I hope you can see what's going on:

It appears that one DW drains into the batch disposal unit & the other drains into the drain pipe after/below it. The sink is a large single bowl with rear left corner drain. Both of our DWs are Miele, with filters. We have the right DW hooked to cold water & the left DW hooked to hot water. (If we were to do it again, I'd hook them both to cold, for the China & Glass cycle.) The white box is an instant hot water heater using a separate faucet. The plumber was not pleased about getting this all into one side, but I love how we have enough room for the pullout.

We don't have instant hot or disposal, so don't know how this might relate to yours:

We have 2 dishwashers and a disposal - I will get pics tomorrow. I was told that it could be an issue with permitting but didn't have a problem (a neighbor had a hard problem getting the permit).

Not to take things off course, but I'm planning on doing this too. We're in the process of getting the building permit, but we aren't specifiying anything about the dw... hope I'm not in for trouble!

Do plumbers balk when they see this setup? I don't know much about plumbing but it seems like if the plumbing is fine for one dw it should work just as well for 2. We had troubles after our Bosch was installed by an incompetent contractor, my husband redid the plumbing and smell/sediment problems were resolved.

A question for all of you...
Where is your trash? I think one time rhome said hers was across the sink in the island. Is that right? I'm interested in hearing placement ideas. Is there any logic to dw, then sink, then trash, then 2nd dw? Or would plumbing that be problematic?

Where is your trash? I think one time rhome said hers was across the sink in the island. Is that right?

Yes, I have my trash across from the sink in the island, so it can pull toward us for scraping plates, etc. as we load either dishwasher. If it was in between the sink and dw, you'd have problems retrieving dishes from the sink, throwing anything away, and loading the dishwasher without a lot of walking back and forth. If I had to, I'd rather use a big bowl or something for trash on the counter, have both dishwashers right next to the sink, and transfer the trash later. It'd be a problem, possibly, to get the hoses from the dishwasher behind the trash cab (or through it?) to the drain under the sink and you'd have to also run the water supply from the sink area, too...or have separate supplies?

I don't know much about plumbing but it seems like if the plumbing is fine for one dw it should work just as well for 2.
Have to make sure they don't drain toward each other, and that sink and dws can drain simultaneously without a problem.

we aren't specifiying anything about the dw... hope I'm not in for trouble

The plans you turn in for your permit should show the 2 dishwashers...At least ours had to, and the price of the permit was altered by the number of plumbing fixtures. They see it when they inspect, and for us, if it varied from the plan, they required a change of plans at the building dept (with fee, or course) and a 2nd inspection.

We only had plumbers to do the rough-in plumbing and dh installed the dws and other fixtures. He had to do some thinking about this particular issue, but this certainly works...and it all passed inspection.

Zillapgh, our trash is in a pullout in the island behind the cleanup sink. I find hat to be a very convenient location because it is handy for prep work (wipe counter off into the trash) and very near the sink.

Gizmonike, your setup is what I think should have been done for us too. However, they put in a T so both DWs are trying to drain into the disposal. Also because both DWs are double dishdrawers, each leg of the T is T'd off as well. This means that there is the opportunity for backwash, and I believe this is the root of our problems.

Here's a picture:

The reason that this most often doesn't pass inspection is that to do it properly requires that you upsize the drain line to 3" to include the possibility of all fixtures draining at once. (The 1 1/2" sink plumbing empties into the 3" drain in the wall.) DW's also should have high loops for the discharge hose before they join the drain, and they should drain into either the disposal or a DW "T" before the trap.

We have our DWs placed next to the cleanup sink. Our trash pullout is to the right of the right DW, putting it closer to the breakfast/family space in the room. The left DW is closest to the cooking area, so it handles cooking utensils, bowls, pans, & overflow from the primary DW. We have another trash pullout in the island, around the corner from the prep sink & across from the refrigerator & microwave.

alku05, I'd get it redone. Even with a high loop, the way the double drawer higher "y" thingy is done pretty much guarantees backwash into one of the two drawers. With the other drawer set being literally lower and the "y" slanting downwards, I don't know what would happen, but it can't be good.

I have one double drawer, the connection to the drain angles downwards now and we'll see how that works, but with even with a high loop and using a straight "T" into the sink drain line, the "y" thingy had to be turned so both drains were parallel to the floor before both drawers worked properly.

To live_wire_oak and anyone else who might be influenced by :

1.
This part is not true:
"The reason that this most often doesn't pass inspection is that to do it properly requires that you upsize the drain line to 3" to include the possibility of all fixtures draining at once. (The 1 1/2" sink plumbing empties into the 3" drain in the wall.) "

Take it to the plumbing forum if you want to know more about drain diameter sizing. Or search using DFU as a key word. You do not need a 3" diameter pipe so don't anyone lose sleep over this.

Here is one table I found on the web
http://www.engineeringtoolbox.com/drainage-fixture-units-piping-load-d_1078.html
which says clearly that a 2" pipe is sufficient.
E.g. in a 12 storey highrise, a 2" pipe will accept the load of 12 kitchens each with a faucet and dishwasher (a load of 2 DFU's per kitchen, since a faucet and a DW = 2 DFU).

2.
The rest is good. Yes the drain hose has to be looped high, and then connect to the drain pipe above the kitchen sink's P-trap, and to make this connection the hose can either go to the disposer or to a "T" or to another connection geometry (in addition to the DW "T", there are other geometries allowed, to connect the DW drain hose to the drain).:
"... have high loops for the discharge hose before they join the drain, and they should drain into either the disposal or a DW "T" before the trap"

hth
-d

I think you all are spot on. Even with the high loops, we're still getting some backwash. It's all about pressure...the high loop is designed to accomodate the pressure caused by gravity pulling on the fluid in the hose. However, once the fluid from the other DW draining comes into play, it's not only the weight of the water, but the force of it being pumped out as well- and that's just too much for a high loop to handle.

We also don't like the disposal hooked up to the smaller side of the double sink- we let the installer talk us out of having it on the larger bowl and have regretted it ever since. Sounds like we need to find a good and flexible plumber to redo this mess. We had one plumber take a look at it while he was over to quote us for a different job and all he did was go on and on about how it wasn't to code because there was no airgap installed (untrue!) and how no DW would drain properly without an air gap- untrue! When I asked about a Johnson T and he didn't know what I was talking about I knew we needed someone better that at least knew as much as or preferably MORE than I do about plumbing.

Also, one more thing: I just want to thank everyone here in Kitchens again for being so amazingly helpful and taking the time to respond. Even though we did our kitchen in 2006-2007 and I was a regular poster through the beginning of 2009, I love that I can get such help after being gone a year, and knowing that I would get the same quality of help even if this was the first time I was ever posting here. I recently posted some plans in baths and building a home (we're planning a large addition) and got almost no responses....that really drove home how amazing this kitchen forum really is. Thank you!

Sorry, I mistyped, it's a 2" drain that is required. A 1 1/2" drain can only serve 3 DFUs. DWs count as 2 each and the sink counts as 2, so you need a 2" drain pipe to be able to serve a 2 DW setup. If you have an instant hot or other water fixture though, you'll need the next size up, 2 1/2" which can serve 12 DFUs.

If a 1 1/2" drain can only serve 3 DFUs, and if a DW counts as 2 DFUs and a sink counts as 2 DFUs also, then no kitchen would ever have a 1 1/2" drain pipe...

So, it is clear this is also not correct. But, we are making progress, because we are going in the right direction! Pipe sizes are getting smaller. Why not leave it to the Master Plumbers? Each field of professional endeavor has its own body of knowledge. Let's not try to act as plumbers when we are not.

As far as I know, a dishwasher PLUS a kitchen faucet equals 2 DFUs.
I wouldn't know "off-hand" how many DFU's a two-dishwasher kitchen would be, and whether or not it would require upsizing from a 1.5" diameter P trap and pipe to 2" diameter. I'm pretty sure that the fine people who posted above have fine pipe sizes. I'm pretty sure that I would have heard about this "problem" many times before, if it were a problem.
... As far as I know, a dishwasher Tee has a very very small diameter for the dishwasher drain hose (!) because it doesn't _ need _ a large diameter to pass that drain water over to the P trap. This can be seen in all the products sold as dishwasher Tee's, without exception, and they are installed in millions of houses and have passed millions of inspections.

I would agree to let a Master Plumber tell me when I need to upsize to 2" in the kitchen.
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