Ideally, you start with enough aperture to capture the rising expanding effluent. Then, having the aperture defined, calculate the CFM necessary to achieve a velocity equal to the rising effluent velocity. This would require 180 CFM per square foot of aperture for 3 ft/s rising effluent if there weren't other considerations. One of those is that with baffles, that velocity only needs to be achieved over about half the area due to the aero shape of the baffles and the flow around them. I didn't figure this out from computational fluid dynamics, but from the behavior of my Wolf Pro Island hood dealing with my induction wok which is offset towards one side (the induction cooktop is offset toward the other side). I expect mesh filters to behave similarly due to the mesh blocking at least half the area of the filter. While a fairly poor analog, if one thinks of the effluent as billiard balls in a zero gravity field hitting the hood filter, one can appreciate that conservation of momentum will cause those hitting a flat surface to bounce away from it such that the angle of exit equals the mirror image of the angle of arrival relative to the perpendicular to the surface that the effluent hits (just like light). The need is to change the momentum of the reflection by entraining the effluent into the hood air flow. Ideally, this is done as the effluent reaches the baffles/mesh and there is little bouncing reflection going on. You may appreciate that the flat surfaces of your nominal hood where there is no mesh will be regions where there will be effluent reflection. Some of that will be captured by the air flow which pulls in from a range of angles. In any case, for best operation, one wants the actual hood aperture to encompass all of the rising effluent and not make the aperture smaller just to get the local air velocity up. (A commercial kitchen hood is an example what is relatively optimal for maximizing capture while minimizing the cost of air handling.) But the bigger the hood, the more CFM needed, and the more make-up air CFM that has to replace the air in the kitchen. This not only adds the modest cost of ducting in MUA, but significant cost if the MUA has to be pushed by a fan and the fan has to be controlled to keep the house pressure in a narrow range. This can be more complex in some ways than large commercial buildings which can run MUA continuously and use pressure differential across a damper to control whether the fan output just circulates around in the fan loop or is pushed into the building. I think my previous post suggested that for the hood aperture you have, the actual CFM achieved will probably be enough. But imperfect capture and containment should expected from the hood aperture design due to the flat areas not being capture areas. 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.

I think it sounds fine. These were where the stove will actually be, right? Not on the counter next to it? If the latter, it's not fair to your poor hood, your hood sucks a treat and you need to stop worrying. :) You'll know for sure in practice, of course. Is your hood the same size as the stove, or is it one size bigger? If it's the same size, I don't think you have any concern at all. You've proven that your hood is powerful enough to capture from the very edges of your cooking area. What it tells you either way is something that is true of all hoods. They draw best in the center. So...if you have something big/smelly/smoky/steamy/greasy, put it as close to the middle as you can. If you're simmering something that's not doing much, on a small flame, put it in the front, on the side. Most of all, unless you have really bad issues with asthma or something similar with your family members, you have a heck of a lot better venting than most people and your new kitchen is going to be great! I agree with Aloha, however. Did you account for make-up air? You seem so well informed about your vent, I didn't think to ask. If you're in an extreme climate with cold, heat or humidity, or if you have a well sealed, modern house, you might want to put in a "conditioned" make-up air system while you're still not finished. That means air that enters the area where it's being sucked out, that's heated, cooled or dehumidified as needed. If you live in a temperate area, however, the open window is just fine if you're not getting enough make-up air from cracks and drafts. 1200 cfm is a lot of suck, and the air it's sucking has to come from somewhere. :)

Unfortunately, most wall ovens do not have their own vent interfaces. What I did was put a pair of registers in the ceiling over the ovens, with their connecting ducts joined in a Y-adapter, and the output duct connected to a roof blower. The blower was a re-purposed NuTone down-blast roof blower purchased many years earlier to provide a boost to the vent of a Litton "kitchen center." The register boxes take (IIRC) 12 x 14 furnace filters to help keep grease particles from the ducts. Note that not even a commercial "eye-brow" hood will capture the smoke escaping a broiling oven when the door is opened. What these registers do is help pull effluent from the ceiling area and slowly clear the kitchen in concert with the main hood. The only way to successfully deal with opened ovens is a large hood right over them, or a commercial porous ceiling ventilation system, such as illustrated here: http://www.kitchen-ventilation.co.uk/heydal Yes, the MUA now becomes more complex because the control has to be "closed-loop" instead of "open-loop" and thus needs a servo control system to control an even larger MUA blower. My kitchen blowers' combined output is 2500 CFM (zero static pressure rating), and likely pull a total of 1500 CFM when there is sufficient MUA. With both blowers having variable control, the actual CFM can be anything, windows can be open or not, bathroom fans can run or not, etc., so the scheme is to control on differential pressure between my kitchen and the vented attic. Not all parts of this system have been purchased and integrated. My excuse is that my honey-do list is very long and keeps expanding. Items in place: Roof down-blast housing on pedestal for air intake interface with attic Hydronic heat exchanger in attic with its separate control loop to my furnace (functioning) 3-ft square diffuser in hallway facing kitchen Sensing system installed Control system components mounted in basement; Fuji computer purchased but not yet incorporated Key elements yet to be obtained (mostly from Fantech, I expect) Big axial blower sufficient to flow around 1500 CFM through the pressure losses of the ducting, air filter, and heat exchanger. It also has to fit through the attic access stairway. Current-loop control fan motor driver Pleated air filter box Damper Various sheet metal interface sections I should add that my only combustion appliance is a blown oil burner that has its own sealed MUA supply, so delay in completing this kitchen MUA system is an annoyance, but not a hazard. In your case, a carbon monoxide warning sensor is suggested. kas