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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?
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.
