Is it on an interior or exterior wall? If interior, do you know if it can vent through the ceiling or do you need to run it through crown/fascia and if so what height do you have? That would dictate whether you can even go with 8" or 10" ducting. What kind of cooking to do you ... do you do wok or other high-heat cooking that needs a lot of cfm or not?

Thanks for replying! It is an interior wall that vents through the attic then roof. If I had to guess it's about 12 ft to the roof. There is existing ductwork to the roof but I don't know much about it. We did look outside on the roof and saw where the roof cap is. I'm sure that's not the right term for it, but it's a boxy looking thing where the range exhaust exits the roof (one story house). My contractor will do whatever is necessary to get this going. It looks like there is a rectangular box-like metal duct leading out from the range area into the wall. What happens up in the wall and attic, I'm not sure. The previous owner had a microwave with built in exhaust fan over a range in that spot. My contractor can duct straight up through the cabinets if need be. They will be ceiling height after crown molding is installed so you will not see anything. I do not do wok or high heat cooking. I sautee, boil etc, over the cooktop and use the oven. I cook every day. FWIW I am Italian LOL and growing up with the smell of food cooking was always a good thing :) I'm amazed at how many people think it's bad. Now the smell of burnt food and a smoked-out kitchen, that's bad! But I don't recall cooking anything on a daily basis that generates a lot of smoke over my current cooktop. I am currently, get this, UNVENTED, yes completely unvented in my condo. Cooktop is over an island. It's a large open floor plan, 10 foot ceilings, and in the 6 years in my current set up, no lingering smells and my ceiling is perfectly white :) Thanks so much for all of your help! Any advice you can give is much appreciated.

Can the vendor from whom you purchased the induction stove or the manufacturer give you information about the ducting and ventilation requirements? I'm not necessarily recommending a Viking ventilation system, just that you see what specs they recommend and go from there.

Here is a link that might be useful: Viking custom ventilation

A lot of people here are happy with their Vent-A-Hood, Broan, ModernAire and Trailwind vents. They are not cheap, alas! But I suspect you can get a decent Broan Best model on sale somewhere. Check out the Appliances forum here on gardenweb for lots of detailed threads on ventilation.

Thanks everyone. My GC says it's best to lose the in-wall vent and go straight up through the attic. It opens up way more choices in brands of vents. My problem is the "cave" size, for lack of better word. It's 53 horiz and 24 deep. Can the 53 width be built up to a smaller width with wood? Is that safe? I was thinking of shrinking the interior width to a more standard size to avoid a custom size vent. Has anyone done this? After it's done I can go with a 36" vent and line the rest of the ionterior with a stainless liner. What do you think?