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Shopping around for a cabinet for my alternate box, I've seen cabinet/case that place the SMPS/PSU at the bottom of the case. This is surprising because I'd have assumed putting it at the bottom would affect the cooling effort within the PSU.

Why is the power supply sometimes placed at the bottom of the case?

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up vote 2 down vote accepted

Actually this is primarily to address the cooling issue. The general idea is that you've got more open space around the bottom of the case, leaving the power supply more (and cooler) air to pull from inside the case. Additionally, at the bottom of the case, it won't be fighting with the processor for cool air quite so much.

Some cases also allow the power supply to be installed in either the top or bottom (or both) to suit your needs (for example, a high-end rig with multiple video cards might use two power supplies).

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I'm still confused ... wouldn't the bottom of the case be almost flush with the intake fan at the bottom of the supply? How would this improve cooling? – Everyone Apr 10 '11 at 3:48
The mount at the bottom of the case would be upside-down, meaning that the intake fan on the "bottom" of the power supply would be facing up. See for example this picture I took of a computer I built a few years ago: – Fopedush Apr 10 '11 at 3:50
Whoops! Thanks (+: I'm brain-dead. Love the photograph of the case; very sleek - what is it? – Everyone Apr 10 '11 at 3:55
Lian-Li PC-A71A. – Fopedush Apr 10 '11 at 3:58

I don't see how this would improve cooling at all. You'll note the Lian-Li PC linked above still has a top exhaust vent (heat rises, remember?) and placing the PSU at the bottom just guarantees anything that falls or any dust in the system will go through the PSU.

My best guess for the reason is to make cable management easier (spare cables will sit on the bottom of the case) and to help make the system less top-heavy.

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Good point ... I didn't think about the dust either – Everyone Apr 10 '11 at 5:39
"Heat rises", yes, but this is an extremely weak influence on air movement. Compared to the air pressure and flow of even a small cooling fan, the effect is completely negligible. – Jamie Hanrahan Mar 6 '15 at 22:34

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