Can the J bend in a p-trap be installed "backwards"??? J-Bend Lo Inlet

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by zbdd2, Jul 9, 2009.

  1. zbdd2

    zbdd2 New Member

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    I am hooking up the drain lines under a relatively deep bowl sink. The installation includes a garbage disposal. In order to get the J-bend of the p-trap to meet the arm of the trap, I have to attach the long side of the J to the arm and the T that combines the disposer and the second bowl to the short side of the J. This is backwards to me but seems like it should work. Will it? Am I creating other problems?
     
  2. Terry Love

    Terry Love Plumber

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    It will work, it can raise the water level in the disposer though.

    Many kitchen remodels should have the drain at the wall lowered 3" if you are using a deep bowl kitchen sink.

    The last kitchen remodel I worked on, the contractor was going to just flip the p-trap.
    It would have left some water in the disposer all the time, if I hadn't caught it for the homeowner before the cabinets were installed.

    [​IMG]

    [​IMG]
     
    Last edited by a moderator: Apr 6, 2013
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  4. zbdd2

    zbdd2 New Member

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    Lowering the rough in would have been the best thing to do but we had no idea what kitchen sink we would be using when that part of the project was done and the difference in the rough in never occurred to me. Disposer installation is new to me as well as how those affects things. The house was all but gutted and all of the galvanized supply and all of the cast iron dwv lines except for one run in a concrete slab were replaced. This is the last fixture that I have to finish the plumbing on and I feel fortunate that this is the biggest issue that I have come across. Live and learn.....

    Thanks for the info........
     
    Last edited: Jul 9, 2009
  5. hj

    hj Moderator & Master Plumber Staff Member

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    trap

    Doing it will not change the level of the water in the disposer, as long as the pipe in the wall is not above the disposer's outlet. IF it is you MUST lower the pipe or you will have constant problems, not the least of which will be premature rusting of the disposer. One problem you will have is that the "outlet" of the trap has a very shallow insertion for the tailpiece so it will fall apart very easily if bumped or vibrated.
     
  6. Redwood

    Redwood Master Plumber

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    Even Watts made this mistake on their website product pictures... :eek:


    [​IMG]

    Problem is like HJ said they fall apart easy and leak...
     
    Last edited by a moderator: Apr 30, 2014
  7. Terry

    Terry Administrator Staff Member

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    I've never been able to get the plastic traps to stay together when reversed.
    I can sort of make it work with the chromed traps.

    But me, if I know there is going to be new cabinets, the first thing I check is draing height. Almost all new remodels are going with deep sinks.
    And that means lowering the drain while you have access to to the wall behind the cabinets.
     
  8. Terry

    Terry Administrator Staff Member

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    They do make a low-inlet J bend

    [​IMG]

    A standard trap with a bowl that is too low for the trap arm.


    [​IMG]

    J Bend with low inlet
    http://www.keeneymfg.com/

    [​IMG]

    J bend with low inlet, installed.
    They make these for 1-1/4" and for 1-1/2"

    [​IMG]

    Putting a granite counter top on a 30" cabinet and undermountng a Kohler Caxton bowl pushed the drain a bit lower. The lo-inlet J-bend fixed that.
     
    Last edited: Apr 6, 2013
  9. Terry

    Terry Administrator Staff Member

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    Here is a reversible p-trap that works!

    [​IMG]

    Dearborn makes a p-trap with a J Bend that can be reversed.

    Dearborn Brass P9703W
     
    Last edited: Apr 30, 2014
  10. sydflash

    sydflash New Member

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    This thread was helpful, but I want to make sure that I understand.

    There is no harm in reversing the "J" bend for a "P" trap, correct?

    I am trying to install an offset in my kitchen sink drain to prevent the dishwasher and the waste water from the R.O. filter from audibly trickling into the trapped water.

    This is what I have roughed in:

    IMG_0015.jpg IMG_0016.jpg

    I think what worries me is that the lower nut on the "J" will always have water above it.
     
  11. Reach4

    Reach4 Well-Known Member

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    sydflash, you have a couple of problems. One is that you have what is called an "S trap". I suggest you search out that term.
     
  12. Terry

    Terry Administrator Staff Member

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    Sydflash,
    You can install a santee with an AAV for venting there, and air straight at the drain. That would fix the p-trap situation too.
     
  13. sydflash

    sydflash New Member

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    New Hampshire
    It's actually an unusual set up with a wye in the basement:

    IMG_0017.jpg
    No idea why they did it this way, but the branch to the left is a dedicated vent for the sink, with the other branch being the drain. The vent is only a vent and no other fixture uses it as a drain. You can't see it but the branch to the left goes to the wall in that bay and then up.

    I wouldn't have done it this way myself, but I think this is a proper vent. The sink never gurgles even if I fill it with water.

    TAt one time it exited the roof on its own, but it was the front of the house. So, when I added an addition, I tied it into the stack near the roof in the back.

    Will I still have the syphoning problem of "S" Traps with this vent?
     
    Last edited: Dec 9, 2015
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The problem with an S-trap is that they can drain TOO well and won't leave enough water in the trap to maintain the seal all of the time. A proper vent must come off of the trap arm before it starts to go down and within specified distances based on the pipe diameter used. This will cause the siphon to break before the trap can be evacuated enough to cause problems.
     
  15. sydflash

    sydflash New Member

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    Location:
    New Hampshire
    Also, it's probably not clear in the pic that there is six inches of pipe between the bends before it goes through the floor - the part with the purple cement on it. Is 2 x the diameter of the pipe still the standard?
     
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