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Thread: Garbage disposal install in double sink w/dishwasher... why do I have two drain pipes

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    DIY Junior Member s_mack's Avatar
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    Default Garbage disposal install in double sink w/dishwasher... why do I have two drain pipes

    Hi. I found this forum via Google seeing others with similar questions but I've yet to see any reference what I see under my sink. This is a new construction home, not a flip or reno so it should be done "right" I would think (but who knows). I don't quite know what the PVC high loop on the left is and why I seem to have two drain pipes going through the floor. My guess is this is some way to address venting when the sink is not at the wall? Does that PVC function as the high loop for the d/w drain? If not, does the d/w drain tube look like its "high" enough?

    Anyway, I'd like to install a garbage disposal. I'd *prefer* it go on the left side of the double sink, but that loop is in the way. My only reasons for wanting it on the left side is that is the one I usually peel potatoes into, and also the garbage can is on the right side and it won't fit if the disposal is there. In other words... no big deal. It can install on the right if the left is too much trouble (or if there's some other reason why it should be on the right).

    I'd like to DIY this. I think I'm capable. I just want to make sure I've got the theory right.

    But first thing's first... what is that "extra" piping for... can I move it? Shorten it? I imagine there's a reason it goes right up to the counter.
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    Thanks.

    - Steven

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    The layout is correct. What you have is an "Island" vent for the kitchen. The vent goes up as high as possible and then drops back down and reconnects to the waste below. Before the connection of the two is made, they would have pulled off a foot vent that went up a different wall and up through the roof.

    Normally the waste from the santee that holds the p-trap would be 2" on the vertical.
    Yours looks to be all plumbed in 1-1/2" black ABS. I don't know how they size in Canada, so maybe that is all just fine.

    The drain from the dishwasher will high look to the counter before it enters the wye. That's normal too.
    Here in the States, we would either high loop it , or use an air gap fitting on the counter. Again, that's a local code thing.

  3. #3
    DIY Junior Member s_mack's Avatar
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    Thanks... so how/where would you install the garbage disposal? And can I move that vent loop to accomodate the disposal on the left side, or should I just install on the right?

  4. #4
    Test, Don't Guess! cacher_chick's Avatar
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    You would save yourself a ton of headaches by putting the garbage disposal on the right basin. There is no easy way to put in on the left side with the required venting in place.

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    Moderator & Master Plumber hj's Avatar
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    If you remove the "loop" you could install it properly. Installing it "as high as possible" means up behind the sink to the countertop, NOT even with the bottom, or in your case BELOW the bottom, of the sink. That is NOT the proper location for the dishwasher drain connection although it WILL work for the drain as shown, but will NOT be proper once you install the disposer.
    Licensed residential and commercial plumber

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    What he said

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