Long Trap Arm and Undersized Branch in an Apartment Laundry Conversion
A three-story, six-unit apartment building is being converted from offices to residential use. On the ground floor, the contractor is adding a laundry room with three stacked washer/dryer units, each on a 2-inch standpipe and 2-inch P-trap, plus a new mop sink. The only available drain is a 2-inch horizontal branch that runs 12 feet from the mop sink trap to a 3-inch stack; the branch also receives condensate from two condensing furnaces. The existing branch has no vent between the mop sink trap and the stack. During the first test, the mop sink drains slowly and gurgles loudly whenever a washer discharges; soap suds appear in the mop sink after the washer pump-out. The framing is closed and the ceiling below is finished. The local code is the IPC with local amendments permitting air admittance valves only for individual fixtures and only where accessible. The owner wants the cheapest fix that will pass inspection and stop the gurgling.
1. Identify the likely cause of the gurgling and sudsing, and state the code issues you would cite.
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The gurgling is a venting symptom, not a drain-cleaning problem. The mop sink trap arm is 12 feet long, which exceeds the typical IPC limit of 8 feet for a 2-inch trap arm. With no vent between the trap and the stack, the washer pump discharge creates a pressure surge in the branch; the mop sink trap self-siphons or is blown through, producing gurgling and suds at the fixture. The suds are a strong clue that the branch is carrying a sudden discharge and has no relief. I would also check branch sizing: three residential washers at 2 fixture units each plus a mop sink at 2 fixture units is roughly 8 fixture units, and a 2-inch horizontal branch may be at or beyond its allowed load depending on the adopted table. The air admittance valve amendment does not solve this automatically because it permits AAVs only for individual fixtures and only where accessible; an AAV on the mop sink would not vent the washer standpipes or the branch. The code issues are trap arm length, lack of trap venting, and possibly branch load.
2. Recommend the least-cost compliant repair, and explain why an under-sink air admittance valve is or is not acceptable.
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The least-cost compliant repair is to get a vent within the allowable trap-arm distance. If the ceiling is closed, the practical options are to relocate the mop sink closer to the stack, or open the ceiling and install a 2-inch vent from a point within 8 feet of the mop sink trap and re-vent the washer standpipes to that vent or to the stack above the highest fixture. An under-sink AAV may be acceptable only for the mop sink if the local amendment truly permits individual-fixture AAVs, the valve is accessible, and the manufacturer's listing and installation instructions are followed. It would not be acceptable as the only fix for the branch because it does not provide relief for the washer discharges and does not correct the trap-arm length for the mop sink unless the AAV is installed at the trap. The cheapest fully compliant fix is usually to relocate the mop sink or add a short vent, not to rely on an AAV for a branch serving multiple fixtures.
3. What testing and inspection steps would you require before the ceiling is closed?
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Before closing, I would require a DWV water or air test per the adopted code, with the test witnessed by the inspector. I would verify that every trap is vented, that the vent terminates outdoors above the roof and not in the attic, and that any AAV is accessible and installed in the correct orientation with no downstream restrictions. I would check the slope of the 2-inch branch, confirm the hanger spacing, and verify that the washer standpipes have the required trap weir distance and standpipe height. I would also run a full-flow test with all three washers and the mop sink operating to observe gurgling, suds, or backup, and I would photograph the vent connections before concealment. If the branch load is marginal, I would upsize the branch to 3 inches before inspection rather than risk a repeat call.