How to Plumb a Sink Drain With a P-Trap and Air Vent


All homes and businesses contain vent stacks that allow air into the plumbing system. This is necessary because without a source of air intake, the negative air pressure inside the plumbing system would not allow water and waste to flow down the drain. Sinks that are too far away from these vent stacks must be vented with an automatic air admittance valve, or auto air vent. These devices allow sinks to be installed in areas that otherwise could not access the vent stack.

  • Locate the drain pop-up valve under the sink. This is the plastic or metal piece that sticks out of the base of the sink basin.

  • Place the topmost section of a P-trap assembly over the bottom of the pop-up valve. Screw the nut on the P-trap onto the pop-up valve by hand until tight.

  • Place a T-pipe over the back of the P-trap. Screw the nut on the T-pipe over the P-trap until tight.

  • Place an automatic air vent into the top of the T-pipe. Thread the pipe into the T-pipe by hand until tight.

  • Place the bottom of the T-pipe into the drain line under the sink. Screw the nut on the bottom of the T-pipe until tight.

  • Run the water in the sink for about two minutes. During this time, make sure the air vent makes a slight sound of air rushing into the pipe and that the water drains smoothly without backups. If the water does back up, loosen the air admittance valve slightly until the clog clears.

Tips & Warnings

  • Almost all bathroom sinks use plastic drain pipes that are 1 1/4 inch in diameter.

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