How to Install an Under-the-Sink Plumbing Vent

Plumbers install a check vent, commonly called an air admittance valve, under a sink when the sink's drainpipe does not tap into a vent pipe. Water flowing through a drainpipe displaces the air in the pipe. If the sink's drain system does not have proper ventilation, the vacuum created as the water level in the drainpipe lowers causes the water in the sink's basin to gurgle. A check vent provides a pathway for air to enter the sink's drainpipe, solving the gurgling problem.

  1. Cement a 1-1/2-inch T-fitting to the building's drainpipe with PVC glue. Angle the T-fitting's horizontal port toward the sink drain's P-trap fitting, and position the T-fitting's vertical port in an upward position, using a bubble level as a guide.

  2. Cover the last 1/2 inch of the sink's P-trap fitting with PVC glue. Push the end of the P-trap fitting into the T-fitting's horizontal port.

  3. Measure from the top of the P-trap's horizontal pipe to the bottom of the sink, using a tape measure. Transfer this measurement to a 1-1/2-inch female fitting with a stem, measuring from the end of the female threads down the stem. Place a pencil mark at the appropriate spot.

  4. Cut the female fitting's stem at the pencil mark, using a hacksaw. The cut stem's length brings the check valve's air intake port above the bottom of the sink.

  5. Cover the check valve's first three threads with plumber's tape. Plumber's tape protects and seals the pipe threads.

  6. Twist the check valve's threads into the female fitting's threads. Hand-tighten the check valve in a clockwise motion.

  7. Cover the bottom 1/2 inch of the female fitting's stem with PVC glue. Insert the stem end of the female fitting into the T-fitting's vertical port. Hold the bubble level against the stem and adjust the stem until it rests plumb in the T-fitting.

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