Originally Posted by E-Peen
A few posts back I explained it, but I'll do it again
If you have your PSU upside-down for better cable management, it's going to exhaust out of the bottom of your case obviously since your fan is going to be upside down.
Since the fan is exhausting from the bottom of the case, you need to elevate your computer for the air to exhaust and for it to get air, period. This is where your case feet come into play. In most cases carpet isn't bad but it's not great, however, in most cases the case feet will sink into the carpet providing no elevation, which equals no exhaustion, and no air really.
Basically, you're choking it of air/not allowing any air to exhaust.
The fan is intake on PSUs unless you modified it to be exhaust. Hence why there's a honey comb mesh at the back. Positive air pressure will force the hot air out through it.
The only disadvantage depends on the carpet. I have a very thin layer so I could more than likely put my case on the carpet and it will have the same clearance as wood, maybe a little bit less but not enough to make a difference.
If I was to put it in my living room where the carpet has more depth to it and is more soft, then the intake would be blocked completely, so the PSU cannot suck in cool air.
If you have a top mounted PSU you shouldn't have any problems, same if your PSU fan is sucking in air from the inside of your case rather than the outside.
As for the OP, although I just said my carpet was thin, what you could do is get an old box an flatten it. That's what I did with my HAF922 (was too big for my desk) or you could just go mATX
mATX = FTW