I would strongly recommend against using any type of flexible duct for cook vent. Grease will build up on the inside becoming a mess, and a fire hazard. Try to get the two units to align as closely as possible, or use adjustable elbows to create the offset you need.

Another option is to have a sheet metal plenum made to adapt the two.

Thanks so much, cook. I'd done a ton of reading and noted that flex duct isn't recommended. That said, we've used the same setup for the past 24 years (downdraft vented w/ flex metal); and when we recently dismantled the old ductwork, there really wasn't that much noticeable buildup (we don't do any deep fat frying - light sauteeing, at most, a few times a week).

If I could get away with the elbows, I'd do it. I spent some time fiddling w/ the various adjustable setups at HD/Lowes, but those suckers were rather difficult to turn and reshape well enough for me to get an idea of what their capabilities might be.

I think the best idea (which allows me the most flexibility for positioning) would be to have a sheet metal plenum made: is that something I would just have someone local fashion? (And what type of business might I google?)

A dedicated sheet metal shop, or possibly some capable HVAC contractor.

Thanks for your (always) sound advice, kas. I let my fingers do the googling yesterday, contacted a local sheet metal shop (or, at least, a company that came up in the search!), and was told they don't do that type of work...Strike One. I've got a lead on a couple of A/C suppliers, but not sure if they fabricate or merely have a more extensive supply of elbow-y configurations than I might find at big box.

I should clarify this is all part of an unintended reno due to DW leak. We're replacing cabinetry and might as well widen the island while we're at it (also because I was told it'd be a knucklehead move to just keep my perfectly function coil cooktop of 24 years, rather than purchase new appliances). Yes, we considered (and thoroughly researched) installing a real hood. However, with 10' ceilings, DH didn't like the visual. The expense (including installation, of course) put this out of our budget.

I understand all the pros and cons of DD vs real hood, so this wasn't an easy decision. But something had to give. Our rationale:

• Our current DD has served us well over all these decades: home cooked meals for family of five, and no discernible greasy walls and surfaces. (I'm not being naive: I'm sure it's all there, but IMO, it's minimal.) It's just the two of us now, and we don't plan on moving...the grease/steam 'load' is far less than it used to be.
• The Bosch DD rises 14-15", and I purposely got the Flexduction cooktop so I can position the larger pots/pans (for boiling pasta and sauteeing) towards the rear, right up next to the DD.

From what I'm seeing, the horizontal distance is not really an issue but the vertical may be. I've seen duct connectors that can twist and turn to allow most any shape. And two 90* turns shouldn't be that hard. Not that great, but not that hard.

something like this will adjust from 0* to 90*. Two of them with something in between should do the trick.elbow