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Thread: Toilet bowl empties when flushing another (back-to-back) toilet

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    DIY Junior Member Emily Paxon's Avatar
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    Default Toilet bowl empties when flushing another (back-to-back) toilet

    I have replaced to old model toilets with Kohler Cimarron 1.28 gpf using the existing plumbing. The toilets are installed back-to-back. When I flush one toilet, water is sucked out of the other toilet bowl. I have a 2" vent and appears to be clear. I'm wondering if something different needs to be done with the actual plumbing drain lines to accomodate these newer toilets. We did replace the copper with PVC but the configuration stayed the same.

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    How do you know without a doubt the vent is clear? Also, did this happen with the old toilets and/or, did you ever see the water in the bowl move when someone flushed the other toilet?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Hopefully, you got some money back when you recycled the copper!

    The newer toilets flush faster than the old ones, and require a proper fitting in the wall. It must be a double combination wye, NOT a double sanitary T or sanitary cross. Otherwise, flushing one can shoot to the other side and create problems as you've discovered. A wye turns the waste at a 45-degree angle down, whereas the sanitary cross, you can basically see from one side to the other. The wye directs the water down with no chance of it shooting to the other side.

    Last edited by Terry; 05-07-2012 at 04:51 PM. Reason: added picture
    Jim DeBruycker
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    Forum Admin, Expert Plumber Terry's Avatar
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    The water sill skip across and creates a wave of air that forces the water higher and then as it recedes, goes down the drain. The newer toilets with the large flush valves can do this on some back to back fittings.

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    Quote Originally Posted by jadnashua View Post
    Hopefully, you got some money back when you recycled the copper!

    The newer toilets flush faster than the old ones, and require a proper fitting in the wall. It must be a double combination wye, NOT a double sanitary T or sanitary cross. Otherwise, flushing one can shoot to the other side and create problems as you've discovered. A wye turns the waste at a 45-degree angle down, whereas the sanitary cross, you can basically see from one side to the other. The wye directs the water down with no chance of it shooting to the other side.

    Jadnashua,

    Are you saying that what you described can cause siphoning because that is his problem?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If vented properly, no, it isn't siphoning, but if you 'rock' the boat, some will end up going down the drain as the water shoots across, sloshing the water back and forth. The wye's shape prevents it from shooting across, and means you could actually run a snake down there if you wanted to (otherwise, it would likely just go out the other toilet).
    Jim DeBruycker
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    DIY Junior Member Emily Paxon's Avatar
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    Thanks. This may be the problem. Just looked at the plumbing and it appears I have the tee/cross rather than the Wye. The toilets were installed just about a week ago and the vent was not plugged at that time. So, I'll check out this solution.

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    DIY Junior Member Emily Paxon's Avatar
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    In your opinion, will this problem be resolved by switching from the tee/cross(which I have installed) to the Wye?

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    DIY Junior Member Emily Paxon's Avatar
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    Quote Originally Posted by jadnashua View Post
    Hopefully, you got some money back when you recycled the copper!

    The newer toilets flush faster than the old ones, and require a proper fitting in the wall. It must be a double combination wye, NOT a double sanitary T or sanitary cross. Otherwise, flushing one can shoot to the other side and create problems as you've discovered. A wye turns the waste at a 45-degree angle down, whereas the sanitary cross, you can basically see from one side to the other. The wye directs the water down with no chance of it shooting to the other side.

    New to this forum and not exactly sure how to post a reply! I do have the tee/cross and not the wye. Hopefully by changing this the problem will be resolved. Thanks!

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    DIY Junior Member Emily Paxon's Avatar
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    Quote Originally Posted by Terry View Post
    The water sill skip across and creates a wave of air that forces the water higher and then as it recedes, goes down the drain. The newer toilets with the large flush valves can do this on some back to back fittings.
    Do you think replacing the tee with the Wye will solve the problem?

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    Moderator & Master Plumber hj's Avatar
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    The "double combination Y-1/8 bend" is NOT an approved fitting for back to back installations either. Its construction, since the connections are at 45 degrees, creates TWO "3/4 S traps". The proper fitting is a "back to back fixture fitting" which is an amalgum of 30/60 bends raising the center lines to the point that "S" traps are not created. AND, it is shorter "face to face" so it fits into the installation better.
    Licensed residential and commercial plumber

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    I guess the big question here is, is the water in the bowl siphoning or is it rocking?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by Cookie View Post
    I guess the big question here is, is the water in the bowl siphoning or is it rocking?
    Rock a cup of water, and some eventually will overflow...you'll end up with less water there. Direct a jet of water there from a hose (pipe) and most of it may end up down the drain.
    Jim DeBruycker
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    DIY Junior Member ramsey76's Avatar
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    Default TOTO back to back install

    I went to purchase a Toto Clayton today from a local dealer and I ran into an issue at the end of the sales process.

    The sales rep / consultant asked me if the new Toto toilet is replacing an existing unit that is back to back with another toilet


    I said yes, and this my be a problem because of the G max feature of the Toto. The concern is that the Toto may draw water from the other toilet every time the Toto is flushed if a Tee connection was used to connect the two toilets in the intial installation.

    ( the house was bulilt in 1990 in and is in Massachusetts)


    My understanding is that a double Wye connection is ok


    Is there a way to get some insight into what type of connection we have with out ripping open a tile floor or gambling on buying a toilet ?



    Thank

    T
    Last edited by Terry; 05-25-2012 at 11:15 AM. Reason: spell check

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    Forum Admin, Expert Plumber Terry's Avatar
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    You can pull one toilet, and then flush the other one. If you see water skipping across and coming up the other arm, then your fitting is allowing water across.
    I have something similar in my home, and yes, it will lower the water in the bowl some. It's not a big deal.

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