I have bought a Seasonic S12 600W and a P180. Now I'm wondering if I should mount the PSU regularly or upside down? I've tried to google this and been looking at the forums here, but I haven't been able to find a clear answer to this question.

That's probably because the general consensus is that it doesn't really matter which way you mount it. Putting it upside down will give you maybe a centimetre longer cables, while the usual way will let you see the pretty spec sheet -- that's about it.

kamina replied to my question: "I have the combo with an Asus A8V motherboard. The psu is upside down, and the cable fairly tight. It still fits."

It does matter, I learned that while moding small pcs.

OK. But what on Earth has that got to do with the P180?

Hmm... Seems it's not quite clear. One thing is the cable length, but I'm more worried about the capability to get reid of the heat. I'm thinking about covering the holes around the PSU, so of course I'm wondering if it matters which way I mount the PSU.

That was kind of my point -- theoretically, due to convection, there might be a tad less hot air circulating in the PSU chamber if the fan is on top, but in practice there's hardly any difference. It could also be argued that because the spec sheet is oriented the way it is, the PSU has been designed (consciously or not) to be working with the fan blowing upwards. Either way, the operating conditions of the PSU will remain more than tolerable, the lifespan of the unit won't be affected, and the HDDs will be cooled just as efficiently.

Moreover, have a look at this thread.

chill dude, it does matter which way you mounth the S12 in P180, if you do it upside down there will be a graeter space for the fan to suck in air in, and I learend this while installing a fan in a shuttle because when the CD rom was removed temps got lower, only because there was more space on the top of the the CPU fan. got that?

I think the connection you're making between the P180 PSU chamber and a Shuttle SFF barebone is really faint. The way I see it, the fan has just as much air to use whichever way you mount it. Either way it will be sitting vertically right in the middle of the space, and there's no CD-ROM or whatever blocking its intake. So no, I don't get it. I start to feel like we're talking about different cases here.

One thing that sounds suspect to me about mounting a 12 CM fan PSU upside down, is that the fan has to fight the rising heat.

Correct, upside down it will never be as efficient as when its mounted the way it's supposed to be.

It'd be interested to see it mounted normally, with the bottom of the case beneath the fan removed. I guess that would be ideal, if you'd filter the incomming air, because it would suck up loads of dust (and dust increases heat).

It's true that heat rises, and when a thermal cooling design is carefully designed to take advantage of it, convection can be enough to move lots of air. I used to think this was very important in a PC. But then, I started playing around with the gear--- and have kept playing for 5 years. I've also watched the results of many experiments by SPCR forum members with chimney convection schemes for PC components. (Try a search for chimney).

Now, this is what I know to be true:

Any significant movement of air using convection alone requires high temperature differentials, a large amount of heat, and a real chimney. None of these conditions prevail inside any PC.

A single slow spinning fan moves enough air to easily overcome rising hot air due to convection in a PC case. Period.

In other words, for all practical intents and purpose, you can forget about air convection in a PC. It can be almost totally disregarded if you use a single fan at low speed in the right spot.

In the case of any PSU in the P180, the PSU's own fan is more than enough to totally offset any "rising hot air" movement in that space. Remember, the PSU chamber is only ~6" high!!

Listen to the man. could take years to learn this.

OK, so I get the feeling that it doesn't matter how I mount the PSU in the P180. I guess I'll just leave it right way up and tape those holes around the PSU shut.

hmmm, it does. As said before there will be a greater space for the fan to suck the air in if mounted upside down. But as mike said, the fact that heat rises will not make any difference in such a small chamber, so you can still do it upsidedown.