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Thread: Double vanity plumbing options

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  1. #1
    DIY Junior Member nawyner's Avatar
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    Default Double vanity plumbing options

    Hello everyone, I'm sorry for posting another double vanity question, but this forum truly is the best place to go for all things plumbing related and as I am not a plumber this was the best place to go. I have been able to find answers to all of my questions so far…which leads me to this one…which happens to be my first post. I think I have it figured out but I would like to make sure I am thinking about it correctly. I live in Georgia and our plumbing code our county uses is the International Plumbing Code. I am doing a guest bathroom remodel on a house built in 1970 which originally had one vanity initially installed in the guest bathroom, but it was also plumbed with the sink from the master (on the left side, other side of the wall). I want to convert this right side single sink set up to a dual vanity set up. The total number of fixtures on this 3 in vent line are at most three (there is on downstairs sink that may be vented) and once the new sink added would be four.

    The way I see it there are three options,

    Option 1: just hook up the drain from the new sink to the existing drain line. (I think this is incorrect b/c of siphoning issues with the other sink, but since this would be the easiest option I wanted to see if I was missing something.)

    Option 2: Hook up the drain from the new sink to the existing drain line and then run a 1 ½ vent line straight up through the roof.

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    Option 3: Hook up the drain from the new sink to the existing drain line and then connect into the stack (see picture).

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    I think option 2 and/or 3 would meet code but I’m not sure, also any other ideas/options would be very helpful. Thanks again

    Jason

  2. #2
    Moderator & Master Plumber hj's Avatar
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    None of your options meet code, and ALL could cause siphoning of one sink or the other. #2 and #3 are exactly the same and could siphon the left hand sink because of water flowing past its connection without a vent. To do it correctly, you would have to insert a new tee lower than the existing one, then either pipe both sinks to it, using a back to back fixture fitting, or the new sink into it using the piping design from either 2 or 3 with a tee in the vertical riser.
    Licensed residential and commercial plumber

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    You can't run waste past another trap without it being separately vented...once a drain, always a drain. Once a vent, always a vent, you can't mix them together.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Junior Member nawyner's Avatar
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    Thanks for the advice so far, I really appreciate it. I've redone some of the drawings so please check them out. The first two correspond to the two option of what I think HJ was describing above. The third option was to raise both sink drains and tie back into existing drain. The fourth option ties into existing drain but inserts a vent in between the two sinks. Do these options meet IP code? Also am considering redoing the entire cast iron stack upward w/ pvc since it would be relative easy to do. Final question, when attaching pvc to existing cast iron on a vertical run such as this are the shielded fernco's (the one's completely encased metal) the best way to attach the pvc to cast iron? If so where would be the optimal place to cut and attach.

    Thanks again,
    Jason

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  5. #5
    Test, Don't Guess! cacher_chick's Avatar
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    The 2nd and 3rd pictures both work.
    Which one will work for you will depend on how high the trap arm coming off the sink will be.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Placement and design is a dance...some of it depends on the height and configuration of the vanity (drawers, shelves, etc.) AND the depth of the sink. Then, you have to work around existing plumbing. Since you have to put the trap under the sink, and the trap arm needs to go into the drain without going up, there will be some height that becomes too high. Some people are using kitchen cabinet height vanities (nominally 36") verses the old standard of 30" or so - that can make a big difference on where your trap arm can go.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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