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How to Use a Plumbing Vent to Clear a Slow Drain

Chris Deziel

The plumbing code requires venting for every fixture in your house, so if your house is properly plumbed, you shouldn't have to add a completely new vent to get a sink to drain. Accidents happen, though, and the plumber who roughed in the sink may have omitted the vent or installed it incorrectly.

You may have to install a new vent through the ceiling.

A vent that is too far from the trap, incorrectly routed or too small, won't work properly. If you know the rules for venting, you can make corrections on your own, but seeking out a plumber's advice may not be a bad idea.

  1. Install the vent for a standard 2-inch sink drain no farther than 5 feet from the point where the curved part of the P-trap turns horizontal, which is called the weir.

  2. Install the vent in a section of drain pipe that runs horizontally or has a slight slope. A common way to do it is to cut the pipe with a hacksaw and glue in a sanitary tee with its sweep facing up and sweeping toward the sewer. Use all-purpose pipe cement to glue either a polyvinyl chloride, or PVC, or acrylonitrile butadiene styrene, or ABS, tee.

  3. Run pipe vertically until it is above the overflow line for the sink. After that, you can run diagonally or even horizontally, as long is it maintains a minimum slope of 1/4 inch per foot toward the sewer. Use pipe no smaller in diameter than 1 1/4 inches, which is the minimum size the code allows for a 2-inch drain. Plumbers usually use 1 1/2-inch pipe to ensure there's enough venting area. Moreover, 1 1/2-inch pipe is often less expensive.

  4. Extend the pipe up through the walls until it is higher than the overflow line of the highest plumbing fixture in the house. From this point, you can extend it to the main vent and connect it with a vent tee. Cut int the stack, glue in the vent tee and glue the sink vent onto the tee.

  5. Use an air admittance valve to vent the sink if your local building department allows it. This is a mechanical device that usually goes under the sink. An easy place to install it is on the horizontal outlet arm of the P-trap.

  6. Cut the P-trap arm with a hacksaw and glue in a sanitary tee with a 1 1/2-inch outlet. Glue a 4-inch length of 1 1/2-inch pipe to the tee and glue on a male threaded adapter. Screw the AAV to the adapter and tighten it with adjustable pliers.

  7. Tip

    If the sink appears to already have a vent in the proper location, it may be blocked. To open it, you may have to climb on the roof and clear a blockage from the main vent outlet. If the sink already has an AAV, it may not be working. It is easy and inexpensive to replace.