i have a 8x24" vent 6 inches from the bottom with an ultra allergen filter across the back of it. Thats why the vent is so big, since the air will come in more slowly because of the filter. On the ceiling of the cab is a 7 1/2" hole with a 7" fan that I took apart for the motor and blades. It has a high and low setting like most household fans. I used it assembled on my last grow as a vent fan and it seemed to work just fine. Then again I was using 2 70w hps lamps in a different cab then too.