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Dishwasher plumbing connection

Brent_F | Posted in General Questions on

Just bought a dishwasher to install in a house that didn’t previously have one.  I know they typically attach to a garbage disposal but don’t have one of those either.  I’m curious if I can cut this horizontal line connecting the two sinks and tie in in here or do I need to take a different approach?  I know it has to be above the trap but not sure what options I have. Thanks

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  1. Expert Member


    Your best bet is to do a Google Image search for "Dishwasher connection". The diagrams are easier to follow that any description we could give here.

    I've never seen them on a horizontal pipe, only a vertical one.

  2. Expert Member
    Michael Maines | | #2

    I agree with Malcolm. You will need to drop your trap lower, with longer tailpieces. One tailpiece should have a T where you connect the dishwasher drain. The drain needs to be above the P-trap.

  3. Andrew_C | | #3

    RE plumbing a dishwasher -
    don't forget about venting requirements (ie, air gap vs high loop methods). Again, Google is your friend, "dishwasher venting requirements". Some communities require one method or the other, so it's worth checking on that. Using the high loop method avoids having to put a hole in your countertop.
    BTW, if you're capable of doing your own plumbing, putting in a garbage disposal is not the worst job, and a garbage disposal is not overly expensive and is very convenient, so you might do some planning for how you'd plumb that in along with the dishwasher. IMO.

  4. Patrick_OSullivan | | #4


    Just FYI... the current drain would not be code permitted anywhere I'm aware of anymore. The pipe going straight through the floor with an elbow taking the input of the P trap effectively creates an S trap which is prone to siphoning.

    If it works without siphoning, I wouldn't worry about it. If you ever hear gurgling noises coming from your sink that you can't account for, now you know why.

    The easiest fix would be to replace the elbow on top of the drain going through the floor with a sanitee taking the input from the P trap. The top of the sanitee would have a stub of pipe running as high as possible with an air admittance valve installed on top of it.

  5. Brent_F | | #5

    Thanks for the replies fellas. I have seen some images of people tying into the horizontal pipe but they are always the "what not to do photos." Patrick, thanks for updating me about the s-trap vs p-trap. There is no vent on this line, the sink is on its own line so to speak, it goes straight down through the floor and straight under the basement floor and then ties into my floor drains before it leaves the house. I have never heard it siphon or gurgling my guess is because there are no other drains on the line that would be pulling air out of the trap? Is that correct?? It appears there is enough excess pvc coming from the bottom of the cabinet that I should be able to just cut it and lower the entire assembly. This dishwasher retrofit is beginning to be more of an ordeal than I originally planned, but what DIY project isnt! haha

    1. Expert Member
      BILL WICHERS | | #6

      The air admittance valve Patrick mentioned is your easiest “fix” here, you have plenty of room to add one and its easy to do. In certain situations it’s possible to have a trap siphon down even without other drains on the same line. There is a fancy way of venting that looks like an upside down “U” sometimes used with kitchen islands, but that doesn’t appear to be what you have here.

      BTW, there are two kinds of washers available for the trap connection rings: the hard milky-colored ones and rubber ones, usually red. The rubber ones seal much better and are less prone to loosening and leaking on kitchen sinks where you sometimes pour hot and sometimes cold water down the drain. When you put your drains back together I’d make sure to use the better washers. The better washers have solved a lot of kitchen drain leak issues for me.


      1. Expert Member
        MALCOLM TAYLOR | | #8


        I prefer Loop and Chicago vents under island sinks. They take up bit of space, but don't rely on mechanical parts to work. They can either be attached to a remote vent, or if the pipe that they drain into is large enough, no vent is necessary.

        For the OP it's too late to add a Loop vent, so an AAV is probably the only option.

    2. Trevor_Lambert | | #7

      The siphon effect is created by the water going down the drain in question, not due to air entering from somewhere else. What might be the saving grace for this set up is the line for the left side provides a vent for the right, and vice versa. This is not the case if both sinks are filled, obviously. Since you are redoing it all anyone, now would be the time to install an AAV.

      Don't install a garbage disposal unit if you happen to be on a septic system.

      1. Expert Member
        MALCOLM TAYLOR | | #9

        "Don't install a garbage disposal unit if you happen to be on a septic system."

        I'd add: Don't install a garbage disposal unit on any house. All they do is masticate things that shouldn't be put down the drain, and add more solids for the municipality to extract and treat.

        1. Andrew_C | | #10

          @ Malcolm, r.e. garbage disposal installations in general -
          If you don't have composting provisions (apartment, no pick up, whatever reason), would it be better overall for the environment to put trimmings (mostly fruit and vegetables, say) into your garbage or your garburator? If they go into the garbage and the dump, I think they end up generating methane. Serious question, don't know answer, and apologize to original poster if this ends up hijacking the thread.
          Minimizing food waste is obviously the most important factor, but inevitably there are some organic bits that you just can't eat.

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