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

The short answer is: Yes there are formulas. I have posted the references in some threads (maybe now timed-out) in this forum. Search online for "Thermal plumes of kitchen appliances." Rules of thumb can be found in the ASHRAE HVAC Application handbook, Greenheck's website, and Wolf's website, to name a few. They don't necessarily agree, and most are intended for commercial application. In any case, you may not like their implications re your present hood plan. But like audio equipment, significant improvements cost exponentially more significant funds. A much longer answer would require interrogating you on cooking style, range type, sensitivity to odor, tolerance for grease condensation on walls, etc. The two key concepts are capture and containment (which implies removal). Higher mounting makes capture harder while improving visibility, lower makes capture easier but degrades visibility (and may lead to hardhat requirements). Wider and longer make capture easier. Cfm and other hood features such as depth determine containment success. Cfm is not so important to capture because the air flow velocity drops very quickly as one measures farther away from the hood. kas

Aesthetics aside, 30 inches from stove top (not from top of grills) to vent hood bottom is the minimum height typically advised for the bottom of the mounted vent hood. Vent hood height may be higher if your range/cooktop manufacturer stipulates so and you plan to avail yourself of the warranty. Wood mounted over stainless brings a new considerations into play: stainless is far less combustible than wood. Height matters. Two vent hood surfaces, one slightly higher, yet close to the lower vent hood, may result in a non-vented grease pocket on the wood vent undersurface, require nimble fingers to clean if even possible, and pose a fire hazard which is why the motor system is a main component of the kitchen ventilation system. One other thought is weight limit on your back wall mounting surface with two hoods, as that's a lot of weight on the back wall. It's good to ask and do your research first. Hope this helps.

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.