Hi, I’ve seen some cases with a design that holds the psu at the bottom. What’s the idea behind this design? As far as I know the hot air goes up not down.

You can mount the PSU upside down. Or you can buy a PSU that blows the hot air out the back, like the Silencer 750W.

Yeah. Exactly, cases like Antec Nine Hundred have a big fan at the top to take all the rising hot air away. It also keeps all the hot air away from the PSU, also I think it just makes it easier to build.

hot air goes up slowly, so a slight draft generally overcomes it, separating the heat sources will help to reduce the overall heat levels, and therefore reduce noise, and stress on components.

Thanks guys, now it makes perfect sense to me

Just one final note - many cases that are designed for a PSU at the bottom have a grill under it. This goes with a trend in PSUs to have a 120mm fan (or just an opening) at the bottom with the air blown out the back. This means that cool air gets sucked into the PSU from under the case and blown out the back. Maximises PSU cooling while also minimising the PSUs effect on the rest of the system

ummm another final note......if that's ok Kraynor?

If you are building a system with a PSU mount at the bottom of the case.....make sure your cables from your PSU are long enough to reach all of your parts, or you'll need extentions.

this can also increase the amount of dust that comes into the system.
something to be carefull for as well

Cosmos has this.

Is there any reason why mounting the PSU upside down on the bottom of the case would be a bad idea? The Lian-Li A70 I'm currently using allows mounting the PSU either at the top or at the bottom, I opted for the bottom, beneath my 8800GTX. I figured the PSU would help exhaust some of the GTX's heat since its fan is facing the GTX's case-side exhaust. There's 2 x 12CM fans at the top of the case.