It is the internal aperture that matters. Because the rising cooking effluent expands as it rises, the hood needs to overlap the areas of the bases of the pans doing the cooking. This leads to the 3-inch rule of thumb, but that is only approximate, as the fact of the expansion angle means that the higher the hood the larger the aperture has to be for full capture. While the expansion angle varies with what is cooked on what surface at what temperature, a nominal value of 22.5 degrees half angle can be used measured from the edge of the heated part of the pan (not necessarily the edge of the cooktop). This value may be found in some reports referenced in my Clippings for commercial cooking devices. I observe lower dispersion angles using a wok on a Cooktek wok hob. The 22.5 degree angle will usually lead to a larger aperture than anyone will tolerate in a residential setting, hence the 3-inch rule. To the best of my knowledge, inserts and liners are terms that mean the same thing. One purchases what is essentially a hood that does not have a finished outside surface - a liner. This liner is covered by (typically) cabinetry, making the visual hood larger. But it is still the internal aperture that has to collect the effluent. Hoods do a very poor job (at any air flow rate anyone could stand in a home) at pulling in effluent from the sides, hence the need to overlap the pan areas. Capture from outside the aperture will be only a few inches at most, and that requires high specific flow rates -- perhaps at least 90 cfm per square foot of aperture. 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!

Those vents are for a recirculating unit. If you are venting to the outside you don't need the vents.

Last year, I made the same switch you are now considering, so I know how much there is to learn about kitchen ventilation! I felt like I was studying for a Ph.D. in HVAC!! It was extremely disappointing to discover how very little most HVAC & appliance professionals in my area know about this subject. I literally had one HVAC company tell me, "You just pick out whatever color hood you like, and we'll hang it. But we don't run ductwork for hoods, because all you got to do it buy a recirculating one." If it weren't for @kaseki & @opaone, I might've believed him, but at that point I had learned too much to accept that answer! Keep reading their threads on Houzz. They'll steer you right!