I got the 10cfm as well. Figured if I ever upgrade the pit to a 22.5 I won't have to upgrade the fan. I almost never damp mine down, the guru knows when to apply the blower, it just does it in shorter bursts when the damper is wide open. I've never had a problem holding 225 with the damper wide open...but I do have all the other vents and the door sealed off with high heat tape so the only air getting in is through the blower.