My grow room is outside, in the backyard, on a covered patio. The room is 4x8x8. On the roof is a 740cfm 8" vortex fan. The intake is a 10" duct that goes out the back of the room and under the house. The duct goes about 20' under the house, on the end of the duct is a 12x 18x24 box with a 12x20...