I'm not a plumber but I recently changed the seal between the wall and the drain pipe. Now the water drains extremely slowly from the sink.

If I move the drain pipe to allow air in between the seal, the sink starts to drain (but some water escapes from the seal). Is there some particular procedure that I haven't followed that allows air through??

  • "If I move the drain pipe to allow air in between the seal, the sink starts to drain" When you do this, is the water that draining just coming out from the seal 100%? If you don't connect the drain pipe to the wall and pour water down the sink, and it all drains-- then you probably have a clog further down the line. Why do you think it isn't blocked?
    – A O
    Mar 14, 2015 at 16:43
  • Tighten up the seal, fill with hot water, use a plunger on it. Vigorously.
    – Ecnerwal
    Mar 14, 2015 at 17:47
  • 1
    What do you mean 'changed the seal'? Did you change the plumbing under the sink? Was it a p-trap? Do you have pictures?
    – DA01
    May 29, 2015 at 21:32

1 Answer 1


Assuming the waste line is still at the proper angle (it does need to have slight decline or gravity won't work for you):

It sounds to me as if you have an airflow problem in the vent stack. There must have been some reason you changed the connection between the drain and the waste stubout? Was something slow before that? Or was there a leak? It's possible something (leaves, dirt) got blown in through the top of the vent stack. Or perhaps it got jostled somehow. If you had positive or negative pressure building up in the pipes due to a lack of airflow, that might have been causing other problems.

Waste lines need to be attached to a waste vent in order to maintain neutral air pressure in the pipes so that the water will flow out due to gravity. Otherwise as the waste water flows in it puts pressure on the air in front of it, which pushes back against the water...causing it to drain slowly or not at all.

Typically the sink waste line is connected to the toilet waste line and that is connected to the vent stack. If you don't have a slow toilet as well, perhaps you have just enough venting to handle the toilet flow but not enough to handle the sink.

If you are in a single family home, the vent stack is probably up on the roof. I believe universal code is or used to be that the vent stack has to be within 5 feet of the waste pipe, so look directly above wherever your slow sink is. You need to go up there with a flashlight and see if you can find any blockages at the top of the vent stack (leaves, dirt, other junk). Towards the top you can stick any kind of long pole in there and try to dislodge any partial blockage. If there is a blockage farther down then can also run a garden hose up there, feed it down through the vent, turn on the water and try to blast out any blockage, which should just go back down and into the sewer.

If that doesn't work something might have put a kink or something in the vent pipe and you're probably going to have to call a pro unless you want to start cutting into the wall to get access.

  • 1
    How would someone "put a kink" in the vent pipe? It's not like they use a garden hose or even pex for those things.
    – BMitch
    Apr 29, 2015 at 19:44
  • I probably misused a slang version of "kink" - I meant "something hinky", that is, something really strangely out of the ordinary, crazy, weird - not a physical bend. Although if nothing is blocking the line or the vent and everything is attached properly then I can't think of anything else, can you? Maybe there's a poltergeist and they should call a priest...
    – nbm
    May 5, 2015 at 19:57

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