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Thread: Back to back sinks don't drain properly

  1. #1
    DIY Junior Member cowboyfunkie's Avatar
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    Default Back to back sinks don't drain properly

    I have two back to back sinks through a wall in the basement that drain too slowly. Old house. I've opened up the wall to expose the drain lines (see photo). The stack is behind wall to the left, vent on the right, distance between is 28 inches. I'd like some advice on how to reconfugure drain lines with ABS or PVC.

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    Moderator & Master Plumber hj's Avatar
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    The stack may be on the left, but the sink DRAIN line is on the right. The vent is the portion of the vertical riser above where the sinks connect. Whoever installed the drains was not a plumber. What you have to do, if anything, depends on what you mean by "drain slowly". Does one sink back up into the other when they are draining? Then the vertical line needs cleaning. Do both drain slowly but not back up" Then it could be a sink problem, not a drain stoppage. In either case, replacing the drain line is probably not the "cure".
    Licensed residential and commercial plumber

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    DIY Junior Member cowboyfunkie's Avatar
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    Thanks hj, the front facing p-trap overflows when the rear facing sink is filled (never noticed this with front sink in place). I removed the front p-trap at the cast iron joint and noticed a lot of grease/fat build-up in the cast iron line. Even a small trickle into the back sink overflows out of the front opening, so I suspect a restriction somewhere between this point and vertical riser to right. I will clean out and see if that fixes the problem.

    A city inspector told me years back that the angle of the pipe may be too steep to allow proper venting, so originally I thought this may have been the problem.

  4. #4
    Moderator & Master Plumber hj's Avatar
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    It doesn't allow "proper venting" but it also would make the sink drain BETTER, so that is not the problem. Without a vent the sink drains by gravity and ALSO by siphonage. The vent is to prevent the siphonage, so it slows the drainage somewhat, depending on how the piping after the trap is installed.
    Licensed residential and commercial plumber

  5. #5
    DIY Junior Member cowboyfunkie's Avatar
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    Snaked out a pretty big hairball between the p-trap and riser, which seem to have done the trick. Front p-trap no longer overflows when back sink is draining. So looks like I don't need to mess around with the plumbing now. Thanks for the advice hj, saved me time, trouble and money.

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