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My bathroom sink was draining very slow. It would fill up while washing your hands and drain over the next few minutes. Today, I took the drain (trap and PVC to the wall) apart, cleaned it all out, and found no blockage. A bit of build-up, but no blockage.

When I put it back together again, I tightened all the connectors (the drain goes from the drain attached to the sink > extender > trap > extender > drain pipe sticking out of the wall), and then turned the water on. It didn't drain at all. It was no longer slow to drain, now it wasn't draining water at all.

Thus, I started to take the drain back apart to see if I messed up something obvious. As soon as I adequately unscrewed one of the connectors, the whole sink drained (remarkably fast I might add). I maneuvered the drain some, thinking maybe the way I attached the various extenders/trap was causing some sort of bad angle (although I couldn't see how), and tightened everything back down.

Again, I turned the water on, and again the drain didn't drain anything. Again, I loosened one of the connectors, and again, the whole sink drained immediately.

So ... with my extremely limited understanding of plumbing, I can only think the drain is having the same issue as when you take a straw, put it in liquid, put your finger over the top of the straw, and then remove it from the liquid. The liquid won't drain from the straw until you remove your finger, and the bathroom sink drain won't drain until there's somewhere for the air to go ... maybe?

This doesn't make sense to me, but plumbing is very much not my strong suit. Does this make sense to anyone? Am I missing something obvious here?

Thanks in advance!

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    Clear pictures of the dubious-sounding and -behaving drain arrangement might enable some advice on correcting it. The text is too vague to see what's actually done wrong, there. Please edit your question to add some.
    – Ecnerwal
    May 11, 2023 at 0:44
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    When you loosened a connection below the sink, did the sink then drain through the loose connection into a bucket (or the floor or whatever)? Or did it drain down the drain? Or a bit of both?
    – jay613
    May 11, 2023 at 1:08

3 Answers 3

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Most likely, the issue is due to back pressure in the drain pipe, preventing water from going down. First, check that the plumbing vent pipe is clear, then check the sink overflow path.

Most sinks have an overflow hole, which also is a path for air trapped inside the pipe to escape, as shown below. Note that the cut-out in the porcelain sink bowl lines up with a cut-out patch of drain pipe.

When water in the sink enters the drain, air is already inside that pipe, and the water must push past it... unless there is an opening that lets air out of the pipe. If that opening is blocked, e.g., by tape dope or by the drain pipe cut-out being rotated away from it, it takes far longer to drain. It also defeats the other purpose of the overflow: preventing water from overflowing the sink, onto the counter and floor.

It's easy to test if that is part of the cause of the issue: pour water into the overflow hole -- if it fills the overflow channel instead of quickly draining, the passage through to the drain must be reestablished.

Sink Overflow Hole

If there is an air vent nearby, and the roof vent pipe is clear, an overflow is not absolutely necessary, but can still speed escape of air.

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  • If the drain pipe is free of blockage, the water will push the air down ahead of it, there do not need to be vents for that, although they do facilitate. There can be no "back pressure" in a clear drain, that requires some sort of blockage, even if just a full bagged-down pipe or an extra trap in the line (house trap?).
    – kreemoweet
    May 11, 2023 at 4:06
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I would start by cleaning out the vents from the roof. I usually would place a garden hose in it with the nozzle doing a strong stream of water. I have been lucky and it has worked each time. This is not recommended unless used as a last resort: you could add an AAV (Air Admittance Valve) to the drain.

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  • If you do run water down the vent like that you should have someone watching the sink and other connected fixtures to sound the alarm if a fixture starts flooding.
    – Hot Licks
    May 12, 2023 at 3:04
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Your straw experiment leads you to the right problem solving.

The drain works by gravity and air pressure. As the water flows down the drain it will creates negative pressure behind it. Negative pressure will hold the water back from flowing. For that reason the drains have a vent pipe to supply the air to allow water to flow.

This might involve a trip to the roof, to check the drain vent pipe sticking out of the roof. It could be also that your vent pipe is blocked just at the connection to the drain, close to the sink.

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    There can be no vacuum in this case, the water in the sink is open to the atmosphere. Drains do not require vents to function, the vents just speed things up. The only thing that can prevent draining is a blockage downstream.
    – kreemoweet
    May 11, 2023 at 4:06
  • @kreemoweet interesting, so what is holding the liquid in the straw
    – Traveler
    May 11, 2023 at 6:43
  • @ruskes, surface tension, which only applies in orifices of very small diameter. A drain pipe is much too large.
    – isherwood
    May 11, 2023 at 13:04
  • @kreemoweet the p-trap acts as a plug
    – Traveler
    May 11, 2023 at 18:02

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