Underneath one of my upstairs sinks there is a connection between the trap and sink basin to what seems to be a plumbing vent (similar connection to how my dishwasher is plumbed to kitchen sink drain). I say this because when the wind blows from a certain direction I get a flow of air out of the my sink basin into the bathroom (no bad odor though) along with various gurgling sounds. If I remove this connection water drips out from the wall/vent side connection (maybe only when wind blows, not sure). This is the only type of connection on any of the sinks in our house. My home was built about 20 years ago.

Why would someone make this type of connection and/or what is it's purpose ?

Can I just cap off this vent connection at the wall ?

Thanks for any replies,
Kirk

It could be connected to the air conditioner for condensate drainage, but we would have to see a photo of it to see how it is connected to the drain.

Interesting suggestion - have not investigated that. Will try to post a pic of the connections under the sink early this evening.

Capping it off would likely be a bad idea. It is serving a purpose.

Here is the pic of the arrangement under the sink: [tinyurl.com]

Turns out it was a bad idea.

Before work yesterday I pulled out the black flexible pipe in the pic and put a cap on the outlet at the wall. When I came home, had a slow drip coming from the seam between two drywall sheets in the ceiling downstairs. The actual location of the drip in the downstairs ceiling was not below the upstairs sink so the water likely ran from the leak location to the drip location, pooled and then came through the seam.

When I saw this I removed the cap on the outlet under the sink I had placed that morning. I expected a rush of water, but only a dribble came out. Going up into the attic, I traced the drain line from the air conditioner to where it turns down to go between the walls. Very near this location is a plumbing roof vent, all of which is in the right location for this sink. So it seems likely that the black flexible pipe in the pic is acting as a connection to the air conditioner drain line.

I put the black flexible pipe back in place last night and this morning there was no more dripping from the downstairs ceiling and the spot seemed on it's way to drying.

There are a couple mysteries, in all this to me:
1. Why is there an air flow *out* of wall connection, i.e. out of the air conditioner drain line, for the black flexible pipe ? I suppose I could imagine some wind pattern creating a pressure differential between the attic and the inside of the house that would drive an air flow. Other possibility ? An unseen connection to a plumbing roof vent ?

2. While I had the outlet capped off, the air conditioner drain line water seems to have escaped to leak onto the downstairs ceiling (no large volume of water when I took the under sink cap off as if a long line had backed-upd, ceiling drip stops when I replaced black pipe). But this seems to imply that the drain line has an open path that is apparently behind drywall. True ? I'm not a plumber but if that's the case it seems like a bad idea.

I'd be interested in any thoughts on why there is an air flow out of the air conditioner drain line and how the air conditioner condensation water could have escaped to form the leak through the downstairs ceiling.

Thanks in advance.



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