# Where does air in the pipes go?

**URL:** <https://boards.straightdope.com/t/where-does-air-in-the-pipes-go/429543>\
**Category:** Factual Questions\
**Created:** [December 10, 2007, 9:57pm UTC](https://boards.straightdope.com/t/where-does-air-in-the-pipes-go/429543 "2007-12-10T21:57:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\
**Post date:** [December 10, 2007, 9:57pm UTC](https://boards.straightdope.com/t/where-does-air-in-the-pipes-go/429543/1 "2007-12-10T21:57:25Z")

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Let’s say I step into my bathroom, put a stopper in the bathtub drain, and fill it up. I then remove the stopper. There are a few tiny little bubbles, but other than that, the water drains smoothly, with no air coming out of the drain.

Presumably, other than the water in the U-bend, the wastewater pipes are filled with air most of the way down to the sewer system. So where does all this air get pushed when I drain my bathtub?

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**Author:** ![masterofnone](https://avatars.discourse-cdn.com/v4/letter/m/4bbf92/32.png) [@masterofnone](https://boards.straightdope.com/u/masterofnone)\
**Post date:** [December 10, 2007, 10:06pm UTC](https://boards.straightdope.com/t/where-does-air-in-the-pipes-go/429543/2 "2007-12-10T22:06:14Z")

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If the plumbing was done correctly, it will go out the roof vent. If the vent was not there, the u-bend would empty and sewer gas would come into your house.

See wiki for the full description:

> **[Drain-waste-vent system](https://en.wikipedia.org/wiki/Plumbing_drainage_venting)**
>
> In modern plumbing, a drain-waste-vent (or DWV) is a system that allows air to enter the plumbing system to maintain proper air pressure to enable the removal of sewage and greywater from a dwelling. Drain refers to water produced at fixtures such as sinks, and showers; waste refers to water from toilets. As the water runs down, proper venting is required to allow water to flow freely, and avoid a vacuum from being created. As the water runs down air must be allowed into the waste pipe either t...

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**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\
**Post date:** [December 10, 2007, 10:53pm UTC](https://boards.straightdope.com/t/where-does-air-in-the-pipes-go/429543/3 "2007-12-10T22:53:17Z")

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Yeah, the pipes don’t all go down. There are also some that go straight up, and usually poke out your roof a little bit.