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Thread: Back to back bathrooms without a double fixture?

  1. #1
    DIY Junior Member junger's Avatar
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    Default Back to back bathrooms without a double fixture?

    I'm in the process of remodeling my bathrooms that are now back to back (originally had one) and I need some help. I'm under UPC. Originally, when I spoke with the inspector he recommended a double fixture tee for the toilets and then wet venting one tub on two lavs and then separately venting the other tub. I've read feedback that Terry and others aren't big fans of double fixture fittings and this leads to three vents (2", 2", 1.5"). Is there an easier way to plumb the bathrooms w/ two, 2" vents with 2 flat vents? I'm still learning so please bear with me. And, I've also been having trouble with the common wall and venting because there is a 2x8 joist directly underneath the wall. Is there a maximum vertical distance for a trap under a tub? See illustration #2. Even squeezing in the other 2" vents is going to be difficult because of this joist. I can move the joist slightly if there is a better design.

    Thanks for the help.

    EDIT: I'm under UPC in Idaho but they've amended 908 to add horizontal wet venting - "24. Section 908. Exception - Vertical Wet Venting. A horizontal wet vent may be created provided it is created in a vertical position and all other requirements of Section 908 of the UPC are met."

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    Last edited by junger; 05-20-2010 at 06:32 AM.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    The way you have it is the way I would install it. There is NOTHING wrong with a back to back fixture fitting, as long as you do not mean a sanitary cross or double combo. IF you put the toilet one low enough you can use a 3x2 or 3x1 1 1/2" street sanitary tee into the top of it, and run from that to the tub's "P" trap.

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
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    If you install any toilet with a 3" flush valve, Cadet 3, Champion, Toto Drake, Eljer Titan and some others, then the double fixture cross will allow the flush to skip over the fitting and force water out of the second bowl.

    Since hj doesn't install these kind of toilets, he hasn't experienced it yet.
    Since you are installing new pipes, you have a chance to do it better then code, and make sure you maintain water in the bowls after a flush.

    I would treat each toilet as a separate fixture, with it's own arm and vent.
    I doubt that the inspector is aware of the problem either.
    It's okay by code to double fixture them, but it doesn't work with all toilets.
    Even the manufacturers are changing the pdf spec files to mention this.

    The lav is fine with a double fixture fitting.

  4. #4
    DIY Member CharlieM's Avatar
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    I am facing a similar situation with back-back toilets that were built with a double-fixture fitting. We are installing two new toilets with 3" flush valves. There is not much room underneath for additional plumbing. An additional run as Terry describes would involve drilling new 3" holes through the joists below with concern for weakening joists in this area. Not sure if this sketch helps. Am curious what options for replumbing you might choose.


    Dumb question is why not use a double combination y-1/8 fitting which seems like it would be less likely to have blow-by issues.

    Thanks.
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    Last edited by CharlieM; 05-21-2010 at 08:00 AM.

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    Forum Admin, Expert Plumber Terry's Avatar
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    You can use the double wye fitting, and they would drop the connection down lower.
    Or you can use a wye and street 45 below in the wall below and come up twice, using a santee that is vented out the top.
    That forces the flush downward, and no chance to skip to the next arm and force water from the bowl.

    A double wye with 1/8" bends isn't going to clear the ceiling, so you will need to box down.
    I prefer to come up twice and not have to box out the framing.
    Spend a bit more on plastic fittings, and less on framing and drywall.
    My way is better anyway. By a country mile.
    If you're going high performance, then the pipes in the wall should be designed for that performance too.
    Or you can go ahead and use double fixture fittings, and live with lowered bowl levels for the duration of the time you live in the home.
    Your choice, they are both up to current, (without a clue) plumbing code.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Terry, you misquote me. I NEVER said a sanitary CROSS was appropriate, (nor do I approve of using a double combo), and I have never used one for back to back fixtures of any kind. A back to back fixture fitting is basically a double combo, modified so it does NOT create "S" traps at the fixtures. Instead of using 45/45 angles for the branches, it is more of a 60/30 branch.
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    Last edited by Terry; 05-21-2010 at 10:47 PM. Reason: added picture, is this the fitting?

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