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My kitchen sink drains very slowly. From full it takes more than a minute to fully drain, and it is hard to perceive the water level dropping.

Listening in the cabinet underneath I do hear the trickling of water. I removed the S-trap to clean it, and to see whether it was blocked, but there was no significant debris on or in it, just some slime on the side.

After putting it back, I tried whether opening the aerator cap (at least, that's what I think it is, marked in red on the picture below) on the S-trap helped with the draining. It did improve the draining speed significantly, but water also leaked through the cap, so clearly it's not intended to be opened.

What else can I do to improve drainage speed?

enter image description here

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    1) Where in the world are you located? Plumbing codes vary. B) that sharp kink at the top of the trap is unusual for the US and could cause issues. C) It's quite likely that there's a clog further down the line than in the sink trap - do any other drains run slowly? D) that circled item could be an AAV, and they tend to fail and need to be replaced somewhat regularly.
    – FreeMan
    Feb 23, 2023 at 19:00
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    1) Netherlands. C) when I run the tap, the water backs up pretty much immediately, which leads me to believe that the bottle neck is pretty early in the drain. What does AAV mean?
    – JAD
    Feb 23, 2023 at 19:08
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    Ah, Air Admittance Valve. Yeah, it might very well be. When I opened it up, there was a rubber circle on a pin inside. How might I diagnose whether it failed? Or maybe the symptoms are an indication that it already failed?
    – JAD
    Feb 23, 2023 at 19:10
  • It looks replaceable, so why not just get a new one?
    – Huesmann
    Feb 24, 2023 at 13:39
  • A bad air admittance valve will not cause this problem, it will cause siphoning and gurgles in the trap.
    – Jasen
    Feb 25, 2023 at 0:18

1 Answer 1

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I ended up doing some asking around at local stores, and the advice I got was that likely the AAV in the siphon was insufficient. A bigger vent should be installed.

Upon closer inspection, a second bigger vent was already present on top of the vertical grey pipe in the background of the picture in the question. Completely removing the cap on the valve didn't help anything, ruling out a bad valve. Instead, it appeared that the pipe leading from the main drain to the valve was completely clogged.

Removing the clog appears to have solved the issue.

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