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Old 01-31-2013, 10:24 PM   #1
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Siphoning sink


The pic shows a kitchen sink drain which was replumbed to accommodate a dishwasher. Feeds into a 2" pipe which goes to an upstairs sink, properly vented above. When the upstairs sink is full and then drained, this lower one gurgles and siphons the p-trap resulting in sewer gas. Doesn't happen except when full sink is drained upstairs.

Not sure if the setup is equivalent to an s-trap - should the p-trap be lowered to allow a straight run to the wall (plus normal drainage slope)? e.g. 22.5 out of the wall, lower tailpipe, eliminate up and down after p-trap.

Thanks for any advice
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Last edited by profcolli; 01-31-2013 at 10:28 PM.
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Old 01-31-2013, 11:47 PM   #2
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Siphoning sink


Maximum drop center-to-center, from the p-trap 90 to where the drainage connects to the vent piping, in this instance is 1.5", anything more and you have an S trap... and you sir... have an S trap.

You need to cut that 45* out behind the cabinet there, place a coupling and connect straight into the p-trap(after lowering it of course). That will make it to code, and stop it from being siphoned.

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Old 02-01-2013, 12:27 AM   #3
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Siphoning sink


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Originally Posted by joeplumber85 View Post
Maximum drop center-to-center, from the p-trap 90 to where the drainage connects to the vent piping, in this instance is 1.5", anything more and you have an S trap... and you sir... have an S trap.

You need to cut that 45* out behind the cabinet there, place a coupling and connect straight into the p-trap(after lowering it of course). That will make it to code, and stop it from being siphoned.
Thanks Joe! I thought that was what it was, good to have confirmation
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Old 02-08-2013, 06:22 PM   #4
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Siphoning sink


Fixed!!
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Old 02-08-2013, 06:53 PM   #5
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Siphoning sink


From the looks of that pipe coming out of the wall in photo 1, New drain lines are indicated.
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Old 02-08-2013, 07:08 PM   #6
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Siphoning sink


Wrong fitting in wall still a three quarter s trap should be a Ty in wall not a wye
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Old 02-08-2013, 07:09 PM   #7
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Siphoning sink


hate to tell you this but if you want it correct- remove the 45 in the wall. It should connect through a tee. Looks like you have a wye/45
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Old 02-08-2013, 07:23 PM   #8
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this will help explain it, yours is the bottom drawing- you want the configuration in the middle
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Old 02-08-2013, 07:54 PM   #9
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Siphoning sink


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this will help explain it, yours is the bottom drawing- you want the configuration in the middle
Great post!
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Old 02-09-2013, 12:46 PM   #10
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Siphoning sink


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this will help explain it, yours is the bottom drawing- you want the configuration in the middle
Thanks ePlumber. That is the original (1954) 2" cast iron pipe - I used a threaded abs connector and a 45 to go over to the trap (pic 1). Guess that was allowed back in the day, because they have the same setup for the downstairs apartment on the other side of the 4plex

It would require removing the kitchen sink and cabinets and opening the wall to replace the wye with a santee and fernco fittings above and below. It would make sense to replace the bottom part of the cast iron pipe (pic 3) down to just above where it joins the remains of a crazy diy plumbing job by the previous homeowner in the basement below (pic 2).

Just to clarify, Figure A means a horizontal wye on its side with a 45 (is that 1/8?) and you cannot pipe straight into the wye, is that correct?
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