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Thread: trap arm question, UPC

  1. #1
    Writing, constructionDIY Member Yersmay's Avatar
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    Default trap arm question, UPC

    In a recent thread concerning a washer stand pipe, the length of the trap arm was briefly discussed. One plumber stated that if the trap arm was too short (less than twice the diameter of the trap arm) then an 'S' trap would result. Another expert said that a trap arm could be as short as you need it to be as long as the trap is vented. I'm hoping this can be discussed a bit more. As a plumbing enthusiast, I've always been mindful to keep the length of the trap arm twice its diameter because I thought the weir needed some amount of air to prevent siphoning. However, it stands to reason that if a trap is vented, the length (or lack of it) of the trap arm becomes irrelevant. Or does it? Could the experts re-visit this to further illuminate the requirements? I'd like to understand this better. Thanks in advance.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    IF the arm slopes enough so the connection to the vertical pipe is below the weir of the trap, it WILL create a partial "S" trap, but whether it automatically causes siphonage depends on whether the flow is adequate to fill the pipe. The minimum length requirement has NOTHING to do with it becoming an "S" trap, and I have NEVER had an inspector fail an installation because the trap was "fitting to fitting" with the tee. The length of the tee's branch, plus the distance from the end of the bell to the downward facing elbow, is almost the 4" distance anyway, regardless of how long the pipe piece is.

  3. #3
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    The minimum length on a trap arm isn't to reduce s-traps from being installed it's to give the trap some space from the venting so that the air moving through the vent doesn't cause the trap to evaporate too quickly.

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    DIY Senior Member Hairyhosebib's Avatar
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    Are running traps not legal in Arizona?

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    Plumber Winslow's Avatar
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    A trap arm that is less than 2x the diameter of the arm is considered a crown vented trap, which is illegal.

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    Moderator & Master Plumber hj's Avatar
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    Why would anyone INTENTIONALLY install a running trap?

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    DIY Senior Member Hairyhosebib's Avatar
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    I would be using it outside for my swimming pool drain. My kitchen sink is at the corner of my house and just outside is a 2" clean out. Just around the corner is my pool filter and pump. I put a 2" trap with a short length of pipe and then a 2X3 coupling with about a 2' length of 3" pipe. I then ran 2" pipe from my pool valve to the pipe. I cut a piece of pipe at an angle and have airgap space between the top of the stack and the bottom of the pipe. It's just that when I backwash the pool the stack can't take all the water the pump puts out. I just thought a running trap would give smoother flow but I will increase another coupling that is 4 or 6 X 3" and just make it taller. I have a salt chlorinator on my pool so all I am putting down the drain is slightly salty dirty water.

  8. #8
    Moderator & Master Plumber hj's Avatar
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    A running trap creates MORE restriction because it has additional bends. Your cleanout is probably 2", and that is why the pump discharge overflows, NOT because your "funnel" is too small. Making it larger and/or longer will just mean it takes a few seconds more before it overflows.

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    Plumber Winslow's Avatar
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    deleted, duplicate thread
    Last edited by Winslow; 07-13-2010 at 01:03 AM.

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