3" drain for kitchen sink?!

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by gdog, Jul 3, 2021.

  1. gdog

    gdog write diagnostic firmware for embedded industrial

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    Haven't been around here in awhile, but know this is a great forum for plumbing advice, and now I could use some please, so TIA.. :rolleyes:

    Recently bought new (to us) house and remodel is in process. Tearing out old kitchen cabinets and saw something under the kitchen sink cabinet I've never seen before, and I've seen quite a few older and newer houses in my time. BTW this house was built new about 20-25 years ago.

    Typically I see 2" drain for kitchen sink coming up through the wall and then connecting to 1 to 2 sink drains, but this place has a 3" drain pipe coming up from basement, through the floor, and bottom of cabinet to serve two drains. WTH??! :eek:

    Plan to replace it with 2", but has anyone ever seen this before? Will try to post a pic

    [​IMG]
     
    Last edited: Jul 3, 2021
  2. wwhitney

    wwhitney Well-Known Member

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  4. gdog

    gdog write diagnostic firmware for embedded industrial

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    Thanks for the quick reply and link Wayne, but I wasn't too concerned about it being up to code.

    It's just I've never seen a 3" drain inside the kitchen sink cabinet before; has anyone come across this? It takes up a lot of the cabinet space! And note the 3" clean-out in there to boot! There's another clean-out on same line in the basement. Oh, and it's vented in the basement. Will be a couple of days before I can get a pic of that up.

    Will try to make it look more like some here: https://terrylove.com/forums/index.php?threads/install-garbage-disposal-in-double-sink.1736/
     
  5. Reach4

    Reach4 Well-Known Member

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    I forget what that is called, but some code allow that with the big pipe because it is so big, it is not going to get filled with water, and so siphoning like in a regular S-trap won't happen. So from a functional point of view, I think it should be good.
     
  6. Terry

    Terry Administrator Staff Member

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    The trap to the disposer should have gone straight over, not dropped down like that.

    [​IMG]

    The 3" is their way of creating an Island Vent.
     
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  7. Jeff H Young

    Jeff H Young In the Trades

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    the 3 inch pipe is fine its everything else thats wrong. vent comes off the top of the santee for a sink, the wye going to disposal is wrong. but the good news is it can be easily fixed and a cheater vent (aav) can go in eliminating any need to do it right. Whoever plumbed it just didnt know how not that big a deal, but kind of ticks me off that kind of slop just gets dumped in an unsuspecting homeowners lap. Good thing your handy and get this all working properly
     
  8. gdog

    gdog write diagnostic firmware for embedded industrial

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    Thanks for all the great input!

    Here's what it looks like under the kitchen floor:

    [​IMG]

    Note the vent pipe on the right.

    Do I really have to use a cheater vent under the sink? Are those things reliable (never used one)?

    Dumb question: Why can't I open up the wall behind the sink cabinet (all the cabinets are out now; new ones on order..) and hook up a new 2" drain from sink(s) to existing vent? Like Terry's illustration? Then I could get rid of the whole horizontal 3" leg above.
     
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  9. Jeff H Young

    Jeff H Young In the Trades

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    sure cut all that out and run 2 inch that's how I would do it. that 1-1/2" vent serves no purpose now , but go ahead and use the section above as a vent for kitchen sink
     
  10. Reach4

    Reach4 Well-Known Member

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    OK... that's what it's called.

    https://up.codes/viewer/michigan/mi-plumbing-code-2015/chapter/9/vents#915 says
    915.1 says
    combination waste and vent system shall not serve fixtures other than floor drains, sinks, lavatories and drinking fountains. Combination waste and vent systems shall not receive the discharge from a food waste disposer or clinical sink.
    So the disposal makes that not allowed.
     
  11. wwhitney

    wwhitney Well-Known Member

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    Yes, that's why the part of my post that you trimmed from your quote reads "Were it not for the garbage disposal."

    Cheers, Wayne
     
  12. jadziedzic

    jadziedzic Active Member

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    The 2018 IRC was revised to allow a disposal on a combination waste & vent system, and it looks like the 2021 IPC was as well. And of course, local codes (or local AHJ) may allow/disallow as they see fit.
     
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  13. Jeff H Young

    Jeff H Young In the Trades

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    differant codes there it sure looks hack from here. combination waste and vent Ive never seen like that. Iguess s traps and a vent 5 foot below flood level is the local way . sink likely under a window but seems a horrible way to plumb to me. dont know if that install was motivated by code
     
  14. Reach4

    Reach4 Well-Known Member

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    With that big diameter, and that vent connecting below, there is no way that 3 inch column could fill with water assuming the drain pipe below was not blocked. So no siphoning as could occur as could happen with a regular S-trap. I like it.

    But then I would like the drum trap in theory for some things like softener drains despite not being able to snake them.
     
  15. Jeff H Young

    Jeff H Young In the Trades

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    . yea it probebly is ok with no vent or having a vent that offsets horizontally 5 foot below flood level. There are a lot of ways to do things that work but waste material , look horrible and dont meet code. I would definately rip it out if remodeling kitchen not a second thought . but many homeowners probebly would leave it.
     
  16. Reach4

    Reach4 Well-Known Member

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  17. Jeff H Young

    Jeff H Young In the Trades

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    I probably pick at it too much . But don't see it as a professional install. of course different regions what's normal isn't somewhere else. I thought that kind of work and s traps went away a long time ago
     
  18. gdog

    gdog write diagnostic firmware for embedded industrial

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    As Jeff mentioned, drain is under a window, and unfortunately, this is an exterior wall, and they're only 2x4 thick walls. You'd think in this day and age, 2x6 walls would be standard?

    I.e. I'm leery of drilling a 2" horizontal hole (1.5" vent pipe has 1.9" OD or ~ 2") in 3 or 4 wall studs to attach drain to existing 1.5" ID vent pipe to right of window.

    Could I go with smaller diameter vent pipe off of the drain?
     
  19. Jeff H Young

    Jeff H Young In the Trades

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    plumb homes every day in my construction years with a window over this without s traps and all that. I've never seen anything like it and wouldn't look into it in my code, If I was in your area and customer was replacing cabinet and wanted me to do the sink Id ask what they want done . advise that it could be reworked. I could leave it alone if they weren't paying me, but prefer a standard rough in
     
  20. gdog

    gdog write diagnostic firmware for embedded industrial

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    Just to follow up, ended up going with 1.5" drain since that what the vent was. Some pics; yeah, not pretty but think got the job done:

    Before:

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]


    After:

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    Yeah, a bit messy with the primer/glue and vent slope was more than I had planned (looks worse in this pic than it is), but think it's functional. Pretty obvious I don't do this for a living!

    Main thing is I can now patch the giant hole in the floor where the original drain pipe was! :D

    Thanks again for everyone's input!!
     
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  21. Jeff H Young

    Jeff H Young In the Trades

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    gotter done < you'll like having space under cabinet
     
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