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During our kitchen remodel, we replaced our double sink with a single bowl. There was no drainage issue before the remodel, now it drains really slowly. The drain was open for 2 months during the remodel, uncovered, with no sewer gas odor. Dishwasher drain enters just below the sink, followed by a p-trap. Drain pipe goes through the floor into the basement and runs along the ceiling, emptying into the main drain about 20 feet away. An old vent pipe branches horizontally from the sink drain about 10 feet and a couple of turns down the line, but it was disconnected and capped off in the basement years ago, before I lived here. I added an aav vent under the sink, to the top of the vertical pipe that goes through the floor, 5 inches above the horizontal pipe from the p-trap. No help. I snaked 25 feet. No help. I used drain cleaner. No help. 2 other sinks and a washer empty into the same part of the main drain line with no issues. I don't know what else to try. Anyone have any ideas?

  • Do you have kids? – Edwin Apr 20 '17 at 1:14
  • Kathy, is the aav located so that the top is well above the level of the water in the full sink? Does pipe to the aav join the drain above the P-trap like an extra sink or on the other side of the the P-trap? Where is an aav pipe supposed to be connected in general? – Jim Stewart Apr 20 '17 at 10:59
  • Does just the sink drain slowly, or the dishwasher as well? – Carl Witthoft Apr 20 '17 at 19:00
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The problem here is that now with just a single drain hole you are experiencing a suction lock as a sink full of water tries to go down the drain. It will most likely require the relocation or addition of another sewer vent pipe up through the roof near this sink to break the suction path.

  • Wouldn't the aav be equivalent to the second sink? Could it be that the aav is faulty? Could it be jammed open or temporarily taken off to test? For it to function properly does the top of the aav have to be located above the level of the water in the sink? Presumably there is a check valve in the aav so that if the water rises in the tube leading to the aav and reaches the aav a check valve seals to prevent the water coming out. – Jim Stewart Apr 20 '17 at 10:17
  • There are tailpieces that have a connection for dishwasher drainage. (Used when there is no disposer.) One of these could be used to make a vent which would provide an air intake path equivalent to that with a double sink. Just connect a flexible hose to the dishwasher tailpiece and direct it up above the level of water in the sink and see if this speeds draining. Personally I think something else is happening here and the change from the double sink to the singel sink is not the problem. But I must admit that I do not understand the principles of venting of sewer drainage. – Jim Stewart Apr 20 '17 at 13:11
  • So I ran the water with the aav off and the pipe open to see what would happen - it overflowed. I opened the cleanout in the basement and snaked again. I removed the plug from my old, abandoned vent pipe that was cut off in the basement. Still slow drain. I opened the tap in the next sink down the line, in my laundry room, then turned on the kitchen water. It still backed-up. So I turned off the laundry sink and let the kitchen sink keep running. When the sink filled an inch or so, something gave way, and WHOOSH. I ran to the basement expecting a flood. No flood. And no slow drain anymore. – Kathy Apr 21 '17 at 11:55
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Install a cheater valve, as Kathy said or other wise known as an AAV air admittance unit, very easy to do if u have basic plumbing experience u can google it, if not calls plumber very quick and cheap for them to do

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