Do Sink Drains Require an Air Pipe?

The Need for Venting

• Whenever you fill a sink and then open the drain, enough water rushes into the drain to completely fill it. As the water flows, it pushes air in front of it and creates a vacuum behind, and the vacuum can be strong enough to pull the water out of the P-trap. Even if it isn't strong enough to do that, the vacuum can slow water flow and cause frequent blockages. By allowing air into the pipes, the vent serves two important functions: it protects the trap and it keeps water flowing so blockages don't develop.

The Drain/Waste/Vent System

• The drain plumbing in a typical house includes a network of connected pipes that drain into a common drain pipe (the home's main drain, or soil stack, that leads to the sewer) and another network that connects higher up on the main drain pipe (at which point it's called the vent stack), which vents out through the roof. These two networks are connected in such a way that every drain in the house -- including toilet, bathtub, shower and sink -- is vented. The plumbing code has specific rules governing the placement of the vents, the size of the pipes you use for them and the trajectories of those pipes. Because there are so many ways to configure vents and drains, the rules regarding venting are complicated and nuanced.

Basic Sink Venting Requirements

• The pipe that you use to vent a sink drain must be no less than half the diameter of the drainpipe, and can in no case be less than 1 1/4 inches in diameter. Most sink drains are 2 inches in diameter, and for drains of this size, the vent must connect to the drain no farther than 5 feet from the bottom of the trap. It must rise vertically until it is above the top of the sink. After that, it can run horizontally, as long as it maintains a 1/4-inch-per-foot slope toward the drain.

Air-Admittance Valves

• Although they aren't legal in every plumbing jurisdiction, air-admittance valves -- sometimes called cheater valves, or Studor vents (after a common brand of valve) -- provide an easier alternative to running pipes through the walls or the roof if you just want to vent one sink. Negative pressure in the drainpipe opens the spring-loaded AAV to admit air, and when the pressure normalizes, the AAV closes again. Plumbers typically connect the AAV to the horizontal P-trap extension with a PVC tee and a short length of 1 1/2-inch pipe. Whether you mount an AAV on the back of the sink or under it, the valve must be accessible and in a well-ventilated location.