One comment among those above that was passed over needs some expansion. Range manufacturers can only make requirements for distances to flammable materials (typically 30 inches), so their advice to the AHJ applies to cabinets and hood surrounds. It would wisely be considered relevant to hood motors also. Hood manufacturers provide (or should provide) height requirements based on capture and containment, along with a minimum safe height for fire resistance of any exposed grease on meshes or baffles (probably where the 24 inches comes from). The meshes and baffles are supposed to provide a firestop for interior grease. Most hood materials are stainless steel (very high melting point) except aluminum meshes (not so high) and wiring (degrades at fairly low temps). So one should think about having a grease fire on the stove top and what it means for all the materials within range of the heat and flame. kas

Given adequate capture aperture, effectiveness is based on having an air velocity at the baffles that is commensurate with the uprising smoke and water vapor velocity, about 3 ft/s for hot cooking. This requires at least 90 CFM/sq. foot over the aperture so that the baffle gaps see around 180 CFM/sq. foot. I find this adequate for wok cooking on my induction wok unit. I can imagine possible higher needs, particularly from 100k BTU/burner commercial wok ranges, but for induction I think the 90 value will suffice. Higher will pull smoke in from the edges of the hood a bit more, but it is a small bit. As to the question of what size blower to use, we need to consider a lot of things, but lets start with the 2 x 2.5 = 5 square foot aperture, and multiply by 90 to get 450 actual CFM. The baffle loss at these flow rates may be 0.1 in. w.c., and there will be other losses in hood transitions, duct friction, and imperfect make-up air (MUA) supply. The blower flow rate drops with pressure loss around the loop from outside to outside. These losses can require the rated capability of the blower to be around 1.5X the requirement, or sometimes even higher. So we start with the derivative assumption that we will need a blower rated for 450 x 1.5 = 675 cfm. For this case I would pick a nominal 800 CFM rated blower, if a compatible one exists for hood installation, in-line mounting, or roof mounting depending on other design and compatibility factors. A 1200 CFM rated unit will also work (given a reasonable fan curve), and provide more margin. Where larger than necessary helps is not particularly in effectiveness, but in lower noise from blade tip turbulence than a smaller fan running at higher speed to move the same amount of air. A 1200 should have a 10-inch duct, and if roof mounted with an in-line silencer, (which is about 14 inches in diameter) careful consideration of duct routing may be needed. An 800 may get away with an 8-inch duct and the silencer will be commensurately smaller. Ideally, duct size should provide a duct area such that the velocity of the air and effluent at the actual flow rate is in the range of 1000 to 2000 feet per minute. This minimizes grease precipitation on the duct, depending on duct temperature. I'm not going to calculate all of the flow rate vs. duct diameter options here, but it is straightforward to do. The big gap in this plan is whether the hood aperture sufficiently overlaps the wok that one is cooking in. If the wok is used on an edge hob, (which I think is all you get with 30-inch induction cooktops) then a 30-inch wide hood will not capture some of the rising effluent, which spreads out as it rises. Better would be a 36 inch hood, which in turn requires 540 CFM actual, or nearly 810 nominal. In my experience, one needs at least 10 degrees of angle between wok cooking zone edge and hood overlap, both to the side, back (on an island), and front. So when this is not easily achieved, then more CFM will help clear the room sooner, and may narrow the rising effluent angle a bit, depending on counter configuration, but will not allow full capture making up for too small of a hood aperture. kas

The key fact is that with the system* pressure losses, even with reasonable MUA, the actual flow rate of the 1500 CFM fan on maximum will be a lot lower -- perhaps only 1000 CFM. I once calculated for my 1500 CFM-rated Wolf/Broan roof fan, with silencer, hood baffles, hood transitions, duct, bends, and some slight house negative pressure would intersect the fan curve at about 900 CFM. (This calculation was done in 2007 and I'm a bit weak on the details now, but active MUA is likely easier done controlling for a slight negative value than trying for exactly zero.) I use 10-inch duct, and aimed for the higher end of the 1000 - 2000 ft/min range due to the duct being in a cold attic (at least in winter). You can find representative fan curves somewhere on the Broan/NuTone web site. Wolf will send you some if you ask the engineer you eventually get to. Fantech has curve tables on their web sit for their various in-line blowers. All commercial up-blast systems have fan curves somewhere. If you want to get an idea of how fan curves work, try this exercise for the student. Take a piece of graph paper (if it still exists) or print one from a blank Excel plot. On the abscissa (representing CFM) establish a linear scale of 0 to 2000 and put a point at 1500 CFM. On the ordinate (representing pressure across the fan, i.e., system pressure loss) establish a linear scale of 0 to 2 inches, w.c., such that the two axes are the same length. Put a point at 1.5 inches. Draw a diagonal straight line between those two axis points. Note that these values do not represent any particular fan, only the nature of fan curves. Take a protractor and draw a quarter circle, centered at the origin, between those two points. Real fan curves for blowers designed to be operated at various speeds between min and max without hunting will typically fall between those lines and be monotonic everywhere (no dips). You will see for any curve between those lines that as the pressure loss increases, the CFM realized will fairly rapidly decrease. The fan curve typically represents full power operation. At reduced power the curve shrinks toward the origin. Pressure loss is a function of actual flow, so the effective resistance of the system loop is generally approximated by a straight line intersecting the origin and a CFM value point where the pressure loss was calculated. Because the baffles may be the biggest source of pressure loss and the least documented, this whole process is more of an estimation effort than an exact calculation, even when real fan curves are used and real duct calculations are performed. kas *System means the air-path loop from the cooktop to the outside and back to the cooktop.

Sue Hawkins related 10Jan2019 and again 19Jan2019: "It is the world's easiest hood to clean as ALL the grease accumulates on the baffles." This cannot be true. There are no commercial baffle filters that collect all of the grease, and I doubt that Tradewinds discovered a secret airfoil shape to achieve it for residential hoods. Grease particulates contained in a cooking plume encompass a range of sizes. The larger are centrifuged out onto the baffle surfaces when moved through them at sufficient velocity. The smaller particles do not and ideally are intended to be blown out of the duct system into the outside air. Generally, some intermediate particles will impinge and stick to duct surfaces, as well as interior hood surfaces. These often (in residential cooking) form a very thin film that doesn't drip or flow. In commercial cooking where the rate of deposition is much higher, the build-up can be significant. This is why commercial cooking exhaust ducts require cleaning and inspection once a year. In the plot below, the range of particle sizes vs. what the baffle collects is shown for a particular baffle at a particular airflow for a particular cooking condition. When stringent particulate emission requirements are imposed on commercial kitchens, a series of filers is used, each type optimized for a particle size range. If grease molecules are to be treated, UV is one solution.