My kitchen sink drains slowly even after I snaked it 20ft without hitting any consequential clogs. I'm thinking there could be a couple of issues:

  1. The trap diameter (2.5") is too wide for the inlet diameter (1.5") and therefore the pressure from the draining sink is not great enough to force the water past the trap. In which case should I just put in a trap to match the inlet diameter?

  2. The air ventilation in the system is insufficient. If so, maybe a vent on the outflow side of the trap would help?

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  • Physics drives the emptying of the trap regardless of inlet size, that is not your problem. Also, one might assume that the vent configuration is appropriate or there would have been historical issues that forced remediation in the past. You have an obstruction of either the drain line or a vent component. – Jimmy Fix-it Jul 21 '19 at 23:54
  • Thanks for your response. Not sure about the full history, but this slow drainage problem has existed since we moved in 3 months ago. – cjn Jul 22 '19 at 0:11

There is no problem with the trap being larger, in fact a larger trap and drain line will reduce the chance of a plug. I have found several homes that the vents have gotten plugged from everything from leaves to birds and squirrels getting in them and plugging the vent. If your vent is plugged it may drain quickly for a few seconds then go slow one telltale sign that the vent is plugged or not there, is if the sink drain starts gurgling after the water almost gone or none left in the sink a few seconds later. For an older home I would be looking into checking the vents because that larger line it should drain quickly assuming there is not a big chunk of food waste or grease just below the drain or at the dishwasher T.

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  • Turns out it was a clog much further down the pipe, consistent with what you and Jimmy Fix-it suggested. I hired a specialist with a large motorized drain auger to flush it out. – cjn Jul 24 '19 at 13:48

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