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Last Updated: , Created: Thursday, January 31st, 2002

How do you put the vent pipe in an island sink?

Tom from Toronto, Ontario wants to know how to go about putting a vent pipe into the drain of a kitchen island sink.

Good question Tom, because you don't have a wall which will simply allow you to go from the drain pipe uphill to the vent stack as we usually do.

The first drawing shows a simple method, but one that is not accepted by all municipalities or plumbing inspectors. You simply have a long vertical run for the drain and put the trap in the floor below. That permits having the vent pipe come off the top of the horizontal run and go up as it should do. Two problems with this: the trap is under the floor not under the sink for cleaning out problems; and the long drop of water can create so much momentum that it will blow the seal out of the trap.

The second drawing shows a much better method. Install a normal trap under the sink and then run the drain down as usual. Run a vent pipe up out of the horizontal portion of the P trap, as you would with a regular installation. Run this pipe up as high as you can under the counter. It works best if the highest water in the sink is never higher than the top elbow of this vent. The vent then turns down, goes through the floor and along the drain pipe, then turns back up to join the main stack several inches above the sink level. You must tie the bottom of the vent loop directly into the drain under the floor to allow for any condensate to drain out, otherwise this pipe could fill up with water and block. Another good detail is to install a clean-out just above the P trap, so that both directions of the drain can be snaked if necessary. This will keep a free air path to the backside of the P trap under all conditions, which is what a vent pipe is supposed to do.

Keywords: Kitchen, Drains, Vent Stack, Plumbing, Ventilation

Article 1707