In the 2nd floor bathroom (sink, toilet, tub), I noticed a flexible hose coming into the sink drain. It's below the counter, above the trap, very similar to a dishwasher draining into a disposal. There are no other plumbing fixtures in the second floor, and from what I can see the flexible hose connects to standard pvc pipe at the wall behind the cabinet. I don't know where it goes after that.

I do not see this on any other sink drains in the house. Is it for venting? Or something else?

view under sink

  • 2
    It's a tap into the drain. Do you have an Air Conditioner in your attic perhaps? If so, this is likely were the condensate from the A/C drains into your house drain.
    – Tyson
    Commented Oct 26, 2016 at 16:13
  • @Tyson Yup, AC in the attic, that's gotta be it. Make it an answer and I'll accept it
    – mmathis
    Commented Oct 26, 2016 at 16:15

1 Answer 1


It's a tap into the drain. Likely for Air Conditioning (condensate) located nearby in the attic.

If this is proven to be the case you will want to familiarize yourself with how it works. Is it all gravity? Is there a pump? Regular (annual) cleaning/flushing is important, as you don't want this to overflow.

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