Psu (bottom mount) fan direction

If you have dust filters below your PSU then its really your choice, if its pointed down the PSU will stay a bit cooler, if its pointed up it will help the case stay a bit cooler, the effect on the PSU is pretty insignificant, the effect on the case temp is also likely insignificant. Since you can, i would probably point it down if you are putting your case on a wood floor or a desk, if its on carpet point the fan up.

Fan facing down at the filtered vents as the chassis designer intended. Just keep the vents unobstructed as @hunter315 said.

You may find that you don't have a choice; the screw holes on the back panel may dictate the orientation, and that may well be with the fan pointing down. As hunter says, it won't make a lot of difference, so I wouldn't worry.

The power supply fan is the intake for the power supply - air is sucked in through the fan and blown out the back. If the fan is pointing upwards it is sucking air out of the case, but it won't have much effect, because heat rises. It might affect a card in the bottom slot or two, or chips on the bottom edge of the motherboard, but it's unlikely to affect the main video card.

I forgot to mention that my psu has 135mm fan and the vent below is longer in length but width, so the area of the blades would be obstructed by case on two arcs of the fan. Do u (with this knowledge) think it should still be aimed down?

Personally I would aim the fan up, and that's what I've done with the last 5 machines I built with Antec 300 cases. These cases are vented pretty well and I add fans in the front too, so if anything it would blow air through the PSU and out the back.  

I think floor mounted or desk mounted is also important. Downward facing vents can get blocked by carpet and possibly clog from dust faster than the rest of the case. I would look at the case you have and determine which orientation will allow the PSU fan to get the most air and do it that way.

Depending on how sensitive a power supply's efficiency is to temperature it is always better to draw cooler air from outside of the chassis through the power supply instead of the warm air from inside the chassis.

Some power supplies' efficiency will drop with a rise in cooling air temperature. That's why reputable power supply review sites will subject the power supply to hot box testing to determine just what kind of loss in efficiency occurs.

Also some of the chassis designs with a bottom mount power supply isolate the chamber where the power supply is mounted from the rest of the chassis. It is also designed this way to prevent disruption of the general air flow from bottom-front to top-rear of the chassis.

Best answer selected by Geis.

This topic has been closed by Mousemonkey