I'd assumed my PSU's (OCZ StealthXStream 400W) 140mm fan was an intake fan and was therefore planning to filter it's intake on the bottom of my case (or fit it fan-side up). Having just replaced the fan (with a very quiet Noiseblocker PK2), I was surprised to find it's actually an exhaust fan (yes, I was careful to fit the fan the same way as the old one!) which makes life easier and I can use the filter I purchased for the front case intake fan instead.

So is it normal for PSU 120mm/140mm fans to be exhaust? I just assumed they'd use the large fan to draw in the air and push it out the vents on the back of the PSU next to the power switch. I guess most cases would have the large fan pointing outside the top or bottom of the case these days but surely some cases would still have it blowing down heat on the CPU, which doesn't seem like a very good idea?

The orientation of the original fan was an assembly error. PSU fans should be mounted as intakes, with the fan blowing air over its struts into the PSU and out of the rear exhaust grill.

I'm not so sure as on the inside of the part of the fan nearest the rear/power switch it has a piece of plastic blocking it, which makes sense if it's drawing air in the rear vents to prevent it going straight out the fan, thus forcing it to flow over the far part of the board before exhausting out the other half of the fan but not if it's meant to be an intake, in which case I'd expect the plastic to be on the outside of the fan to force air only to be drawn in by the furthest part of the fan. Having the plastic where it is if it was meant to be an intake fan would seem strange, as then it would be drawing air in that half of the fan only for it to hit the plastic at the bottom.

As I said, with many cases (including mine) the fan is blowing directly out of the case (the bottom in my case but some will be the top) so perhaps they had that in mind when designing it and didn't take into consideration the low-end cases where the fan would be blowing onto the CPU.

That plastic piece on the fan is quite common, it's designed to stop short-circuiting of the case exhaust fan when the PSU is mounted in the traditional top position. There are two good illustrations of this arrangement and what your fan setup should look like on this page of the SPCR review of the Kingwin Lazer Platinum 550W http://www.silentpcreview.com/article1203-page2.html (the fifth and last pictures).

Yeah, I think you've convinced me. Pretty shoddy quality control by OCZ then!

However, as the PSU in my case will have the fan facing out of the case, I wonder if it might not be better as it is. If I use it as an intake fan I'll have to get another filter as originally intended, which isn't a problem but obviously the filter will reduce the airflow and thus might lead to the PSU running hotter. If I run the fan as an exhaust, I won't need a filter on it, although I guess then it's going to draw dust in the rear vents along with the air and if I'm going to do that it would make as much sense (from a dust point of view) to run it as an intake with no filter. The other option is to have the fan facing up running as an intake, drawing air from within the case which would eliminate the need for a filter but obviously the in-case air will be a lot hotter than the external air so that probably doesn't help either.

It still seems like it would make more sense to have the plastic piece on the outside of the fan rather than at the bottom in the PSU if you want to prevent that half of the fan drawing in air but I guess the engineers know what they're doing

I just opened the PSU to flip the fan over and found that it is already the right way round, with the wires and label on the inside, so I have to take back my comments about OCZ's quality control.

The reason I thought it was the wrong way round (exhausting) is because I could feel a draft when I put my hand over it but maybe that's normal even when a fan's working as an intake?

It might be worth removing the piece of plastic designed to prevent short-circuiting and seeing if that makes a difference.

Yeah, maybe that's causing blowback although I could feel a breeze from both halves of the fan, so maybe the air bounces off the circuit board anyway. The only thing I'd be concerned about removing that is if it might cause most of the air to go in that side and out the rear vents, causing the other half not to get as much air over it.