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...