Island Sink Venting: AAV Versus Loop Vent

Why the drain alone fails: the vent must connect to the drainage system within the allowable trap-arm distance, measured from the trap weir to the vent fitting. For 1-1/2-inch pipe at 1/4 inch per foot that limit is about 6 feet. The proposed 7-foot run from trap weir to the stack exceeds it, so the trap would be unprotected and would siphon on a slug of waste.

Does the AAV fix it? Only if the valve is the vent connection point. With the AAV teed off the drain between the trap and the 7-foot run, the trap arm is inches long and the 7-foot pipe downstream of the vent takeoff is a branch drain, not a trap arm, so the length limit no longer applies to it. That part of the plan is sound.

Conditions the AAV must still meet:

• Listed for the application and installed in the vertical position its listing requires.
• Mounted above the trap weir so it cannot be flooded or submerged.
• Accessible for replacement without demolishing the cabinet.
• Sized for the fixture it serves (1-1/2-inch drain, 1-1/2-inch vent riser).
• The branch drain it protects must still be 1-1/2 inch minimum and properly sloped to the stack.

The alternative is an island fixture loop (bow vent): the vent is taken off the drain downstream of the trap, carried horizontally under the floor to a point where it can rise, and then returned to a vent that terminates outside or reconnects to a vent stack above the flood level rim of the highest fixture.

Why it works, and the conditions that make it work:

• The loop is a closed circuit: the rising vent leg and the falling "foot" leg both connect to the drainage system, so the loop drains itself and condensation cannot collect and block it.
• The foot of the loop must drain back to the drain by gravity - it is graded toward the drainage connection, not trapped.
• Minimum size for the loop is generally 1-1/2 inch, and it may not be reduced below that.
• The loop must remain below the flood level rim of the fixtures it serves where it passes under the floor, and the vent portion must not be subject to backflow of waste.
• The drain from the island still needs its cleanout at the base of the run, and the 7-foot horizontal branch is fine once the vent is taken off at the sink.

If a loop is impossible (no path to a vent through the post-tensioned slab), the remaining option is to relocate the sink or accept the AAV, which is why the first question is whether AAVs are permitted.

An AAV that has failed, is misapplied, or is being subjected to positive pressure will pass sewer gas; the dishwasher discharge is the clue because it dumps a large volume fast and can pressurize a restricted drain.

Likely causes, in order of probability:

1. Valve stuck open or diaphragm fouled - grease and food debris from a kitchen sink are the classic killers. Open the cabinet and check whether the valve seals.
2. Valve installed below the flood level rim / trap weir, so it can be flooded and then cannot reseat. Verify the height relative to the trap weir.
3. Drain partially blocked downstream, so the dishwasher discharge fills the 7-foot branch and briefly pressurizes the AAV. Rod or camera the branch and check the slope - a branch installed flat will hold water.
4. Dishwasher drain improperly connected - it should discharge through an air gap or a high loop into the sink or a wye, not directly sealed into the drain without protection.
5. Trap seal lost by siphonage from another fixture, which points back to an inadequate vent path.

Sequence: verify trap seal depth, check and swap the AAV, camera the branch for slope and obstruction, then confirm the dishwasher connection. Do not simply install a second AAV or tape over the valve.