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A plumber came to my house to unclog my kitchen sink. As he was leaving, he glanced at the plumbing under my kitchen sink and said something was not done correctly, but I don’t remember what he said was not done correctly. Can anyone spot any obvious problem with the piping under this kitchen sink? Thank you!

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jeffrey
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  • Out of interest, exactly what and where was the blockage? Directly related to the plumbing in the pic? – Tim Jun 15 '21 at 14:08
  • I could be mistaken, but it also looks like the garbage disposal is wedged in at a bit of an angle. – End Anti-Semitic Hate Jun 15 '21 at 17:05
  • @Tim The plumber came and stuck a sewer machine snake into the cleanout hole at the side of the house – jeffrey Jun 16 '21 at 15:46
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    Too much pipe or fittings space could cause the disposal to not hang plumb. It also could also just be the sink sheet metal is warped to one side a fraction. As long as all gaskets and seals are leak proof it wouldn't be a serious problem. –  Jun 16 '21 at 17:03

2 Answers2

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The correct plumbing is one P-trap for both sinks like this:

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The horizontal slopes down to the P-trap by 1/4in per foot, and is above the exit into the wall.

There should be no chain of P-traps as your picture seems to suggest. Trapping each sink separately and joining the drains after the traps is allowed.

IRC 2017:

P3201.6 Number of Fixtures Per Trap

Fixtures shall not be double trapped.

When emptying the far sink (the one further from the exit), so not when just running the tap, the velocity and siphoning can empty both traps, and there might not be enough residual trickle to re-fill the near trap (the one near the exit, second in chain) through the far sink, exposing the home to sewer fumes through the near sink.

P2000
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    What is the problem with having 'too many' traps? – MikeB Jun 15 '21 at 10:29
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    Flow is impeded to such an extent that debris tends to build up, causing clogs. – isherwood Jun 15 '21 at 14:04
  • @MikeBrockington a) OP wondered what the plumber meant. Trap chain not allowed in NA. b) when emptying the far sink (far from exit) there may not be enough residual to re-fill the near trap (near exit, second in chain) through the far sink, exposing the home to sewer fumes through the near sink. c) what isherwood wrote – P2000 Jun 15 '21 at 14:53
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The disposal side of the sink is "double-trapped" since there is another P trap in the other sink outlet, that this one empties into.

There should not be a trap between the two sink halves. The only way to correctly have a trap per sink bowl would be if they each directly connected to the drain, NOT one output connecting to the other input.

Ecnerwal
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    It may not be the correct way - but I don't see anything wrong with it like that. If either trap is going to get clogged, it'll be the disposal unit one, which means the other sink would still be usable at that time. I'd prefer two traps, both joining the waste pipe after, and so separate till into the final waste. – Tim Jun 15 '21 at 07:39
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    The trap between the two sinks serves no useful purpose whatsoever, so I'm really curious how this came to be. Why did anyone go through the pain of installing an extra P-trap just to isolate two drains that are both open to the atmosphere right next to each other… – TooTea Jun 15 '21 at 07:52
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    @TooTea Seems obvious to me that the trap is there in order to deal with the difference in level between the start and end of that segment of pipe. But may also be related to the sink on the left having a right-angle fitting - maybe no straight out fitting was available? – MikeB Jun 15 '21 at 11:11
  • @tim, sequential traps slow flow and result in more debris accumulation. They're likely to cause more problems than they prevent. Thus, illegal. – isherwood Jun 15 '21 at 14:05
  • @isherwood Good point. The aerator side of that second trap would have plenty unless extra water was run. 2 traps are bad. Having the disposal drain through 2 traps even worse. –  Jun 16 '21 at 18:32