The effluent doesn't rise vertically from the cooktop, but rather expands like a funnel. Its also buffeted by any movements you make and sensitive to slight breezes in the kitchen.

So I think its smart that your central hob is the one most likely to be producing smoke, and also that your hood extends out from the cooktop edge just a few inches on all sides.

Obviously it depends some on how and what you cook. You could cut corners here (to a 36"x24" hood) -- but I bet you'd regret doing that.

The only thing that makes some difference is if you will have cabinets up against both sides of the hood. If so, you could go for a narrower one. If open on the sides, then keep the larger one.

Personally, I would kick up the CFM to 700 - 900. This gives me the option to use a lower power (quieter) most of the time and higher if needed.

@chac_mool & @weedmeister: thanks for the input.

There will be uppers on both sides of the vent. So what i am getting here is better do 42x27" like as if i were to do with gas rangetop.

Do whatever looks good. :) I had a 36" hood over a 36" induction cooktop and it was fine. It didn't even stick out that far and had 650 cfm. I'd have to turn it to max every once in a while and it was always adequate, whether I was burning popcorn or burning bacon.

Unless you do major stir-frying or stinky foods, what you have planned is way more than adequate.

(I would not need the big hood you have planned and maybe you do but probably not.)

If you want the nitty gritty calculations, you can look it up on The Induction Site. They have fancy equations for figuring out cfm recommendations based on size and power, like you'd do with gas.

For most people who don't have a tame dragon, like the CC, 600 cfm is adequate. I agree with what Weedmeister said about going a bit more powerful and using the lower settings. It's supposed to do a better job, besides being quieter. Using the equations for both gas and induction, I figured that I'd need about 900 cfm (actually a tad less) when I have all hobs going full blast, which I do a few times per year when I most want the air cleared. MA had 1000, which is what I planned on. Then, just at the time I was placing my order, it seems, they changed to 1200. That's way overkill for my daily cooking on the induction, and sometimes I wish I had a lower low setting (it's two 600's working together so low is the low of two 600's), and I have to be doing something really intense to need to crank it up near high. (Should mention that deep frying is a rare occasion, and fish is a never (allergic). I'd crank it up more if I did either of those with frequency.) Still and all, 600 should be fine for most people's normal cooking.

Chac_mool is right about the "plume". Since you'd planned it, having the extra capture area is a good thing. The more coverage you have, the less escapes, and the less has to be sucked backwards. (My hood is strong enough to pull steam back and disrupt the shape of the plume, though I don't know if it does that for heavier particulates).

Make-up Air (MUA) doesn't get discussed enough. If you live somewhere cold and/or hot or very humid, where you use HVAC a lot, or if you have a very well sealed, newer house, you might want (or might be required to have) a make-up air system and might want heated or cooled make up air. Check with your HVAC engineer about that. I live in a very moderate climate (even more moderate than the general area), so it wasn't a concern. When I do have all 1200 cfms pulling (it's right out the wall, so very little suckage is lost), I sometimes feel like the air is being sucked out of the room. Perhaps it's real, perhaps it's all in my head. I just open a little casement in my garden window behind the sink, which I can do in any weather. At that point the kitchen will be hot, anyway, from all the cooking, and rain won't come in there. Rarely happens.

jmith, the above posts covered everything very well. My CFM is smaller than what you are planning, and I'm happy with it. Completely! My inline blower is quiet, and when maxed out the sound I hear is rushing air, and only that. Air is wind when moving = some inevitable perceptible noise = normal.

So far this story of mine is like all the others above. Two aspects of my installation are different however. One is that I have my ideal plume capture canopy shape made out of transparent material and I've placed it at about 18" above the cooktop. Think "restaurant buffet sneeze guard" if you want to see web images of a readymade product sold as a product. It's not overhead, it's between your eyes and your forearm. I'm building it myself. I'm in no hurry, as the suckage is fine without much of a canopy to speak of. I'm reworking t to be made of two parts so that it will be a slide-out. Web search pull-out or slide-out kitchen exhaust canopy hood to see real life examples. I've spent time researching materials (glass-like and plexiglas-like) so that I can factually answer future objections about fire resistance. I've tried out shapes with high sides and without. The idea is to rework my canopy shape to see what seems to capture plumes best, in my kitchen space. In the final analysis, it seems it's easy to adjust one's cooking style and splitsecond fry-handling to ensure there is no massive smoke, and that this counts as much as anything else.

I have wall ("upper cabinets") cabinets above my cooktop. The idea was to have a pullout hood, a slide out. There are several in the market. Before getting a ready-made product, I'm experimenting with my own canopy shape. I've also observed how the air movement happens when I stand large cutting boards on the countertop, as air barriers to each side of the cooktop. (... and, within a few years (!) I'll have some strong conclusions to write about.)

Here is a 600 CFM exhaust hood with a glass slide out that slants down so you can see through it.
I have not seen it.
A curved glass visor slides out. http://www.faberonline.com/velvet.html

Hth.

Here is a link that might be useful: general web search for slide-out-range-hood

@p111og: As usual very helpful in your post and i think you... will stick to the 42 x 27 hood. As for MUA anything above 600 requires to be mua and thats a lot more money obviously and a newish house is almost impossible to install but doable... so i am sticking wiht the 600cfm.

My current kitchen is horrific for venting so lot many times when i do cook foods with heavy odours it usually takes a while to get rid of the smells, so i try not to cook too much of a smelly food.

Having 600cfm would be good for me and failing that i will have to invest in a MUA.

@davidro1: thanks for the input. Intresting venting you have going on.
For me i want to have the SS canopy as it will add into my kitchen's "look".

While local codes may specify a requirement for MUA system installation for a given rated CFM, the functional requirement is to keep the house pressure from falling enough that any combustion appliances (furnaces, hot water heaters, gas dryers, fireplaces, etc.) do not backflow. Backflow can begin to occur for very slight negative pressures, e.g., -0.02 inches of water column. So it comes down to what utility you might have as a combustion effluent hazard, and how much house and window leakage there is to offset the pressure drop from running the ventilation. (Note that combustion appliances that are sealed off from the kitchen part of the house and which receive separate MUA are effectively absent and not a hazard in this context.)

If no oil or gas heaters are in use, and no fireplaces are expected to be operated when cooking, then MUA only helps the kitchen ventilation flow the cfm that you expect. Otherwise, we are in the territory of safety, and arbitrary requirements for MUA vs. CFM should be considered arbitrary.

One should also understand that there is always MUA. What actually flows out has to be replaced somehow. With no MUA system installed, the house leakage CFM equals the actual ventilation CFM, which may be much less for a tight house than the rated CFM.

kas

@kaseki: Good point.. since all the new homes are "energy star" rated i am assuming they are fairly tight in terms of how much air is "leaked".

I am not sure how to come up with a cfm number for the house that will be vented in my scneario but a quick maybe to have a crack of window opened in the kitchen area to have a bit of an airflow?

yes, to leave a window open is better than not.

Oct 9th:
"Without going overboard praising induction cooking I will say everything people have mentioned in terms of lack of heat loss, efficiency & cleaning of these cooktops have been very humbly stated."

Here is a link that might be useful: Induction vs. Gas

More than a crack is needed at the window to flow any significant cfm in a tight house. Unless aided by a strong wind, the window aperture should be of the order of the duct area, at a minimum, and larger if screened. Otherwise, one has to accept reduced vent cfm as the house interior pressure drops.

The opened window should be such as to not cause cross winds near the hood.

If it is cold outside, opening several windows more than a crack in several rooms will allow the heating system to have a better chance at keeping the interior temperature tolerable. (Room heaters are not sized for massive air replacement rates. Kitchen venting requires 100k or more btu/hr of air heating, whereas baseboard hot water heaters, for example, are closer to only 1k btu/hr per foot.)

I don't know scaling for hot air furnaces, but if that type of system exists, then it should be relatively easy to add MUA, albeit with possibly insufficient air heating rate for the higher cfm demands. After all, a tight house is intended for reduced heating demands, and the furnaces are scaled appropriately.

kas