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We've converted our kitchen to an open plan by tearing out the wall separating kitchen from dining area, so that the sink and dishwasher are now in an island. The wall that I tore out, of course, contained the kitchen sink's vent to the roof. After a good deal of research, I settled on a Studor Mini-Vent AAV under the counter; it's mounted as high as I could manage without scraping the underside of the countertop, so it's 20 inches above the P-trap and about 15 inches above the bottom of the sink. (The minimum requirement is 4 inches; I think I've got that covered!)

The problem is that - with the faucet running full-blast - the sink simply fills up until there's about an inch of water in the bottom; at that point the drain rate catches up to the fill rate, and the sink doesn't fill any further unless it's plugged. If the water is turned off, the sink will drain, but it takes several minutes to empty.

If I unscrew the AAV, the sink drains almost instantly. If I then put my hand over the top of the pipe - sealing it - the sink again fills to about an inch. I've tested the AAV by trying to breathe in through it - there's no noticeable resistance - and breathe out through it - it plugs, as I would expect. As far as I can tell, it's working properly - but it won't let the sink drain.

Random facts that may or may not be pertinent:

  • The old sink had two basins; the new sink only has one deep basin.
  • The drain riser that comes up through the floor connects to a 10-foot, nearly-horizontal run of 2" pipe along the ceiling of the garage below. That run used to be nearly flat, and I used to have to snake it every few months; it's been replaced with a new section of pipe and the slope has been increased slightly. The pipe is brand new, and has never had anything but clear water run through it.
  • That horizontal run dumps into a much larger 5" vertical riser that's shared with our next-door neighbor's kitchen (we live in a townhouse complex).

As requested, here's a picture of the top of the stack: Top of stack

I took pictures of the whole run, from sink to sewer pipe; they're available here if needed for clarity.

How can I get the sink to drain normally?

Update: The problem appears to have resolved itself, and it appears to have been some sort of very minor obstruction - not in the drain, but in the garbage disposal. At the time I posted the question, I had finished the plumbing but not the electrical wiring for the island; the first time I ran the garbage disposal, the drain ran freely and hasn't had an issue since. I still don't understand the physics - why would removing the AAV have had an effect when the garbage disposal was the issue? - but I'm satisfied that the problem isn't/wasn't a venting issue.

  • A picture of below the counter would tell the most. Does the drain exit thru the floor as one would expect? – Tyson Nov 2 '16 at 1:38
  • Post your update as an answer and I'll upvote it – ThreePhaseEel Nov 6 '16 at 23:39
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As suggested, I'm posting the (accidental) resolution as an answer.

Update: The problem appears to have resolved itself, and it appears to have been some sort of very minor obstruction - not in the drain, but in the garbage disposal. At the time I posted the question, I had finished the plumbing but not the electrical wiring for the island; the first time I ran the garbage disposal, the drain ran freely and hasn't had an issue since. I still don't understand the physics - why would removing the AAV have had an effect when the garbage disposal was the issue? - but I'm satisfied that the problem isn't/wasn't a venting issue.

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