First time building a PC. Manual says to mount it so the fan faces upwards due to its 'fanless' operation, but an article on here recently said that was a bad idea.. I guess I should follow the manual if that's what it says, but I'm a bit paranoid
A fanless PSU will not exhaust the heat to the case exclusively. If you had a fan the fan should exhaust the heat from the case. But follow the manual until you prove that you can get better cooling. In other words, experiment with it facing up and down and see which is better.
The manual says to mount it so the fan faces upwards due to its 'fanless' operation mode because heat rises and needs to escape the power supply unit's case.
If the fan faces down the heat gets trapped against the circuit board side of the power supply until the temperature rises high enough for the intake fan to kick in and bring cool air into the power supply's case.
Heat decreases the efficiency of the power supply and the life of electronic components so you don't want heated air lingering inside the power supply.
I hope you realize that my explanation is for a case that uses a bottom mount power supply unit.
If the case uses a top mount power supply and there is a vent in the case located above the power supply then my explanation also applies.
It is when you use a case that uses a top mount power supply and there is no vent in the case located above the power supply where you will have to flip the power supply so that the intake fan faces down.