I attached exhaust fans to the front grill on the inside of my case. I just wired them up to the grill. It really helps to quickly pull the hot air out of the case. I can't understand why KNC didn't do that in the first place. All PCs use a push-pull configuration for air circulation. So why not do the same for an equally hot ASIC miner?
I should think the hash-unit fans create enough positive air pressure. Do you mean you put fans on the back of the unit to pull air out? The front already has fans and it's the input, not output...To me, the back of the case is where the cords come out... pretty much like everything else that we plug in.
The back fans pull cool air in and the heat sink fans push that air through the fins of the heatsink. By the time the air has traveled though the fins it's hot and the velocity has slowed way down. The logical thing to do is try to exhaust that hot air as quickly as possible before it has time to recirculate through the case again, hence the front exhaust fans.