Not sure about this: Instead of running the 120mm fan off of the PSU's fan controller, attach it to the mobo's fan controller. That way the speed of the 120mm fan is dependant upon the internal case temperature. Basically, the PSU's 120mm fan becomes the exhaust fan. This has the benefit of allowing you to run the fan at a higher voltage than a traditional 120mm exhaust fan because it is inside the case (i.e. less noise) as opposed to being attached to the back of the case. What about the problem of cooling the PSU with warm air from inside the case? Simply open a hole on the front of the PSU (where the wires are coming out from). Attach a 80mm fan over the hole and connect it to the PSU's internal fan connector (i.e. the one the 120mm fan was initially connected to). Run a duct from this fan to the front of the case (through the 5 1/2 bay) or to a top mounted blowhole (which then becomes an intake hole). I believe the front bay duct has been covered a few time in the forums. The top intakehole idea may work a little better as there are cases that alright employ this idea for exhaust. Modding a case for this would also be fairly easy. The 80mm fan would draw in the colder outside ambient air to cool the PSU whenever the PSU complained about being too hot. I believe the Nexus NX-3500 would be ideal for a mod like this.What's the best practical way to use a 120mm PSU in a hot system to maximize cooling without allowing the fan to speed up beyond minimal?
This setup wouldn't be any louder than a 120mm exhaust fan/80mm PSU fan setup because both fans are controlled by the same fan controllers in both setups. The ability of running the 120mm fan at a higher speed because it is internally mounted I believe would compensate for the additional work required to push air past the PSU internal components vs. blowing directly out of the case. As well, the 80mm fan is now on the inside too, pushing cold air over the PSU vs expelling warm air out. As a result, this fan could also be runner a little higher without the expense of increased noise.
The biggest issue in this setup would be the "collision" between the air that the 80mm fan was blowing over the PSU with the air being blown upwards by the 120mm fan. If there is anyone here who knows something about fluid mechanics then perhaps they could shed some light on this.