Plumbing vents allow air out of the waste pipes.Ģ. The waste won't flow well if it's held back by low air pressure or a vacuum in the pipe behind it.

We don't want the water to be siphoned out of the trap every time a fixture is used. It's the water sitting in a plumbing trap that stops sewer gases getting into the home. Vents allow air in to prevent a siphon.Ĥ. Plumbing vents allow sewer odors to escape from the house, venting safely above the roof. Gurgling Sounds and Sewer Gases at Sink Drains, Showers, Tubs Without venting, the sewer gases seep through the water in the trap and enter the house. Gravity pulls the water out of the fixture and down the drain pipe. That's all good.īut when the sink basin is empty, how do we split that solid slug of water flowing through the pipe, so that some stays in the trap and the rest flows down the drain? Gravity wants to leave some water in the trap because it's a low spot.

The venting system allows air to get between the water going down the drain and the water staying in the trap. The illustration below shows this nicely.

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