Unvented Kitchen Sink Trap Arm in a 1925 House
During a kitchen remodel in a 1925 two-story house, the plumber finds the kitchen sink drain is 1-1/2-inch galvanized pipe that runs 14 ft horizontally to a 3-inch cast-iron stack. The sink has a 1-1/2-inch P-trap with a 6-inch tailpiece, and the trap arm has no vent. The homeowner reports gurgling from the sink and a sewer odor in the cabinet after the dishwasher discharges. The remodel will add a 1-1/2-inch dishwasher drain and a new 1-3/4-inch-deep sink. The stack continues full height to the roof and serves a second-floor bathroom. The kitchen is on the first floor, and the ceiling below is finished. The local AHJ allows air admittance valves only where a conventional vent is impractical and requires access. The plumber wants to avoid opening the finished ceiling.
1. Explain the gurgling and odor. Is the existing 1-1/2-inch trap arm likely compliant? Support your answer with the venting principle, not just a code number.
Read the worked answer
The sink trap is not vented within the allowed trap-arm distance. Under the IPC, a 1-1/2-inch trap arm is commonly limited to about 6 ft; 14 ft is far beyond that. When the dishwasher or another fixture discharges, the slug of water pulls air through the sink trap, causing gurgling and siphonage. The trap seal is lost and sewer gas enters the cabinet. The long arm also allows a plug of water to compress air ahead of it. The problem is not the trap itself but the missing vent connection and excessive arm length.
2. Compare three fixes: a conventional vent through the roof, an AAV under the sink, and a revent from the trap arm to the stack. Include access, code acceptance, and damage.
Read the worked answer
A conventional vent runs 1-1/2-inch pipe from the trap arm up through the wall and roof, or connects to the existing stack above the fixtures. It is the most reliable but opens finished space. An AAV must be listed, accessible, and installed above the trap arm; it may be allowed only where a conventional vent is impractical, and some AHJs restrict it. A revent to the stack requires opening the wall but avoids roof work. All options must keep the trap arm within the allowed length; if not, relocate the trap or add a vent at the trap. Verify local amendments before choosing the AAV.
3. Describe the compliant repair you would install with minimal ceiling damage and the tests and inspections needed.
Read the worked answer
Replace the galvanized drain with PVC or ABS, maintain 1/4 in/ft slope, and install a cleanout. Cut the trap arm back to a compliant length. Run a 1-1/2-inch vent from the trap arm up inside the wall to the existing stack above the second-floor fixtures, or use an accessible AAV in the cabinet if the AHJ accepts it as impractical to vent conventionally. Connect the dishwasher through an air gap or high loop. Water-test or air-test the DWV system, inspect before closing the wall, and verify the trap seal remains after the dishwasher discharges.