PSU Fan direction

[Sarel_]

Does it really matter if the fan of a bottom mounted psu faces upwards or downwards and why or why not?

 

[Suztera]

If your case sits on a carpet floor and the psu fan is facing downwards, you're risking damage to the psu.

 

[nostall]

Generally want it facing down so that it is pulling in cool air from under the case. But Suztera is absolutely correct; if the PC sits on a carpet there is the real risk that the carpeting will block airflow and you may overheat the PSU: If it will be on the carpet, mount with the fan facing up. It will be pulling in slightly warmer air from inside the PC (depending on your case and quantity/positioning/rpm of your case fans, but will still get enough 'cool' air to run at proper temperatures.