Rule of thumb is total BTU divided by 100 equals total CFM. That means if you had a range with 4 18k BTU burners, then you should plan for around 720CFM (18000 x 4 / 100). Of course you can just take the kBTU and multiply by 10 (18 x 4 x 10) to reach the same number which is easier math. That said, it's just a rule of thumb. Total CFM required depends on the type of hood you use, your install situation, and the type of cooking you do. Long duct runs reduces the net CFM rating of the hood so a 600CFM hood might only be moving 400CFM of air if there is a long duct run or many bends. If you do stirfry, indoor grilling, or heavy saute, that would up your spec a bit as well. Also, nearby windows and doors that are likely to be open during cooking can be a source of drafts that can carry smoke away so they need to be considered as well. Typically, get the most you can afford and can fit in the space. Sometimes getting a larger hood to increase capture area can actually do more than increasing the CFM. Some would argue that since I never have 2 burners going at the same time that it's ok to have a smaller hood which is reasonable but does reduce your buffer. IMHO the "rule of thumb" already accounts for less than full capacity usage since I doubt 900CFM would be enough if I had 4 full power saute or stirfry pans going. In other words, I would not advise going lower unless you think your usage will be less than "average". Good luck.

As I read your situation you'd have the xost going up then making a 180ú down under then probly another 90ú to horizontal.Something like that anyway. Be sure to size the duct diameter accordingly because each bend will be equivalent to several feet of straight duct. For example two 3 foot straight sections connected by a 90ú elbo might be equivalent to 10 feet of straight pipe. So you'll need to figure the equivalent straight line duct length then size it's diameter to reduce resistance. The instructions with the hood should provide enough info for you to figure it out. A 6 inch dia pipe has 44% more area than a 5 inch dia pipe. 5 inch is pretty much standard so watch it. If the pipe is too small the hood won't work for diddly. Don't count on the contractor to be able to handle that horribly advanced mathematical concept. Don't. We have a downdraft type cook top with 5 inch pipe. I shoulda been 6 inch. Total length is about 15 feet but the equivalent straight length is roughly 25 feet. It runs thru the crawl space and xosts to the outside. Most grease stops in a trap but the fan and housing collect a lot over the years. Not much if any goes into the pipe. It's a centrifugal fan so the grease gets thrown to the housing instead of being impelled along the duct. If your fan is not centrifugal that may not be true.

Best is to do it right and start from scratch, so to speak. However, in the interest of saving brick work, there would be some effectiveness if you (a) removed the blower and put a serious blower on the outside (sort of a side draft configuration) and then (b) put an enveloping hood without duct interface over the cooktop at, say 30 inches max from the hood bottom to the cooktop. It could have lights and fan control that would be wired to the exterior blower. The effect of the hood would be temporary capture, and if the breeze past the bottom was adequate, cooking plume effluent would tend to move to the wall port rather than into the room. You might test this with a cardboard hood shape and the existing blower with light steam generation at the cooktop. The existing blower is probably unable to deflect hot grease particles. (c) The interior wall space from the existing blower port to hood might be used to provide a larger vent area by co-opting some of the inter-stud space. This would require sheet metal covering all combustibles, preferably spaced a bit. (Assume a grease fire.) And on that point, with or without any inter-stud refinements, lack of proper grease capture requires easy access to the duct to allow cleaning. For the outside blower, one of the Broan/Wolf/Abbaka roof blowers, mounted on the wall, will blow downward. Some wall cleaning will be needed. If sightliness is not an issue, then a mini upblast blower could be used blowing outward and the wall would stay cleaner. This scheme will be loud at full power, as will any hood with internal blowers. Best is to go upward to the roof, use a roof blower, and insert a silencer in between.

Yes, indeed, the stove is on an interior wall. It would be simple if it were on an exterior wall, but a remodel to move it is out of the question. I am very happy with the current placement of all appliances and sink and storage. The stove currently vents down and under the floor out to an exterior wall. I will seriously consider induction, just wish I could get it with down draft. I have checked my pans with a magnet, and not one pan I have works, other than my numerous cast iron pieces. All the comments (especially the links that show the process) give me a lot to think about. I have to keep some kind of exhaust as I occasionally make a batch of lye soap. I hadn't thought about a portable coil for countertop, but it would allow tea brewing in the 2 gallon pot and would accommodate the steel pressure cooker. I am not sure I would trust the large heavy pressure canner on it. I could always visit a friend to can! Guess I would find out who my real friends are.