>Does the proposal from the plumber make sense? No, not really. See below.

>Because the water from the steam oven is so hot, I assume the extension has to be copper hose. Correct? The install specs I've seen for Miele CSO ovens always state "Only genuine original Miele hoses can be used with this appliance." And "Do not shorten or cut the supplied drainage hose." This clearly means do not use your own supplied copper tubing.

A few things you wrote don’t make sense, probably because not all details are listed. The main thing is how you phrased what the plumber told you. Did the plumber a.) “suggest” this high loop after looking at your setup? Or b.) did he review the installation specifications and then “suggest” this? Based on what you posted my guess is he did not look at the installation specifications. I do not see that ‘high loop’ as satisfying the installation requirements and specifications.

Here’s my suggestion:

1. Get both of these documents (from the mieleusa.com website) for your exact oven model: a.) Operating and Installation Manual, b.) Specification Sheet. These are two different documents, and depending on the model and model year may use use slight variations on the document name. The first document is about 200 +/- pages and you want the section on drainage connections. The second document will be about 20 +/- pages and it will also have a section for drainage connections.
2. Preferably find a different plumber than the one you talked to above.
3. Tell the new plumber you have a problem with the drain connection on a high-end plumbed in steam oven, and you want to employ him to review the specifications for the oven and tell you if the plumbing on the install is done according to the manufacturer’s specifications. Because you suspect it is not.
4. If new plumber grumbles or resists “we don’t need to read no specifications…” find another plumber. Good plumbers who do kitchen and bath remodels are used to researching things like this and will have no problem doing so.
5. If the install already meets the specs and you still have a problem then it’s time to contact Miele service.
6. Last point, suggest you question who ever designed your arrangement to find out if he/she followed the manufacturer's specs (it sounds like they did not), and if not why not? Isn't that what you paid him/her for?

Why all this talk of and fuss about manufacturer’s specifications? Because no matter if you are talking basic drywall, regular old concrete, hardwood flooring, roofing and rain-gutter systems ... or multi-thousand dollar hi-tech ovens, having something installed correctly -- to exacting manufacturer specs -- is the key to long life, to full warranty coverage, and to minimize failures. Short translation: If you want your oven to work correctly make sure you follow the manufacturer’s installation specs.

California requires dishwasher air gaps at kitchen countertops. They are stupid, ugly, and useless, especially compared to the other 49 states where a high loop, in apparent defiance of the laws of physics in California, work just fine.

Had I followed Elkay's manufacturer installation instructions for a recent sink installation, I would have a nightmare replacement on my hands. As I did it, it will take a couple hours at most.

My point is, the codes and the manufacturers can be wrong. We'd still be using silicone to make seams in Corian, had some fabricators not disregarded DuPont's instructions 40 years ago.

The plumber's suggestion for a high loop is an inexpensive and intelligent first shot. Take it please.

I got a call to enlarge an oven opening in a granite countertop. I looked up the GE manufacturer's instructions, cut to them, and promptly got a call that the appliance wouldn't fit. Having done a job twice and only able to bill for it once, I now refuse appliance alteration jobs unless the appliance is on site.

Designers, architects, and manufacturers can draw and specify anything whether it works or is parctical or is code. It's up to the guys in the field to make it happen safely in real life.

You have to watch that one manufacturer's installation instruction doesn't cost you the warranty on another manufacturer's product.

The Elkay sink installation mentioned previously illustrates studs drilled into the bottom of the countertop for clips to hold the sink in place. Drilling into any engineered stone top voids the manufacturer's warranty.

Disregarding their instructions, I used a strap system that connects clips to the cabinet sides. Now instead of altering cabinet sides to access clips and/or risking the countertop by driving them out and lying on my back in a cabinet for a half hour, I simply unscrew 2 nuts while I sit on my butt. The straps get reused; the estone warranty remains intact.

Manufacturer's instructions and designer/architect specifications and drawings should be viewed as very strong suggestions and pay particular attention to the stuff in bold and all caps, however, they are not the the tablets inscribed in stone from on high as some would have you believe.

@ Joseph Corlett, LLC, you are relating a "high loop" used in dishwasher plumbing configurations for dishwashers having have pumps on their drain lines.

The Miele CSOs drain lines are gravity only, no pumps. Correct me if I'm wrong. One more example of why it's important to follow the installation specifications, IMO.

I didn't say the high loop was the answer; I said it sounded like the plumber made a good, inexpensive, and low risk proposal for a solution.

Thanks all for your comments! I’ll have to do some installation manual studying.

Some more details:

- the oven is installed on top of a 19” cabinet. So not undercounter.

- the sink is around 7 to 8 ft away from the oven. So the provided 9ft hose is on the short side.

- it’s an Eichler house that sits directly on a concrete slab. There is option to do anything in a crawl space because there is none.

"Escept a high loop will create backflow, not prevent it."

How so, please?