Fan down, its how its designed to work....it should pull out hot air as it rises from the cpu and spit it out the back....
Does your psu pull in(from the back?) air instead of pushing it out?
All power supplies should
exhaust hot air from the case (and themselves in the process). Never heard of one that blew air in.
Anyway, you can tell which way a fan will blow just by looking at it. Case fans always blow towards the fixed part of the frame where the motor is attached.