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Thread: The NSPC permits s-traps?

  1. #1
    DIY Senior Member Noth Jersey's Avatar
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    Default The NSPC permits s-traps?

    Am I mistaken, or does the following language permit the use of an s-trap?:

    12.8.1 Venting of Fixture Drains
    Fixture drains shall have a vent so located that the vent connects above the top weir of the trap and the
    developed length of the trap arm is within the limits set forth in Table 12.8.1.

    EXCEPTIONS:
    (1) Water closets and similar siphonic fixtures.
    (2) Combination waste and vent systems. (see Section 12.17)
    (3) Vents may be connected below the top weir of the fixture trap if the following conditions are met:
    a) The vertical section of the drain pipe shall be at least one pipe size larger than the trap inlet size.
    b) The horizontal pipe connected to the trap outlet shall be at least two pipe diameters long.
    c) The developed length of the trap arm shall not exceed the values in Table 12.8.1.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default

    It appears to be what it says, but I would question whether it is what its intent is.

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    DIY Senior Member Noth Jersey's Avatar
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    What do you think they're going for? If I could drop below the floor before I vent, I could avoid butchering a kitchen cabinet and losing a bunch of storage space. I was thinking I could meet the exception by putting 2" traps on my sink and my disposal, since the inlets would be 1-1/2".

    On a different note, can I use the elbow that came with the disposal and put a p-trap on each fixtures, or should I make a straight shot to a sanitary tee connected to the sink tailpiece? I kind of got the impression from something Terry wrote that the disposal elbows should be avoided.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Most plumbers will use a disposer kit and one p-trap. The disposer kit comes straight off the disposer.
    We use the supplied 90 on single bowl sinks.

    I worked on a kitchen sink the other day that had three p-traps. The third trap for the dishwasher was so full of gunk that the dishwasher could no longer drain.
    I cut out the three traps, took out the supplied 90 and ran everything with one trap. Now she can use her dishwasher.

    I don't know why anyone would revert back to old style plumbing. There was a good reason we quit doing things that way.
    On the job above, one of the complaints was kitchen smell. Does a smelly kitchen sound good to anyone? Or is it just me?
    I like smell containment.

  5. #5
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by Noth Jersey View Post
    What do you think they're going for? If I could drop below the floor before I vent, I could avoid butchering a kitchen cabinet and losing a bunch of storage space. I was thinking I could meet the exception by putting 2" traps on my sink and my disposal, since the inlets would be 1-1/2".

    On a different note, can I use the elbow that came with the disposal and put a p-trap on each fixtures, or should I make a straight shot to a sanitary tee connected to the sink tailpiece? I kind of got the impression from something Terry wrote that the disposal elbows should be avoided.
    You don't need a 2" trap you need the VERTICAL section of the drain (IE after it connects to your vent) to be 1 size larger then your trap inlet size.
    Last edited by Doherty Plumbing; 06-30-2010 at 04:49 PM.

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    DIY Senior Member Noth Jersey's Avatar
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    Terry,

    I was contemplating individual p-traps based on this fellow's theory:

    http://www.nettally.com/palmk/GDplumbing.html

    What's wrong with his line of thinking? Does the baffle in the disposer kit constrict drainage that much?

  7. #7
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by Noth Jersey View Post
    Terry,

    I was contemplating individual p-traps based on this fellow's theory:

    http://www.nettally.com/palmk/GDplumbing.html

    What's wrong with his line of thinking? Does the baffle in the disposer kit constrict drainage that much?
    If you have two traps wouldn't you either need:

    A. Both traps protected by their own vet,
    B. Both traps connect to their shared vent via a double fitting so that each trap is connected to the vent symetrically?

    I would think with 2 traps tied togeather like that, and one of the traps constantly has rushing water going through it, that you'd tend to siphon the sink's trap dry when the dishwasher is draining.

    Up here a setup like that would be against code.

  8. #8
    DIY Senior Member Noth Jersey's Avatar
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    Quote Originally Posted by Doherty Plumbing View Post
    You don't need a 2" trap you need the VERTICAL section of the drain (IE after it connects to your vent) to be 1 size larger then your trap inlet size.
    Couldn't I just make everything downstream of the trap 2"?

    Whatever the case, does this code provision sound practical?

    Based on the exception I could run the drain down through the floor (long sweep, sanitary 1/8, sanitary 1/4?) and jog horizontally to the vent as long as:

    1) I size up the vertical drain one pipe size
    2) I make a 3-4" (twice the pipe diameter) horizontal run out of the trap outlet before turning down
    3) I keep the developed length (whatever that is) of the trap arm under 8 feet for a 2" drain and under 5 feet for a 1-1/2" drain (see table 12.8.1 in http://www.phccweb.org/files/Depts/T...stratedWeb.pdf)
    Last edited by Noth Jersey; 06-30-2010 at 06:09 PM.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Plumbers don't run two p-traps on the same trap arm.
    We wouldn't be able to sneak that by a plumbing inspector. And there is a good reason why.

    As a side note, if your disposer isn't grinding well enough to drain through a baffle tee, it's old and worn out. It's time to replace the disposer.

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    DIY Senior Member Noth Jersey's Avatar
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    I'll refer the author of that page to this thread. So, the idea is that water rushing through the wye from one fixture would siphon out the p-trap on the other, correct?

    Any thoughts on the s-trap exemption in the NSPC?

  11. #11
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by Noth Jersey View Post
    I'll refer the author of that page to this thread. So, the idea is that water rushing through the wye from one fixture would siphon out the p-trap on the other, correct?

    Any thoughts on the s-trap exemption in the NSPC?
    Correct.

    As to why they all s-traps who knows....

  12. #12
    DIY Senior Member Noth Jersey's Avatar
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    Going up a size and tying into the vent within about four feet should prevent of siphoning, right? Would you size it up just one size, or would you go to three inch on that vertical piece? Thank you for the invaluable information!
    Last edited by Noth Jersey; 06-30-2010 at 10:45 PM.

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    TROJAN WORLDWIDE SALES RP MACPLUMB 777's Avatar
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    The proper way to run pipe is 2" vertical with a 2"x11/2"x11/2" tee on top !

    That way you have 11/2" for trap arm and 11/2" vent up with a 2" drain out

    MACPLUMB 777

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  14. #14
    DIY Senior Member Noth Jersey's Avatar
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    The unfortunate framing of my kitchen area would end up killing a lot of cabinet space in a small kitchen (2x4 framing, sink under the window, rim joist close to the center of the bottom plate, difficult spacing of the stud bays). If I can design an S trap that won't break the p-trap seal under the worst conditions, we'll be in good shape. I'm sure a 6" drain would do the trick, but I'll bet the threshold at which siphoning takes place is at least a couple pipe sizes smaller. I'm counting on giving up most of the space in my sink base, so I don't mind running a large drain if necessary.

    My main question is whether or not an increase of one pipe size and proximity of the vent would provide a viable system. The code doesn't appear to present a problem in this case, but siphoning would be unacceptable.

    The reason for the 2" vent is to accommodate the washer and laundry tub in the basement.

    Update: I just got off the phone with the inspector. My interpretation of the code is correct except for the size of the trap. If I don't want to go up to 3" for the trap arm and the rest of the drain, I need a 1-1/2" trap, sizing up to 2" on the outlet. The 1-1/2" size trap means that I have five feet to hook into the vent. For anyone else in New Jersey or Maryland (the only two states that have adopted the NSPC), this information should come in handy. According to the inspector, a 2" trap arm/drain is sufficient to avoid siphoning on a 1-1/2" p-trap. Can anyone corroborate with a real-life anecdote?
    Last edited by Noth Jersey; 07-01-2010 at 07:13 AM.

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    Forum Admin, Expert Plumber Terry's Avatar
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    What you describe on the framing is normal framing. We manage to plumb correctly under those circumstances daily.
    A plumber would have it done in less time that it takes to type this. It's that easy.

    Do you want pipes coming up through the cabinet taking up space?
    It seems a shame to cut up a new cabinet for a pipe that shouldn't have been there.
    The finish carpenter is going to love you for making his job harder.

    It sounds like you have it handled with the inspector though.
    Last edited by Terry; 07-01-2010 at 08:22 AM.

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