At the heart of the ventilation system is the fan(s) it uses. An axial fan looks like a ceiling fan, while a centrifugal fan resembles a squirrel cage. A centrifugal fan moves more air than an axial unit does, and is better suited to long duct runs. However, an axial fans is less expensive.
The amount of air the fan can move is measured in cubic feet per minute (cfm). The Home Ventilating Institute recommends a minimum of 40 cfm for every linear foot of your range. That means a 120-cfm unit should be just enough for an average-size range. For a downdraft unit, that figure jumps to a minimum of 150 cfm. A remote-mounted motor, whose fan is located at the end of the ductwork rather than in the canopy so it's less noisy, also requires a higher cfm. How much higher depends on a number of variables, with length and layout of duct runs being the most important. Figure up to 400 cfm for a wall unit and as high as 600 cfm for an island cooktop.
From This Old House.
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