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Thread: Venting Screw-Up - below P trap

  1. #1
    DIY Senior Member chefwong's Avatar
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    Default Venting Screw-Up - below P trap

    I think my *GC* has screwed up. The wall where my sink is is only 3/4" in depth as it's up against my brick wall.

    It was my understanding that the vent must ALWAYS be above the P trap.
    Please see the pic attached. It is basically the underside of the kitchen from below - in the basement. The black pipe on the right is the DWV that is underneath the flooring and goes to the flooring where the sink cab is. However, the vent for the sink is tapped BELOW the P trap. The white APS pipe you see is the vent that the GC did .

    Can anyone confirm is this is fine or WRONG ?


  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    Yes, it's wrong.

    If the black pipe goes up to the p-trap,
    and the white pipe is the vent, then you have an illegal S trap.

  3. #3
    Plumber Cass's Avatar
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    Looks like he just took a stab at what he thought would work. If it is as Terry said it is wrong.

  4. #4
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    I have something similiar on my shower. Is it wrong too?

    Pipe goes from floor drain to trap, and then tees off about 1ft downstream to go up to the vent. Then the drain continues on to the stack. Works fine. Is it wrong?

    Jason

  5. #5

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    Jason-
    I don't believe yr shower is in error.
    As long as the vent is above the trap weir (top of the trap) it's not an s condition. In yr shower, the ptrap is below the floor, so the vent can be below the floor too. In a sink, the trap is up about 18" above floor, so the vent has to take off from the drain before it falls below that. This means sink vents should be in the wall directly behind the sink. if the GC made the wall too thin, then he wouldn't have had room to do it right.

    Looks like the vent runs up parallel to the drain in the neighboring cabinet. Can you tap into that vertical piece through the cab wall out of yr p-trap? That'd solve it. Even if that portion of the pipe had to be resized it'd be no big deal (looks like 1.5", though, so shld be ok).
    Last edited by prashster; 06-15-2006 at 09:09 AM.
    (important note: I'm not a pro)

  6. #6
    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Ah, ok I follow you. Thx
    Jason

  7. #7
    DIY Senior Member chris8796's Avatar
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    Default

    While a little odd, I think your ok. Because the vertical drain line is 2" and your P-trap should be 1 1/2", so you don't have a true s-trap. Since the drain is larger than the trap, it can't siphon the p-trap. There maybe a minor code violation in that the vertical riser and vent, don't look to be 4x the drain diameter apart on the horizontal run. But, I wouldn't worry about it.

    Good luck

  8. #8

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    Huh? Please elaborate. I don't see how xitioning to a larger diameter drain avoids siphoning.
    (important note: I'm not a pro)

  9. #9
    DIY Senior Member chris8796's Avatar
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    Technically, this is wet venting with an auxillary vent.

    In simplier terms, you can do this if you upsize the vertical drain one size up from the horizontal p-trap arm (above the level of the trap). By doing this you prevent the p-trap from being siphoned, since a 1 1/2" horizontal drain with never be able to saturate the 2" vertical drain.

  10. #10
    In the Trades Bob NH's Avatar
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    Default Air Admittance Valve?

    Is there an Air Admittance Valve at a tee where the P-trap goes into the vertical? That would take care of any siphoning.

  11. #11
    DIY Senior Member chefwong's Avatar
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    The vent is not parallel to the sink cabinet. The DW on the right of the vent is actually making a 90 degree corner connection if you look closely. The sink base is about 6 feet away from this corner.

    The main vent stack is the one just right to the vertical vent you see in the pic.
    |
    2" | 1.5"
    -------------------
    |
    |
    |1.5"
    |
    |



    This is the basement vent stack. It is 1.5" coming out of the cleanout hub, vertical pipe is 1.5", then it runs horizontal at 2", and then 1.5" going up from the basement ceiling all the way up to the 2nd floor.

    The ABS pipe you see goes up into the 1st floor and taps into the vent stack going vertical. I do plan to open it up, and fix the problem before the cabinet guys come in.

    So what are my options on fixing-making this work properly

    - Remove the wye - and *cap-close* off the vent how it is now and install a Studor AA valve in the sink base

    - Leave as is and install a AA valve regardless

    - Any ideas/suggestions.

  12. #12
    DIY Senior Member chris8796's Avatar
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    What does the inspector say? It seems if the job is big enough for a GC, it should also have a permit.

    Here is some more reading for you:

    The IRC also allows three other solutions to the island sink problem, including a combination waste and vent, a vertical leg waste fixture drain, and air admittance valves. A combination waste & vent essentially has no vent pipe, and instead has an oversized drain. The idea is that the trap could never fill the drain completely, and the remaining air in the pipe is sufficient to prevent siphoning of the trap. The vertical length of a combination waste & vent is limited to 3 feet, and the horizontal portion may slope no more than inch per foot. The UPC also recognizes combination waste and vent systems, but restricts their use to commercial applications and requires specific approval from the administrative authority. A "vertical leg waste fixture drain" is superficially similar to an "S" trap, except that it does have a horizontal section of arm, then an upsized vertical drop and an upsized horizontal section connecting to an actual drain and vent. The total horizontal length of this system must not exceed the maximum for a trap arm, so the practical application might be for a peninsular sink rather than an island.

    (copied from: http://www.abhomeinspections.com/PlumbingSystems.html)

    It looks like you currently have a a vertical leg waste fixture drain. Whether it is code or not depends on the jursidiction. Each of these methods of venting has pros and cons. I personally don't see a problem with their solution. If you are still worried try one of the other methods, but all three will work.

    Good luck

  13. #13
    Homeowner geniescience's Avatar
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    Default One more photo, please

    wong, you got one picture up. How about another one?

    I have come back to this thread ten times, and tried on different days to see what you have in the top right corner.

    I can't.

    I can see clearly
    a.) white 1.5" going straight up
    b.) black 2" and its two fittings

    Beyond that, I can't be sure what you have. One more picture probably would help everyone. It IS important to see what there is and how it is connected.


    Thank you for the description above. I may be able to figure out what you have based on that alone. But I didn't succeed in my first reading.

    David

  14. #14
    DIY Senior Member chefwong's Avatar
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    I was planning to take some pics and post up

    In the 1st pic, it is basically the branch right before it goes into the basement floor. The main waste stack is in the rear, and in front of it is the vent stack. To the side, you can see where the new waste for the kitchen ties in.

    On the 2nd and 3rd pics, hopefully it give you guys a better perspective on how things are. The vent goes up vertical and is 1.5". On the horizontal , it is 2" pipe that *T's* out to a 1.5". For those that may get confused, the vent stack is T as the other other is going into a soffit that ties into the vent for the basement sink, bathroom, etc.

    The kitchen is on the opposite end of this.

    X - vent stack going vertical

    -------------------------------
    |
    |
    |
    |
    |
    |x - Kitchen Drain
    |
    |


    Keep those responses/advice coming fellas. I don't mind a little gruntwork on my end to get things right !




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