap.
• Step 4: Gurgling occurs.
• Step 5: Trap water may be reduced enough to admit sewer odor.

What makes this pathway plausible

The timing, location and operating or weather trigger should repeat. A diagram shows a mechanism; evidence from the actual house determines whether it applies.

Best next observation

Record the trigger and compare the nearest materials, components and routes before opening finishes or choosing a repair.

Verification after repair

Recreate the original trigger when safe, then confirm the symptom does not return and affected materials are stable or drying.