By careful measurement of wall distances over a path from one side to the other of the duct wall, it should be possible to determine the thickness of the wall. If it is a standard 2 x 4 stud wall, the cavity is 3.5 inches; if a less standard 2 x 6 wall, then the cavity is 5.5 inches. Neither of these will accommodate a 6-inch round duct, so transition to rectangular is likely. The third possibility is a double wall built to accommodate the round duct. It should be possible to distinguish these even with modest error in measurement. kas

advisor - maybe we just are reading this differently: "contractor is saying we can run 10 inch off the hood and then "choke it back" to the 8 inch for the majority of the run. " You see , only an idiot would got to the trouble to install an 8" to 10" transition at the hood and then immediately add another one going in the opposite direction to go back down to 8". So what is likely happening is there is the takeoff at the hood at whatever size to 10" for a run of between 2 and 24 feet at which point there is to be another reducer of 10" down to 8" for the rest of the run - or majority as o.p. says. This leaves a big belly in the middle of the assembly. If you think this is good design then you are certainly entitled to your opinion, but fancy website aside I say you're wrong. You are correct about the transitions 8" to 10" is quite common , you can get one of the shelf at your local home depot, and VAH will even sell you one. They'll even sell you one of their wacky 8" plus 6" dual takeoff to 10" round if you get one of their larger hoods. BUT, I'm not letting any contractor go from 8" to 10" back down to 8" again in our projects. You can do what you want, and I'd suggest mrd gets someone that knows what they are doing locally to advise him/her because 1. neither you or I are on site to see what's actually going on, 2. have not been engaged to design the system, 3. seems that the onsite HVAC guy is not quite on the top of his game based on the quoted gameplan. RE: the 600 cfm, might be enough, but what if it's actually not in this case. Are you going to pay for the swap, refund the consult fee, ect.??? I know 900 + will work because it there is a lot of xtra headroom built in. It might cost more, or maybe not as VAH is proud of their hoods, but that cushion is nice. You also do not mention how loud that VAH is going to be especially on full tilt ! May not be important to mrd as (s)he didn't mention it - but what if it is???? Doesn't seem prudent to make a rec to a stranger that is too conservative - but hey it's a free country.

ditto. it's kind of hard to bump your head, even if you're tall (i'm 6'0') because the range/cooktop is blocking you from moving forward. if you lean forward, your height is reduced. i was worried about bumping my head and had decided to mount my VAH 33" above my cooktop. of course, when the contractors installed it, i wasn't around, and they did it at exactly 30" despite my multiple specifications of 33". i was very upset. six months later, it's fine, and i'm actually happy that it's lower, since that gives me better capture. the only time i've ever bumped my head is when i've purposely stuck my head up there to clean or remove the squirrel cage or something.

Please note that you should ideally have a 36" hood over a 30" range, in particular if you have a gas range. Around here, most of the contractors already know that and this seems the standard configuration in new construction. In other parts of the country, contractors frequently don't inform themselves as much about what it takes to make a kitchen really functional vs. just making it look pretty. So, it is often on the home owner to point out these technical details. As you mentioned that you want a hood with a deep capture area, I'd like to give a recommendation for ModernAire. We have one of their hoods and are extremely happy with it. Due to various constraints with a deck right above the kitchen, we couldn't get a remote blower nor an inline filter. So, things are a little noisier than we'd ideally have liked it. But since ModernAire builds all their hoods to order, we got exactly what we wanted. Our hood it mounted above head height and is extra deep to easily capture the front burners, even when you consider the expanding plume.