Diagnose my leaky pipe!
October 17, 2012 12:44 PM   Subscribe

Can you help me to understand what the problem with my sink and washing machine pipes is? A heavy load of draining water spills out from near the U-bend.

Since having the washing machine fitted a couple of months ago, we have had a problem with the U bend underneath the sink. There is a spare outlet pipe hole at the top of the U-bend, and when there is a heavy load of water draining out of the sink the majority of it spills out of two slanted holes near the top of this cap. The volume of water is not extraordinary, just a full sink. Before we call an expensive plumber, I was hoping someone might be able to point out what the problem is that stops all of the water from draining down the pipes and instead pushing out of these holes. The problem occurs whether or not the washing machine is on.

Here are some pictures: Imgur The second picture shows the connection between washing machine and sink just to the left of the other pictures.

Here is a video of the spill in action: Youtube

posted by tumples to Home & Garden (9 answers total)
You've got a blockage further down the line, and water is backing up..

You can probably just get a $20 snake from the hardware store and do this job yourself.
posted by mhoye at 12:49 PM on October 17, 2012

Not a plumber here but if the water's coming out from those places, then they aren't watertight.

Also, water exerts pressure in all directions, not just down the hole. If you have more water than can go instantaneously down that pipe, like in a full washing machine, you're going to have pressure in the pipe.

It is possible there's a clog further downstream that is increasing the pressure. If so, snaking out your line may help. But I'd address the water comes out here issue by making "here" watertight.
posted by zippy at 12:53 PM on October 17, 2012

I initially thought that the problem was with the cap on top of this spare section not being on properly, but it turns out that there are two inward slants sloping into the middle of the pipe just underneath the cap, which is where the water spills out from. This is an intentional design feature, but I can't get a good picture of it.
posted by tumples at 12:57 PM on October 17, 2012

From the photo, I think the U-bend may be improperly installed. Honestly I'd probably just replace it, because it may be tough to get the old one to not leak if it was abused or overtightened or any number of other things. They are basically standard parts.

Also it looks a bit like the actual piece that is leaking is supposed to be at the bottom of the U-bend, not at the top (it looks like a cleanout that's supposed to be at the lowest point of the U). The fact that it's on the top, and leaking, makes me wonder if someone changed the trap around when it started leaking to move the broken part to the top (where it's normally dry). Again, I'd probably replace the whole thing ... but make sure you install it correctly, don't just duplicate the current configuration!
posted by Kadin2048 at 12:59 PM on October 17, 2012

Based on your followup, I retract my idea that the trap is upside-down, if the leak is definitely coming from what's certainly a design feature of the part and not a crack or out of the threads or something.

It might be some sort of dodgy pressure-release / vacuum breaker that's getting flooded with water because of a slow pipe (partial clog) downstream. Snaking the drain probably can't hurt.
posted by Kadin2048 at 1:02 PM on October 17, 2012

Can you get a better look at the cap that is leaking? It probably says on it what it is (or gives some codes that will provide a clue). My guess is that it is an air admittance valve (which allows air to be sucked in when the pressure is negative, removing the need for a vent). It sounds like the valve itself has failed (it should not let any water out as it should only open under negative pressure) AND you have a slow drain which is backing up and applying water pressure at the valve.

So, check if that is actually an air admittance valve, then replace it and snake the drain. You could also try re-jigging the plumbing there so that the air admittance valve is higher than the drain in the sink or at least higher than it is now (which is, judging from the photos, not much higher than the level of the pipe leaving the trap).
posted by ssg at 2:43 PM on October 17, 2012 [1 favorite]

That may be some sort of cheap-ass air-admittance valve, installed as a "vent" since the sink is no longer vented by itself but now in tandem with the washer. Most likely, the washer wasn't installed far enough down the line to avoid a suds problem. Was this done by a licensed plumber, inspected, etc?
posted by notsnot at 5:43 PM on October 17, 2012

ssg has it. In older homes with already dodgy plumbing these air admittance or under cabinet vents are sometimes the best you can do, and it prevents the problem of not having a vent at all. But you need a plumber to find out if it is a problem with the valve or line problem (probably both).
posted by bartonlong at 6:17 PM on October 17, 2012

I believe the inward slanting slope underneath the cap is where a compression ring should have been installed. The cap would be screwed down tight enough to make the ring squeeze onto the pipe that extends down into the trap and down onto the trap itself, sealing that part of the joint. From what you describe, that ring is missing. Go to a plumbing supply store or a big box store and look at traps like this and see if there isn't a nearly clear little ring inside the cap. Get one of the right size.
posted by Old Geezer at 7:37 PM on October 17, 2012

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