If you're planning a 30" cooktop & island hood, I think 600 cfm should be enough. But if you decide on a cooktop that will generate more heat/smoke/splatter than a 30" unit, you will need more cfm. Make sure the exhaust intakes are directly over or as nearly directly over the cooktops as possible to maximize the overall benefit.

Not an expert, but did just buy an island hood after much research and a lot of help from this forum. There are a lot of great threads that are worth going through regarding this. An attempt to summarize some of what I learned - It does also depend on:
1. whether you are going to do a lot high heat sauteeing, grilling or deep frying which generate more fumes and may require more CFM.
2. How wide your hood is, I believe that at 3" wider than your range on each side is recommended, and 27" deep will give you better capture than 24".
3. Recommended height from cooktop is 24 - 30" - If you are going higher that 30", more CFM are recommended.
4. How high is the kitchen ceiling? How many feet do you have to go from the kitchen ceiling to outside? Is it a straight shot, or are there bends? (basically, how far does exhaust have to travel)

Also, Don't forget to check if you will need make-up air. Best wishes!

600cfm is not enough for island hoods. Island hoods must be over sized and over powered to perform.

This is going to be helpful for me. Thanks chefkev for summarizing what you have learned in layman's language. I've read some terrific and technical threads and felt a little overwhelmed. Would 600 cfm be enough for a 30" induction cooktop with 8' ceilings? It would be for an island installation and a straight shot up through the roof single story home. I don't deep fry, grill outside (Florida living), and basically saute veggies (garlic and oil... YUM!) and some fish though usually I bake my fish. From what you've written I understand I'll need a 36" wide hood that is 24" - 30" above the cooktop. I'm not a big fan of hunks of hood coming down from the ceiling so it's going to take some searching to find something I like. I wish the downdraft systems were more well respected. Asthetically, they would be more to my liking but I've read they are not nearly as functional. As I plan this remodel (whole house) I'm trying to remember to work it out so function will trump form.

The key factors for grease and smell removal, whether residential or commercial, are capture and containment. The hood has to be large enough in aperture to capture, and the hood has to be deep enough (top to bottom) and flow enough cfm for containment.

Island hoods have two "problems." One is aesthetic. Instead of being on a wall, they are in a living space protruding from the ceiling. This makes people want to minimize their size when the other "problem" -- described below -- drives them larger.

Island hoods do not have a wall behind to direct the effluent up to the hood. They do not have a wall behind to minimize the effects of drafts, and to minimize air disturbances from people walking about. As a result, it is easier for the rising effluent to miss being captured.

This leads to the need for making the hood larger in aperture area. If placed higher than the typical 30 inches, whether for better sightlines to the cooktop or for less obtrusive appearance, the hood aperture has to be even larger. To keep the effluent in the hood after capture, the air velocity has to be sufficient, so the larger the hood, the higher the cfm needed.

Bottom line is that the island hood may need as much as 1.5 times the cfm of a wall hood performing the same duty.

If you go to the Subzero site, click to go to the Wolf part, and then go to the ventilation hood pages, you should be able to find the design guide. Near the back of this pdf file are exhaust fan recommendations by hood type and cooktop type. These nominal values should give you a starting point from which you can work as you size your hood for your conditions.

Otherwise, you will have to study cooking ventilation theory, a bit of which I have referenced in previous posts based on having to do this myself.

kas

When I said on this thread the other day that 600 cfm was enough, that was assuming that, with a 30" cooktop you were planning more or less "average" home size meal preparation, and that you were not planning restaurant-style 36" or 48" cooktops with burners that could smelt iron ore. On that assumption, 600 cfm is okay for a 30" island cooktop, which is twice the 300 cfm that such a cooktop would need if it were against the wall.

With 600 cfm, and especially with more than 600 cfm, you should consider that you may generate negative air pressure which you will have to compensate for either by low tech means (opening a window), or by adding a fancy air intake system that brings in fresh air as you run the exhaust, like the ones they use in restaurants. If you plan to run a catering service in your home and plan to prepare several dozen meals per day, you'll should consider such a system.

FWIW, I agree with others regarding oversizing capture area and CFMs in an island configuration. Also, 600 CFM usually is not enough to require make-up air. My last kitchen was 900 CFM with no make-up air with no problems at all.

remodelfla - My home is very similar to yours, 8 1/2 ft (1/2 ft hight than you), single story, straight shot through roof. An "expert" at ABT took these measurements and recommended 900 CFM for me, but that was with knowing I do a lot of high heat cooking. I am planning on practically smelting ore so I went an oversized 42" 1200 CFM hood and make-up air.

How may BTU are your burners - are any of them power burners (15000 or more I think)? I don't know as much about the actual science, but I agree with haus_proud that if you don't do high heat cooking on power burners, that 600 cfm with a 36" hood will work just fine for you. A 27" deep hood as opposed to a 24" one would be a definite plus.

If you have an 8 foot ceiling, depending on your height, be very careful that hood you buy can be mounted high enough so that you're not worried about whacking your head! I just came back from having my hood installed at 32 3/4 inches. (My Best by Broan manual says that for every inch over 30" to add 33 CFM.) That was the maximum it would go up, and that was with my installer drilling new holes, any higher up and they would have needed to shorten the actual metal sleeve (not recommended). Best luck to you and kandkwi on your hood decision.

I'm 5' tall and my DH is about 5'10". Obviously, at my height... I don't have to worry. I do 90% of the cooking, though it would be OK with me if DH did a little more! That poor man though manages to bang his head daily on just about anything within a 2' radius of him. (he has a personal relationship with the light fixture that hangs about our kitchen table! ) Once we get closer to being in the house we'll measure to be sure.

Here's a question... I actually was going to start a new thread on this and still might. In a moment of... I don't really know what to call it.... he bought this HUGE fan for our outdoor grill that he planned on installing at our current house. He was going to install it on our flat deck over the patio area of our patio/pool. Now we're not staying here and he wants to use it as the fan part for the hood over the cooktop at the house we're remodeling. WHen I say HUGE, I mean restaurant size application. The thing looks like Sputnik (as I've so fondly named it) and goes to 2800cfm. It's crazy... I want him to cut his losses and sell it. He says we can use it on the lowest settting. Would this be too stupid? I'll post a pic when I get a chance. I don't yet understanding how it works with the interior portion of the hood. I tease him that our dogs (2 dauchsaunds) will have to run around with weight belts on or they'll get sucked out. :-)

remodelfla - strongly recommend against sputnik. Will be incredibly noisy and probably void hood warantee.