Why a toilet flush can siphon a washer trap dry, and how to fix the vent

A trap stays full only while the pipe behind it stays near atmospheric pressure. Water moving down a drain pulls a vacuum behind it, and if no air arrives from a vent, that suction empties nearby traps, according to valve maker Studor, an IPS brand. On This Old House, Richard Trethewey showed a new washer's discharge emptying the toilet next door in a poorly vented house.

An air admittance valve, or AAV, handles only half of that. The International Plumbing Code defines it as a one-way valve that opens under negative pressure, closes by gravity, and stays sealed at zero or positive pressure. Studor lists the sewer or a septic tank, and a blockage or belly in the pipe, as sources of positive pressure, and says it can push sewer gas past a full trap. That is why the IPC and the International Residential Code require at least one vent in every system to run outdoors.

The codes also fix where the valve goes. Individual and branch AAVs must be mounted 4 inches or more above the horizontal branch or fixture drain they vent, within the maximum developed length for that vent, where there is access and room air, and must be rated for the vent size (IRC P3114, IPC 918). Under IPC 406.2 a washer standpipe needs a trap and fixture drain of at least 2 inches feeding a branch or stack of 3 inches or more, so the tie-in itself is by the book. Under the Uniform Plumbing Code, an AAV needs local approval as an alternate method.

Start at the valve, which the code requires to be reachable. A setup that worked for months and then failed points to a change, such as a valve that sticks or a restriction forming downstream. Swap the AAV, confirm it sits 4 inches over the branch and matches the pipe size, then snake or scope the line below for a clog or sag. If traps bubble when other fixtures drain, that points to positive pressure, which an AAV cannot relieve; the answer there is a vent to open air.