I could not look at the photos. As long as it is above the trap with a high loop or air gap and installed with a wye you are fine. The high loop has nothing to do with gases, it has to do with backflow of contaminated water if the drain gets clogged so water goes in the sink instead of in the dishwasher.

Wye Branch Tailpiece

On a Double sink with wye on the vertical

On a double sink with the wye on the horizontal

Thanks vithdude! Good info! Not sure what the photo err was - those links work for me and the gallery is set up for public access. I'll attach below anyway.

As you can see, there's not enough vertical distance, between the left sink basket and the point on the tailpiece that joins with the horizontal from the right sink, for one of these "wyes", without making a custom one, or somehow lowering the entire assembly (which I'm not convinced would work anyway.) The "Double sink with wye on the vertical" is intriguing - is this a plastic-only part? I've never seen this, but it certainly looks like something like that would work for me. My problem is that I've been looking at brass fittings only, and putting the same thing together piecemeal requires too much vertical space.

I'm intrigued by your last photo - with the horizontal wye. Do you think I can do this in brass fittings?

Finally, what do you think about the other drain - the one below the right sink? Do you think this would have been used originally for both sinks to have separate drains? Would it be to code to use this right drain solely for the dishwasher? The two drains combine together about 5 feet below this point in the basement.

Thanks! That's a really good idea (horizontal slip joint tailpiece/wye/union) for attaching the DW drain with minimal change. I will consider doing this myself.

There are no plumbing fixtures using the other drain/cleanout. It was once used by a sink in the bathroom overhead, which has been remodeled to use a different drain. Below the kitchen drain, it simply combines with the other drain pipes about 5 feet below. Given this, do you still think it's a bad idea to use the cleanout for the DW drain? Wouldn't there be an advantage to separating the DW drain from the sink drain in the "using both simultaneous" use case?

If it was a drain line previously it probably would work. Is that drain line vented properly? Does the drain pipe get larger where they combine? If so it is probably ok to use, but install a trap and an air gap. The air gap will allow water to discharge into the sink in the case of the dishwasher drain line getting plugged.

Also, I just noticed, check the slope on your horizontal drain, it looks like its sloped towards the right instead of the left. Not sure you can do anything about that without messing up the slope behind the trap. Might not be a good idea to do that horizontal wye then if the slope is wrong.

There's not much slope in the horizontal drain - it's pretty level, but just barely slopes the correct way for my configuration. Hasn't been a problem (yet). Duly noted, though. That might make me lean towards using the other drain/cleanout for dedicated DW drain.

I can't really tell if the drain pipe gets bigger where it combines. I think it might but not by much. Here's the only photo I have of where the two drains combine:

I'm not sure how to tell if the right drain is vented properly, now, but I remember seeing the same drain pipe upstairs when I had the walls down and the pipe went all the way to the roof so I think it's vented. But I should probably prove this hypothesis. How would I do prove it's vented? Maybe send a snake up the cleanout and if it goes all the way to the roof then it's probably vented?

ASSuming it IS vented, I would proceed with:

* remove cleanout plug

* make sure there is enough clearance to attach a ptrap there

* install an adapter in cleanout opening to take it to slip joint

* connect ptrap to slip joint adapter at cleanout

* connect tailpiece to end of ptrap

* connect DW outlet hose to tailpiece

Sound plan?

You might have a problem if you drain the sink and the dishwasher at the same time (the pipes are the same size and merge), but if you install an air gap next to your sink you should be ok in that case (water from the dishwasher will overflow and go in the sink). It sounds like it is vented but you can test it as you mentioned.

Go from dishwasher, to air gap, to trap, to drain. Whatever adapters and worm clamps to make it work.

The right cleanout pipe drains VERY slowly. I poured about 1 quart down the pipe and it backed up immediately. I don't have a plumbing snake, but I jammed my electrical one down there and didn't encounter any noticeable blockage. I'm not sure how to proceed now and will probably call an experienced plumber to investigate.

Still, I'm curious for people's advice on the situation. Here are some photos of the deposits on the pipe and plug. The access to this pipe under the sink and where it combines below in the basement is pretty good. So I might consider having the pipe or a section of it, replaced.

Or, I could do the easy option and just do the horizontal wye with brass tailpiece. It's just a mystery how well it's going to drain and whether the sink will backup..

Thoughts, anyone?

Since the slope is going the correct way between the sinks, even if it isnt a lot of slope, it should be ok to use if the plumber forsees fixing the other drain to be a major fix. I was just worried it was sloped the wrong way, which would cause a problem.

I met with a plumber this morning and he routed the right drain via cleanout, and it drains fine now. I'm going to attempt the ptrap - adapter - DW hose - high loop solution first. I believe I do not need an air gap with the high loop. This is a high-end Miele DW that "comes equipped with a
built-in non-return valve on the
discharge side to prevent waste
water from flowing back into the
dishwasher." I don't believe the state of Oregon requires an air gap anyway.

Since I am adapting down already, it seems I have the option of using either a 1 1/4" or a 1 1/2" p trap. Do you have any recommendations which I should use? It would seem to me that bigger is better in this circumstance.

So, I am going to install either a 1 1/4" or 1 1/2" p trap to the unused cleanout (using the appropriate adapter), and am seeking a way to connect the dishwasher drain hose to this p trap assembly. Any ideas? I haven't (yet) been able to find any such thing at plumbingsupply.com or local plumbing places.

If I can't find an appropriate adapter, one option I have considered was to just use a tailpiece with dishwasher wye, connect that to the p trap, and connect the DW drain to the wye, then find a way to just cap off the tailpiece. Any ideas on hoe to cap off a slip-joint tube?

If the cleanout is 2", you could use up to a 2" trap but the 1 1/4 or 1 1/2 will be fine. The part of the trap going vertical you can connect whatever adapter(s) required to go from the pipe size used to a hose barb that fits the DW drain tube, then you secure the tube to the barb with a worm clamp.

For the high loop, I am not sure. My only concern is that it would be a closed system and there is no air behind water before the trap like you get with a sink. Since the water is being pumped it could pump all the water straight pass the trap and leave no water in the trap to stop sewer gases.

But, with the non return valve on your DW, you might not even need a trap and can pump it directly into the cleanout since I believe the trap wont really do anything in a closed system.

I dont know if all dishwashers have non return valves, but normally there would be an air gap and a trap if the dishwasher had its own drain and wasnt connected to the sink plumbing. An air gap allows water to drain normally by gravity. With the air gap, the DW pumps water to the air gap and from there on the water drains by gravity.

And you cant do the installation like that of a clothes washer. The risk of water overflow is a BIG factor. Thats what the air gap is for so if there ever is an overflow it goes in the sink instead of all over your cabinet and floor. If a clothes washer overflows it is 'usually' next to a floor drain and not a big deal.

So I am thinking either do an air gap and trap, or install it to pump directly into the cleanout since your DW has a non return valve.

Here is a diagram of how an air gap works, allowing air behind water before the trap. An air gap should also be installed if hooking a DW to a single sink in case the sink is holding water. Double sinks rarely have water in both sides so I wouldnt put an air gap there.

http://diy.stackexchange.com/questions/27547/how-do-i-keep-water-from-getting-in-and-out-of-a-dishwasher-drain-air-gap