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I snaked my kitchen sink drain to the end of my 20' snake so I know there is no clog for 20'. When I run water into the sink it starts to fill up immediately - not enough time to fill 20' of pipe. When the dishwasher drains it begins filling both sides quickly but only for an inch or two. This has been a problem ever since we installed a double sink. I'm tempted to install an air gap (aerator) for the dishwasher but I don't see how that would help. There is no problem with the vent since my vent is in the wall behind the sink (with a one way valve) because I took the valve off, snaked the vent pipe, and that made no difference.

After the sink fills a bit it will stay full for a few minutes and then at some point when the water gets low enough it will drain a little faster - that is why I think there needs to be an air gap to break a siphon but again I don't see where the siphon is happening.

I'm stumped.

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2 Answers 2

If you're positive that it's not the vent (though this sounds a lot like a vent issue) then you may have a drain that's not properly sloped downward. Without a proper slope, water will build up in the drain line until it reaches a low enough point, at which point gravity will take over and siphon the line.

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I know it does sound like a vent issue but I can take the one way valve off the top of the vent which is in my wall a few inches higher than the top of my sink and still get the same result. I thought of the shallow slope of the left basin drain pipe since that is the main difference in the plumbing since installing a double basin sink. I guess I don't understand why water filling the almost horizontal left drain pipe would cause both basins to not drain since the right basin drops straight down to the p trap. How can water in the horizontal pipe "get in the way" where it runs into the vert? –  Kirby Hopper Jan 2 '12 at 4:35
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@KirbyHopper I was thinking more of a long run in the wall or under the floor than the short bit under the sink. Since you mention the problem started with the double sink install, then my next thought is that the new sink is deeper and possibly below some of the plumbing in the wall. –  BMitch Jan 2 '12 at 13:16

It could be possible that the setup of your piping/p-trap, is not allowing air through, which is causing the water to not be able to flow smoothly.

I just moved into a basement apartment and have the same issue: the sink will drain, but sometimes the sink will fill with water for 10 seconds or so, until an air pocket gets through, after which it will drain fine.

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The p-trap is not supposed to allow air through, its sole purpose is to prevent sewer gas from coming into your house. There is a separate vent for the drain which allows air in. –  Steven Feb 17 '13 at 22:49

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