Isolated Dishwasher

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Scotchy, Aug 10, 2020.

  1. Scotchy

    Scotchy New Member

    Joined:
    May 8, 2020
    Location:
    Illinois
    If I understand this correctly, there are a few things that must/should be followed when installing a dishwasher.

    1) A high loop (attached as high as possible in the cabinet)
    and
    2) Some jurisdictions may require an air gap.
    a) The air gap can be on the counter pointing to the sink​
    or
    b) Connection to a disposal connection in the sink.​

    Now consider this. What happens when the dishwasher isn't going to be connected to the sink or isn't anywhere near the sink that is possible to connect it to the disposal? Perhaps give it a name of a remote dishwasher, dishwasher far away from the sink or satellite dishwasher.

    The solution for this seems to be one of two options:
    1) An aproved Air gap on the counter (which would just pour water all over the floor if it overflowed as its not anywhere near a sink.)
    or
    2) Stand pipe (similar situation if the stand pipe overflowed)
    or
    3) Johnson Tee (apparently used in the upper west coast) Wouldn't water flow out of the house outside or is this just an AAV?​

    With vent stacks for sinks the vent must extend above the sink 6 or more inches 2015 IPC905.5. As I understand it if the vent is a branch, as such in the case of a separate sink and dishwasher, the highest point is only for fixtures served by that branch (not the entire floor.)

    Back to the problem. There is a sink of which is vented to the roof on one side of the room. On an adjacent wall the dishwasher is served on a separate vent joined to the sink branch in the ceiling. Both the sink and dishwasher connect to the main stack in the basement.

    So what to do with the air gap requirement? Here is a possible solution I came up with (or at least haven't seen this suggested before) Air Gap v2.png
    In theory it solves the ugly air gap on the counter and doesn't have an unnecessary AAV that could fail over time or hole through the exterior wall. The dishwasher still has a high loop in the cabinet but directly connects to the wye through a barbed fitting and flows through the p-trap. The wye connected to the vent would prevent siphoning from the drain back into the dishwasher.

    From what I have found most dishwashers have a small bit of water left in the sump. I believe, but haven't determined, that this acts as an air trap. The water from the high loop on the dishwasher side would fall back down the tube towards the dishwasher when the dishwasher drain pump stops. This would then serve the purpose of the sewer gases from the vent not entering the house through the dishwasher hose. If that is true then the p-trap isn't necessary nor is this entire setup.

    The use of an air-gap on the sink only solves the problem if the drain is clogged. If the hose is clogged then the water still will not eject from the dishwasher. It would be pretty obvious that there is standing water in the dishwasher that didn't get ejected when the dishwasher is opened.

    A Johnson tee would be simpler but if there is already a stack there why not use it? Plus there is no hole through the exterior wall nor waste water spewing out the side of the building in the event of an issue.

    What are your thoughts? Is this overthinking/over-engineering it?
     
  2. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    No go. An air gap is to prevent a vacuum in the water system from sucking sewage that has backed up. They want the backed up sewage to spill onto the floor or into the cabinet instead.

    See https://terrylove.com/forums/index.php?threads/would-a-wye-make-an-effective-airgap.85982/

    At least one state does not want you to make your own airgap, even if it is fully functional, but I expect Illinois would be fine with that.
     
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  4. Scotchy

    Scotchy New Member

    Joined:
    May 8, 2020
    Location:
    Illinois
    Exactly. So is there a difference between an air-gap and an air break? I have found numerous articles that state that an air-gap is to prevent non-potable water from contaminating non potable water. Here is the problem. The water enters through the dishwasher through a valve of which is opened to let potable water in. At that point the water non-potable. It has become contaminated with other things. So back flow is going to happen in the dishwasher tub itself to the potable system through the potable water valve on the fill cycle.

    Now, if we are talking about the rinse cycle of the dishwasher, the water would be potable (not sure I would drink it anyway) and be ejected out the sump to the drain. The siphon effect occurs where there is no air-gap/break. So a full sink without an air-gap could siphon water back from the sink when the drain pump stops. However if there is a connection to the vent as I showed above, the air would be pulled from the vent, the same way as an AAV or approved air-gap. This would stop the siphon from occurring.

    QUESTION:
    Why doesn't connecting the pipe back to the vent, which is to the roof, sufficient? Is is really any more likely that the vent will become blocked? The vent is 1 1/2" wide vs 1" in at the air gap. It's 6" above the flood level of the sink in the same room but a different vent.

    I understand when the water is ejected from the dishwasher to the drain the siphon action would happen if the vent is blocked. The dishwasher is below the p-trap and as any kids knows the water will be pulled to the lowest level. This would also happen if the Air-Gap device was clogged with debris.

    I understand that an inspector is going by the book. I am questioning the book and science leading up to the requirement of an air-gap. This is not to say or suggest in any way that I am smarter than the code-book or approval laboratories.
     
  5. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    I had missed that. I am not the one to debate that. My dishwasher does not have an air gap, and I don't worry about it.
     
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    The air gap on the drain is to prevent potential sewage from being pushed back into the dishwasher, potentially contaminating all of your supposedly now clean dishes. It is not about trying to keep potable water from being contaminated, only your dishes and whatever you washed in the DW. Normally, a drain isn't pressurized, so a high loop can protect your WM from sewage from being pushed back into it, but note that a disposal acts like a pump when moving waste, and it goes the path of least resistance...generally down the drain, but if that's clogged, who knows.

    So, keep in mind that an AAV is to prevent a trap from being sucked dry, so you do not need an AAV for the DW itself, only the trap that it empties into needs something to break the siphon so you don't get sewer gasses back into the room, or the DW.
     
  7. Scotchy

    Scotchy New Member

    Joined:
    May 8, 2020
    Location:
    Illinois
    .

    I have read that too.

    Sometimes I think a picture is worth a 1000 words but perhaps it isn't. Because the discharge tube is connected to the wye which above it is connected back to the vent above any potential flood level and behind the wall, it negates the need of an air-gap. There is no siphone because the vent prevents that. I feel like I could even argue the because it isn't open to the air that it doesn't even need a p-trap as no sewer gases would be present, due the small amount of water left in the sump of the DW.

    Interestingly enough I found this recent article in FineHomebuilding regarding the building code for dishwashers in an island. finehomebuilding.com/2019/09/10/a-new-old-way-to-vent-a-kitchen-island. Seems that I appear to be on to something, although I suspect many were on to this way before I was, that the code wasn't tested by science.
     
    Last edited: Aug 11, 2020
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    If you have an air gap from the DW hose to your drain, and the drain doesn't have a p-trap, then it is open to the drain, and sewer gasses will have a path into the room, if not into the DW, then at least into the room. The AAV or an atmospheric vent is to protect the trap, not the DW itself.
     
    Jeff H Young likes this.
  9. Jeff H Young

    Jeff H Young In the Trades

    Joined:
    Feb 27, 2020
    Location:
    92346
    Mostly as a plumber if you dont go through a formal aprenticeship or even if you do we are told what we can and what we cant do. Some things you dont know why or you think you know why you cant do this or that, and I always want to know the reason , but after 30 plus years sometimes we just have to accept that the reason is its the code. Why I can drive 25 but not 30 in school zone? Why some places need an airgap others dont. Why do I need a vacuum breaker on my hose bib come on Im not going to contaminate anything. Bottom line is it doesent matter why We are supposed to follow code not just the ones we agree with but in reality we all bend a bit with trying to provide a safe system and trying to keep cost down when its a non safety issue and it will still work well
     
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