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I recently saw a plumbing setup where a 2" pipe was run from a bend in the 4" toilet return. The 2" pipe bent upwards to meet the floor next to the toilet, where it was capped off with a removable cover.

In case of a clog, apparently the plumber uncaps and pushes air (or water?) through this pipe (or vent? valve?) to purge the clog.

First of all, what is the correct terminology for this apparatus?

And I hope it's not outside of the scope of the question to also ask does this thing function as a vent?

Will it have a one-way valve to prevent the elevated water in the toilet from rushing out when it's uncapped? (cap is level with the floor next to the toilet)

Here's an ugly sketch of the thing:enter image description here

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A capped opening in a sewer line is called a clean-out, usually they are a 4" long bend wye with a cap on one branch so you can pull the cover and run a snake down the whole length of the main sewer line.

In this case, a 2" pipe would be a good size to run a snake down to clear any clogs downstream from the toilet. I'd guess that somebody didn't allow enough drop in the sewer line and added the pipe to allow shoving the solids out of a long, level area.

Sewer Auger (Snake) or a Water Bladder Clog Buster just in case you ever have to use it.

No, it's not a vent.

enter image description here enter image description here

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  • OK thanks. What's preventing the water in the toilet from overflowing when the cleanout is uncapped?
    – Ben
    Jun 1, 2013 at 7:00
  • A Mop LOL (not so funny when its me with the mop tho)
    – UNECS
    Jun 1, 2013 at 10:54
  • You should never leave it uncapped because you'll be letting the sewer vapors constantly enter the house and it can cause you sickness. Water (and the sewage with it) will take the path of least resistance, so it happily travel further down a larger pipe (your drain) than try to go up a narrow pipe (your cleanout).
    – Jason
    Jun 1, 2013 at 12:43
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    The water in the toilet is contained in a trap. The pond of water in the bottom has to flow uphill. If you look at the side of the toilet, you can trace the tube cast into the porcelain and see the "S" shape. You can take the cap off and nothing but sewer gas will come out, unless the line is blocked so badly that water above normal level in the toilet is caused by it and not a clog in the toilet trap. Now when you flush, expect water to come up out of that uncapped hole whether the line is clogged or not. And to reiterate @Monso, it's not a vent. Sewer Gas is explosive and causes suffocation.
    – Fiasco Labs
    Jun 1, 2013 at 14:07
  • Thanks for the info fellas, I didn't mean leave it uncapped, I meant "at the time it is uncapped" for cleanout. The trap example answers the question though. Cheers!
    – Ben
    Jun 3, 2013 at 5:13
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That would be an odd location for a cleanout, since a plumber would likely pull the toilet and run a large auger directly down the toilet drain if you had a clog. It looks like a vent line that was installed but not attached to the vent stack (either in error or because you're looking at something while it's still under construction). When the toilet is at the end of a drain line, rather than installed after the sink/shower in a bathroom, you don't have a wet vent for the toilet so you need to install a dry vent.

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