I suspect the 0.4 factor is intended to account for the fan/blower performance in realistic conditions, particularly conditions that correspond to the small amount of MUA that the formula requires. My comments here have usually assumed a factor of 0.67, but that approximation assumes that imperfect MUA won't lower the interior pressure more than 0.1 inch w.c., there is another tenth lost in the baffles, and a commensurate amount in the hood ducting. One-tenth negative pressure, however, is too much for combustion appliances with air intakes that aren't isolated from the kitchen, or for fireplaces. Note that the Wisconsin formula listed by the OP is for cases without such combustion appliances. If there is an intent to filter the MUA duct for anything smaller than a bat, I would at least taper out to a largish filter so the filter pressure loss will be minimal. A smaller sized furnace filter would do, or a window screen if large enough (perhaps two square feet). When arguing for a four-inch duct because it can move X CFM, one has to consider what pressure loss will occur at that CFM. I think six-inch would be better in this case unless the duct length will be very short. kas

Sorry it's taken me awhile to get back here. Thanks for all your responses....some very helpful info here. Angie_DIY......Here is a link to the GE hood we're considering. After reading the manual I realize that the charcoal filter is only needed if you're using it as a recirculating fan. We won't be. There is also a grease filter that is washable. http://products.geappliances.com/ApplProducts/Dispatcher?REQUEST=SpecPage&Sku=PV970NSS michoumonster.......that makes sense, and I certainly would not be using the fan on high all of the time. But some other comments have mentioned that code often has a limit on how many CFM's are allowed. We're moving to a small little community where they are sticklers about code (we've had fits with them on several other issues), so we'd better check to see what is allowed first. GWlolo.....Yes, I've read that the hood should be slightly wider than the cooktop. Ours won't be unfortunately. We have a 30" cooktop and a 30" hood, but I guess the range hood will extend beyond the actual burner area, so I think we'll be okay. I have to confess that until recently I didn't know that range hoods removed odors from cooking. For years I had a Thermador down-draft cooktop, and the fan was so loud that it honestly sounded like a jet engine taking off, so I didn't use it except on rare occasions. It's nice to know that in my new kitchen I'll be able to get rid of smells like brussels sprouts! A2gemini.....I agree...that's the only way I'll make bacon anymore, and I just learned this method about a year ago. BTW, we do live in a cold climate, and we do have building codes to contend with. But I didn't realize the hood CFMs would have to be considered. See above where I talk about this. I guess I have some checking to do. WileyBob.....I agree about Thermador....I never liked my down-draft, and I had it for about 20 years. I finally replaced it with a GE Monogram cooktop. I like it much better, but I still rarely use the down-draft. I'm looking forward to having a normal hood above the cooktop. I won't need anything as powerful as you need with your Wolf though. I've heard great things about the Wolf cooktops. Detroit burb.....I wish I had read your information before I started my research on range hoods. Good info there! The thing I've had the most trouble with is finding a 'not-too-high' CFM rated range hood with a low sone rating. We need something not too bulky looking, yet we still want a decent performance. So I think we're just going to go with the GE curved profile style in the link I've posted above. It's noisier than I wanted (7 sones on high), but I'm sure we won't have it on high for long periods of time. My husband and I did your calculations on CFM vs. room size, and I think we'll be okay with that one. Thanks for your help! artemis 78.....I also like the Kobe's, and we were considering going to Cincinnati to look at them (they don't sell them where we live), but I fear the building inspector (they really do inspect here) would think the CFM rating is too high. But I'll check on this.....glad to know you like yours. We're moving to a modest little home (we plan to spend most of the year in our RV), and I'm sure we already have the best kitchen in the neighborhood. We plan to stay in the house forever, so we're making it as nice as we can within reason for the type of house it is. Thanks again, everyone!

Contacted the owner of the local major hvac company. He cleared me for not needing make up air. 1888 home. If we ever sell the house in 30+ years and it’s a problem we’ll deal with it then but I’m not going to worry about it now after a general contractor, Aitoro installer, and HVAC person all said we’re good. I’m not taking this lightly as this is a safety thing and expensive, but I’ve done my due diligence for my home and I’m doing everything based on the recommendations of professionals.

We are in need of ventilation for our 31.5" French range. It's not a flame thrower like some newer ranges but we cook most of our meals and often at high heat settings for meat and vegetables. We only have 19" between the bottom of the hood and the ceiling. We have a structural beam running right above the center line of the future hood. This leaves us two options: 1) we can do a double 90 immediately out of the insert--from there it is a 3' straight run to the roof, or 2) we can have the insert or hood custom made to have the duct exit offset 10" or so to clear the beam. This would leave us with a straight shot to the roof. Modernaire charges $240 for the custom work. This leads to my first question: In your estimation, would you get more performance from a 400cfm motor with a straight duct run or a 600cfm motor with a double 90? The cost difference between the 400 and 600 is about the same as the custom charge. The hood will be 36" wide. Second question: If we buy an insert now, can it be installed while we decide what we're going to do about the canopy/surround? Thank you.