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Thread: Minimum Trap Arm?

  1. #1
    DIY Senior Member dlarrivee's Avatar
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    Default Minimum Trap Arm?

    What would the minimum trap arm length be for a 2" trap from a washing machine drain going into a 2 inch santee?

    Is it alright to have the hubs of the santee and the trap connected with just enough pipe to connect them?

  2. #2
    Nuclear Engineer nukeman's Avatar
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    You need at least 2x pipe diameter in distance before the vent. At least that is what it is here. Otherwise, it would be considered a crown vent.

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    DIY Senior Member dlarrivee's Avatar
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    Quote Originally Posted by nukeman View Post
    You need at least 2x pipe diameter in distance before the vent. At least that is what it is here. Otherwise, it would be considered a crown vent.
    I figured there had to be a minimum of some sort, thanks for that.

  4. #4
    Nuclear Engineer nukeman's Avatar
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    No problem. Don't know how much it will help you, but the VA state codes are here:

    https://www2.iccsafe.org/states/Virg..._Frameset.html

  5. #5
    Moderator & Master Plumber hj's Avatar
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    There is NO WAY a trap line into a sanitary tee, no matter how short the branch is, could be considered a "crown vent".

  6. #6
    Nuclear Engineer nukeman's Avatar
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    That very well could be. I'm just reporting what the code says here:

    906.3 Crown vent. A vent shall not be installed within two pipe diameters of the trap weir

    No exceptions listed. It may be that the combination of the santee and trap makes it physically impossible to be within 2x the pipe diameter or you could argue that it really isn't a crown vent due to the drain being in that same location. I figure it is easy enough to just make it 2x minimum and then you are safe. You don't know how the inspector will interpret the codes, so it is best to be covered, IMO.

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    Moderator & Master Plumber hj's Avatar
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    Physically, the closest you could come to a crown vent would be a "P" trap into a san tee on its back and a street ell into that pointing downward. And even that would not qualify since the flow "upward" from the trap would not have a direct path up into the vent, the way it would with a "true" crown vent. The "P" trap outlet, and the sanitary tee inlet distance together are greater than the 2 pipe size dimension. But I did have an inspector who wanted a short piece of 3" cast iron between the tee and "P" trap for a mop sink basin. It made no difference to him that the trap wier was already 6" from the vent, and adding any pipe would either require moving the vent sideways or the basin would have to be away from the wall since the trap and tee was the exact dimension to put the opening at the center of the MSB.

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    If you take a vertical san tee and use a street ell and then the p trap it makes a beautiful crown vented s trap. Therefore the trap arm needs to be 2 X the pipe diameter

  9. #9
    Moderator & Master Plumber hj's Avatar
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    If I read you correctly, the you have to use a piece of pipe between the street ell and the trap's "U" bend, and that creates a "deep seal" trap which will usually get you a rejection slip. I don't know ANYONE who would install a trap by reversing the "elbow".

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    Nope, sorry. confusing. Take a san tee installed vertical with the vent off the top and drain off the bottom. Insert a street ell facing down and then two more to make the trap ( make your own trap ) Or, same thing only use a very short piece of pipe between the first elbow and the san tee. I see the short nipple thing on washer drains all the time and of course, reject them.

  11. #11
    DIY Senior Member dlarrivee's Avatar
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    I wont be doing any of these things, but I have an existing trap in my house without any trap arm unless you include the hubs of the two fittings and it gurgles a bit.

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