As title states, this sink drain in a bathroom has me confused as to the purpose of the higher set pipe above the trap. My assumption is that it's a vent, but I don't understand why it's here. I'm pretty sure this was a previous DIY job given that all the fittings are glued together. Any insight would be much appreciated.


2 Answers 2


Most likely, this is not a vent, or more precisely, this tube will not work as a vent.

F.e., this tube will not prevent that the siphon/trap will lose its water if a low pressure sets in downstream the trap.

Venting is normally done on the other side of the trap (downstream).

It looks more like a drain of a washing machine, dish washer, a condensing water line of an A/C or a gas/oil heater etc.

Maybe behind that wall there are hints of (former) installations?

  • It could be the condensing water line from the A/C, that's the only thing that'd make sense given the location. Cutting into the wall may be in the cards once I remove the vanity, so we'll see. Commented Dec 27, 2021 at 14:13
  • I've confirmed it was indeed the A/C condensation drain line, thanks for the tip! Commented Dec 27, 2021 at 18:03

It is probably a vent but that wouldn't pass code. A vent can't go horizontal until the vent is higher than the flood level of the sink.

In this case if the P trap clogs then waste water and debris can flow into the horizontal portion of your vent pipe and clog it. A code approved install would have this horizontal piece higher than the flood level of the sink so if the p-trap clogs the first thing that would happen is the water flows onto the floor before clogging the vent pipe.

  • 1
    if it's a vent it's connected to the wrong side of the trap.
    – Jasen
    Commented Dec 27, 2021 at 14:10

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