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Thread: Single to Double Sink Vanity (again)

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    DIY Member SAS's Avatar
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    Default Single to Double Sink Vanity (again)

    First, let me apologize as I know that there are many threads on this subject already. But after reading them all, I am still confused. I have what I would have thought was a common and straightforward issue. The current plumbing is in the center of the wall and will need to be relocated about 15" to both the right and left. As usual the supply plumbing is easy, but the drain is confusing. Without opening the wall, is there a simple fitting that could be added to the current centered drain opening and then connected to the trap under each sink? Could you add a wye horizontally to the horizontal pipe coming out of the wall?

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    Forum Admin, Expert Plumber Terry's Avatar
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    NO. You can't just throw a second wye in.
    The point of plumbing, is to prevent trap siphoning.

    Of course if you don't care about smell, then it doesn't matter.
    Each trap needs to be vented.
    When you run waste paste a second sink, it pulls the water from that trap.
    When that happens, well I guess you know what it's like to live in Guatemala.
    You are now becoming your own third world country.

    Third World Plumbing

    It seems that people in developed nations have subsequently either forgotten about or never really been aware of the public health role of the plumbing industry.

    If it is accepted that the plumber is a health professional, then a comparison can be drawn with the education and training requirements of other health professionals. Few would claim that a doctor or nurse can acquire all of their necessary skills and knowledge in a few weeks. Nor is that the case for the plumbing health professional.


    Last edited by Terry; 06-16-2010 at 10:46 AM.

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    Homeowner geniescience's Avatar
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    Tubular Continuous Waste Center Outlet
    Tubular "Continuous Waste" Center

    These are key words to go web searching.

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    DIY Member SAS's Avatar
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    Quote Originally Posted by Terry View Post
    NO. You can't just throw a second wye in.
    The point of plumbing, is to prevent trap siphoning.

    Of course if you don't care about smell, then it doesn't matter.
    Each trap needs to be vented.
    When you run waste paste a second sink, it pulls the water from that trap.
    When that happens, well I guess you know what it's like to live in Guatemala.
    You are now becoming your own third world country.

    I'm not sure I see how the water from one sink would run past the other. If I took your picture and added a tee instead of the wye (sanitary cross?) and then added the Y horizontally, wouldn't that be the same as adding the wye as I suggested?
    Last edited by SAS; 06-16-2010 at 02:03 PM.

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    Moderator & Master Plumber hj's Avatar
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    Right now you have absolutely no idea HOW the pipes are arranged in the wall. You have a very good chance of having the drain pipe go horizontally, rather than vertical and if that is the case you options become much more complicated. Usually two sinks are going to have drawers between them and if that is the case a "continuous waste" or a "second Y" are probably not feasible, but then they are not the ideal first choice in any case.

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    DIY Member SAS's Avatar
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    So if I understand you correctly, the only way to do this is to first open up the wall to see how the current drain is configured?

  7. #7

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    Plumbing for a double lav goes in the wall.
    That's what the pictures show. Have you ever seen it done any other way?
    I haven't.

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    DIY Member SAS's Avatar
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    Quote Originally Posted by Obama the Plumber View Post
    Plumbing for a double lav goes in the wall.
    That's what the pictures show. Have you ever seen it done any other way?
    I haven't.
    Not sure if you were trying to make a joke or just be confrontational. I know that the plumbing "goes in the wall" and obviously if there is a single sink there now, there is a waste pipe that goes into the wall. What I was asking was whether it was necessary to open the wall to determine where/how the plumbing is routed in the wall (as suggested by HJ), or whether I could simply use the existing waste pipe and hook the two sinks to that pipe. And if I can use that existing pipe, what is the best way to hook the two sinks together and up to that pipe?

    If you can help answering those questions, that would be great. If not, I'm not sure why you bothered posting a reply.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The difference is this: see the pipe coming out of the top of the wye? That is the vent. Because it is there, rather than combining the waste outside the wall with another wye (putting the vent further away), it breaks the siphoning effect and waste from one drain won't affect the other's seal in the p-trap. So, no, you can't do what you want and have a reliable result without looking into the wall.
    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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    Homeowner geniescience's Avatar
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    "Trap arm" is key here. Saying "the plumbing" is vague.

    I cannot follow what you mean in your original post.
    I cannot figure out whether or not Terry of "hj" are making sense either.
    Too many assumptions, to be certain.
    Too many vague terms.

    You wrote ".. after reading them all, I am still confused." But you didn't clarify if you were referring to your "plumbing " as being the part prior to the P trap, or the part that comes downstream of the P trap.

    "The current plumbing is in the center of the wall and will need to be relocated about 15" to both the right and left."


    It could do with some clarification. If you please !

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    Forum Admin, Expert Plumber Terry's Avatar
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    If you are asking how to plumb two lavs without siphoning the p-traps, then the answer is to pipe each lav with a separate p-trap and trap arm, each one vented in the wall.
    Does the wall need to be opened up? Yes.

    The method you have been asking about does not meet any plumbing code I know of.
    If you don't want answers from plumbers, then maybe you should have posted it at a true DIY site and a homeowner can answer your question with,

    "I work on computers, but sure, everything works."

    I'm pretty sure that in other fields, not everything works.
    I'm pretty sure that that they have classes and eduction for every job.
    Plumbing requires an 8,000 hour apprenticeship.
    We're trying to save your bacon with some quick answers, but you don't like hearing that the wall needs to be opened up.
    Plumbing is simple, right? That's why they require only 8000 hours, class lessons and then a State required test and licensing, followed up with plumbing permits and inspections.

    Right now, get on the phone and order a plumbing permit like the rest of us.
    Then you can ask your local plumbing inspector if what you are doing will pass "his" inspection.

    We give advice that will satisfy a plumbing inspector.
    If you don't give a rats ass about permits and inspections, then don't bother posting.

    There are other sites out that there will pat you on the back and tell you that everything works.
    Not everything will pass inspection, not everything will drain when you are done, not everything will prevent siphoning of the p-traps, but anything can be done.
    Not everthing works in real life though.

    I love it.

    And this is as easy as it gets.
    We even had a picture to show you.

    Last edited by Terry; 06-18-2010 at 02:00 PM.

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    Homeowner geniescience's Avatar
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    One p trap only, and a continuous waste center outlet. Without opening the wall, this is the "simple fitting that could be added to the current centered drain opening and then connected to the trap "

    SAS you wrote
    Without opening the wall, is there a simple fitting that could be added to the current centered drain opening and then connected to the trap under each sink?

    You meant to write:
    Without opening the wall, is there a simple fitting that could be added to the current centered drain opening and then connected to the trap
    NOT one trap under each sink.

    AM I right about this?

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    General Engineering Contractor ballvalve's Avatar
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    Around here, you can find the second sink hooked up to the original sink with some pool or vacuum hose and some clamps and a tee. Welcome to Afghanistan west, but it actually works. Easy to clean, just hook it back up to the shop vac.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Yes, I'm sure vacuum hose and kitchen sink fittings or rubber hoses will work.
    Keep in mind, that the more pipe above the p-trap there is, the more exposed pipe area creating bad smells.
    The whole point of good plumbing is to reduce smells in home. If you don't mind bad smell, or you keep a plug-in deodorizer in the bathroom then I guess you will be okay.
    A plumber would try to have as little pipe above the trap seal as possible. We have sensitive noses.

    I'm getting hungry just looking at this


    Looking inside a sink drain, you know this stuff smells good.


    Stuff we find inside of pipes
    Last edited by Terry; 06-20-2010 at 11:12 AM.

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    DIY Member SAS's Avatar
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    Quote Originally Posted by geniescience View Post
    One p trap only, and a continuous waste center outlet. Without opening the wall, this is the "simple fitting that could be added to the current centered drain opening and then connected to the trap "

    SAS you wrote
    Without opening the wall, is there a simple fitting that could be added to the current centered drain opening and then connected to the trap under each sink?

    You meant to write:
    Without opening the wall, is there a simple fitting that could be added to the current centered drain opening and then connected to the trap
    NOT one trap under each sink.

    AM I right about this?
    Yes, you are correct and I apologize for my lack of knowledge regarding plumbing terminology. I am not a plumber, but a DIY who is looking for some good advice. I have another bathroom vanity that was converted from single to double about 8 years ago. We used a local plumber. In that bathroom the drain opening was into the wall forming the right side of the vanity. The plumber added a wye horizontally and connected each sink (with its own P-trap) to the two ends of the wye fitting. We have never had any problems with the functions of the drains, nor have there ever been any odors emanating from the drains of either sink.

    In this case I am looking to do it myself and can see two alternatives. One would be to do the same thing as that plumber did 8 years ago; put a wye on the centered drain opening, put a trap under each sink and connect everything together. Another option would be to use either a center or end outlet continuous waste kit and only one trap. Despite all that I have heard on this site, I still don't see why one of those (or both) wouldn't meet my 3 criteria: it works, it's safe, it meets code.

    It works in my other bathroom. Hard to see what could be unsafe about it. And our town uses the National Standard Plumbing Code. I've attached the section on Traps. From what I can make out, the option of using a single trap should be within code. Separate traps are not required for lavatories which are adjacent to each other in the same room. I also couldn't find anything that said that my first option, using 2 traps and a wye, violated code (although it could be buried somewhere and I missed it or didn't understand it).

    So I'm still looking to hear why neither of these options is appropriate. I understand that you wouldn't build it this way if you were starting from scratch, but if there is an option that works, is safe, and meets code and that doesn't require taking the wall down, then that's the option I'm going for.

    If I'm missing something, then please tell me. And if one of my two suggested options is better than the other I'd like to know which is better and why.

    And thanks in advance for any assistance.
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