Without venting, pressure imbalances cause traps to fail. The seal remains as long as the pressure on each side of it is equal (1). But as soon as water drains through the line, it creates negative pressure behind it (2), pulling nearby traps dry (3). Proper venting provides the air needed for the drain to flow freely without creating pressure differences.
Without venting, pressure imbalances cause traps to fail. The seal remains as long as the pressure on each side of it is equal (1). But as soon as water drains through the line, it creates negative pressure behind it (2), pulling nearby traps dry (3). Proper venting provides the air needed for the drain to flow freely without creating pressure differences.

To learn about how to do plumbing vents right, visit the JLC Field Guide.

For more on understanding how plumbing vents work, go to this article (you must register or be logged on, and download the PDF).