In the interest of brevity, (for prolixity search this forum on keywords such as plume, effluent, MUA, hood, etc.), here are a few maxims:

• The air velocity at the baffles has to be high enough to keep the rising and expanding cooking plume from reflecting down and out of the hood. This value is proportional to the velocity of the rising plume, which is proportional to the temperature of the cooking surface (for large surfaces), and of the flame combustion product (for small surfaces, large flames, and obviously gas cooktops). I recommend 90 ft/min for general all-purpose success, but with modest induction cooking, you might get away with only 60 or 70 ft/min. Feet per minute is equivalent to CFM per square foot of hood entrance aperture.

• The hood entrance aperture should overlap the pan surfaces by enough to encompass at least a 10-degree half angle from the contour of possible surfaces bounds (except where constrained by a wall). Obviously, as the hood height is increased, the size has to increase to meet this angle.

• From the above two results, the required total flow rate (CFM) is calculated.

• Duct diameter should be such that the velocity of the air inside is in the 1000 to 2000 ft/min range to minimize grease deposition. The higher end of this range may be best in warm climates.

• Blowers are usually rated for flow rate at zero static pressure, i.e., hanging in the air. In a ventilation system there is restriction that causes pressure drops (called loss). As pressure loss increases, flow rate decreases according to the fan curve of the blower. The CFM rating should be some multiple of the CFM required. The exact multiple depends on many things, including the fan curve, different for each blower, and MUA restriction. I recommend using a factor of 1.5 to be safe with open windows for MUA. (Note that screens will have a pressure loss depending on the open window space.)

I will skip MUA maxims in this message, as open windows in southern CA should be adequate. Please search on MUA to find out about hazards of low interior house pressure with combustion appliances.

kas

I installed the 30" Miele induction a few months ago. I oversized to 36" and used their Ingognito hood, which is 625 cfm. It's been working fine, but I think that's the largest size. I'd put in a 42" over a 36" cooktop if you can.

The short answers are as follows based on how you claim to cook

42" is better than 36"

600CFM should be appropriate granted its a relatively short run and doesn't have any serious turns/curves. If its complicated add a little power.

Given that the miele cooktop will be a total depth to the front of the appliance of roughly 22-23" from the wall (instead of 24-25 like a range). a 22" deep hood would be acceptable if the other conditions are met. 24" will be better...but if its problematic don't stress the 2."

The keys to good hood performance for most is HOW you use them. Turn it on low early, before you start cooking even if possible. This creates positive air flow that most smoke/heat will follow. If you wait until you've burned something to even turn it on...you've lost the battle already. Keep your filters clean. Cook on the rear burners whenever possible.

MUA depends on area, install and inspector. It's on the books in NY but its enforced 50% of the time or less...its really very tricky. If you do have to follow it, I recommend an Elica or Zephyr as some of these can have the power lowered slightly to meet requirements if you get into a messy situation.

Thanks all, at least I am now armed with a little more info when I talk to my contractor. There certainly is a temptation to put in a 400 cfm hood, open the windows, and be done with this. Oh yeah I have about a 7 foot horizontal run of 8 inch ducting inside a 12" soffit ....