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Our kitchen sink (which is like 2 weeks old) is clogging up... and it's really weird, because I can't find a single (physical) clog. When you fill up a basin (let's say you put in 2 liters) and open the drain of the sink, there's maybe like 0.5 liters flowing perfectly, then after a certain point, it gurgles and suddenly stops draining. The water then flows in slow motion...

At first I thought it was a physical clog. I unscrewed the siphon but nothing, it's as good as new (two weeks old at the same time). When I pour water into the tub with the siphon disconnected from the discharge, everything just flows through the siphon and goes into the bucket without a hitch. So the siphon isn't the problem. So I thought the clog was further down, in the discharge. I put some destop in, went in with a ferret (plumbing snake in english? Not sure), but nothing. The 5m ferret went through without a hitch (normally I should already be in the communal dump after 5m). The ferret comes out clean. It's really like there's nothing there at all. Then if I empty a 3-liter bucket directly into the discharge (the place where it becomes vertical on the photo under the sink) there's no problem, everything flows normally, it doesn't clog up or anything. So the physical clog seems to be ruled out...

But when I connect the two parts, the siphon and the drain... it doesn't flow. I tried pouring 3 liters into the drain (I looked with a camera afterwards and the vertical part was totally empty) then immediately afterwards, connecting the sink and putting 3 liters into the tray and after not even 0.5l it stopped flowing. So the problem seems to be the layout under the sink...

I confess I don't understand. Maybe a suction/air pocket/vacuum thing going on... Any ideas?

Further info:

  • originally, the sink was at the place where the pipes go into the ground. We've extended that further to the right. The slope between the new position of the sink and the previous position is like a vertical drop of about 7 cm for 1.60m horizontally. So in my opinion, this is plenty enough. (The plumber wanted to do it in the ground, with like 2 cm of slope so I guess this should be enough)
  • the flexible grey pipe you see under the sink is a dishwasher see also figures

first figure under the sink second figure under the sink full installation enter image description here

Update 1: I tried what you suggested... but it does not seem to help much. It feels like it still helps a bit. I feel like there's more water that flows before it stops. But i'm not sure... Here is what I added (see figure)

update 1

Would it also make sense to put a vent here? Since after that point, there is still like 2 meters to the left (I think??) Before the common discharge.

enter image description here

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    It sounds like it might be a vent problem more than a clog in the drain pipe. There is not enough air getting in.
    – crip659
    Jul 29, 2023 at 12:06
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    Your English is excellent! My French is just good enough to recognize "eau chaude" as "hot water". Could you please annotate your picture to indicate which part you're referring to when you say "siphon" - that's not a term I'm familiar with. Additionally, under the left-hand sink, what is the white pipe leading horizontally out to the right, then up? Where does that pipe go?
    – FreeMan
    Jul 29, 2023 at 12:45
  • @FreeMan I suspect the pipe (with the accordion bend) in between the bowls is some kind of vent. OP, what is that, and does it go above the sink deck? Also, I don't think your dishwasher hose goes as high as it should.
    – Huesmann
    Jul 29, 2023 at 13:51
  • Thanks @FreeMan ! When I say siphon I mean the part of the white pipe that is S shaped and which goes down then up (then eventually ends up in the grey elbow). The second one is indeed a vent. I will add a picture of the sink when I can but it's in the middle of both sinks.
    – mwoua
    Jul 29, 2023 at 14:35
  • We had a slow drain where the blockage was down in the basement below. The sink drain would allow water to flow freely down but then stop as the vertical pipe between the blockage and the sink filled up.
    – mr blint
    Jul 29, 2023 at 14:39

3 Answers 3

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As Ecnerwal stated, you need a vent beyond the trap.

Remove the elbow and replace it with a Tee. Add appropriate sized pipe for your location and an AAV.

Run vertically to as high above the bottom of the sink as practical. (see the yellow in the pic)

That should cure your problems.

enter image description here

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  • I will try that and come back when I know it cured (or not) my problems! Thanks
    – mwoua
    Jul 29, 2023 at 14:29
  • Adding the vent did not help (see update)
    – mwoua
    Jul 30, 2023 at 13:13
  • Are you allowing the water to drain without the strainers in the sink?
    – RMDman
    Jul 30, 2023 at 15:05
  • Usually no. But sometimes things go through of course... would not surprise me if a spaghetti slipped through once or twice.
    – mwoua
    Jul 30, 2023 at 19:32
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The plumbing shown lacks a vent after the trap.

You need either a vent line to the outside, or a "mechanical vent" (air admittance valve) which is a one-way valve that lets air into the drain but does not let sewer gas out (until it fails, stinks, and needs to be replaced.)

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  • I've added that... but it does not seem to help :/
    – mwoua
    Jul 30, 2023 at 13:13
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Well, in the end, it was in fact a blockage. I called a plumber. He came with a quite huge plumbing snake (at least thicker and longer than mine) and which was motorised. He drilled in there like there's no tomorrow. It did not look like he encountered a proper blockage to me. But hey... now it drains correctly.

What he told me was that, in his opinion, since we've been doing some works for 3 or 4 months without using the discharge (we changed kitchen), all the filth that was adhering to the walls of the tubes dried. And when we poured water again, this filth got unstuck and caused a blockage somewhere. Let's hope it's indeed that, so that it never happens again!

Thanks for the help anyways

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    Sometimes you need to call a professional. No shame in admitting another guy's plumbing snake does a better job, its his job after all ;) Glad your problems got sorted. Aug 2, 2023 at 14:46
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    @LioElbammalf Oh yes, definitely! And I'm glad I did :)
    – mwoua
    Aug 10, 2023 at 14:20

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