I have a big, heavy duty chimney fan that I want to install to get a steady, even draft. Who has experience with a power failure while such a device is operating? I know there is room beneath the fan blade plate and the expanded metal at the sides for smoke to escape naturally, but I have a situation now with occasional puff-backs and I want to make this thing as safe as possible with no possibility of soot and smoke in the house if it stops. What if I add some angle-irons to the frame to make it sit higher above the smoke exit. Would that be O.K.?