You don't have to plumb the oven that has moisture plus to use it. You just cannot use the burst of steam function nor the Master Chef Plus programs. I would call Miele and verify, but we put an in-line water filter per the installation instructions on our water supply line. I do not have the plumbed combi steam but you set a water hardness level and that determines how many times you can use the steam function before you need to descale.

Thanks Kim G! I called Miele and they recommended an in-line charcoal filter. They indicated that it would be fine for both ovens.

I have hard water and was hell-bent on the plumbed Miele. I was about to order it, when, to be on the safe side, I called around to a few local plumbers. All 3 warned me not to buy the plumbed one. Yes I know there is the descaler. However, what they did mention is - there is a filler tube that is between the water source and the steam oven, that does not get de-scaled during that cleaning process. That will be the cause of problems down the road and already had to be repaired (a major repair) that one plumber mentioned. So, sad but true I ordered Wolf. Don't have it yet. Hope this helps

The way I understand things, hard water is bad, because it leaves deposits. Contaminated water is bad, because the contaminants can clog things. Demineralized water (distilled or reverse-osmosis) is bad, because complete lack of minerals makes water more prone to corroding metals. And acidic or alkaline water is similarly bad.

So, ideally, you supply soft clean tap water. Where that isn't available, an ion-exchange water softener can remove hardness without removing total amount of minerals. And a carbon or particle filter can remove contaminants, if applicable.

These restrictions are true for all kitchen appliances regardless of brand. So, switching from Miele to Wolf just because the wording in their manual is different doesn't actually buy you more reliability. Switching from plumbed to supplying suitable (!) bottled water would help though. And you can do that with either brand.

And installing appropriate point-of-use water treatment would work just as well.

Thanks for providing an update. I came across this thread since we are considering getting a plumbed version of the Miele Combi steam oven. We live in Southern California and our water is hard (12 grains per gallon). We are planning to install a whole house water filter to filter out contaminants and a salt-less water conditioner (doesn’t remove minerals instead treats the water to transform the minerals so they don’t scale the pipes). Are you using any kind of softener or filter for the water that goes into the oven? How often do you have to descale? Thanks!

Wolf actually explains what the problem is

"unit contains an electromagnetic level sensor to measure the flow of water into the boiler. This type of technology requires mineral content in the water for accurate measurement."

So, you can filter water any way you want, as long as there is some minerals left in it. Some filters actually add back beneficial (for human health) minerals and the produced water should work.

Convection Steam Oven Water Requirements | FAQ | Sub-Zero, Wolf, and Cove (subzero-wolf.com)