EggXpert

This is also a case-related question, but it's about the power supply so I'm posting it here.  I won't be surprised if my 50/50 chance of guessing the right spot was wrong, and if so, sorry!

What are the pros/cons of top and bottom mounted power supplies?  Are there certain types of PSUs that work best in bottom mounts?  I've only worked with top mounts, but bottom mounts seem popular with some case makers.  

A large reason bottom mount is really not optimal is related to heat dispersion, but that's also the benefit.

On one hand, having the PSU at the bottom potentially creates more heat in the PSU because through induction the heat goes up to the core computer. But since the fan is spinning, it gets [hopefully] pushed outside the case. The hot air wants to go up, the fan wants to push it out, so there is some [a small amount] of tension there, not significant by any standard. BUT any heat that emanates from the PSU will bleed into the core computer, which definitely could be bad.

If you've got a PSU that has a fan pushing the air through and out the PSU without a secondary fan, try using a top-mounted case if possible, because now the heat will want to move up more. If you've got 2 fans both pushing the air out, you can safely use either top or bottom.

It's really not a big deal regardless, but there is some slight variance as I mentioned. 

On the OTHER hand, heat is reduced inside the PSU because it is no longer pulling hot air from the core. Then comes those people [including myself] who like using the PSU fan(s) as extra cooling. That is hard to do when the PSU is at the bottom because it is no longer taking in so much hot air, it's taking the much cooler air. This reason is why cases always have fans at the top back, it's where all the heat is (technically the top has more and is more convenient, but can be difficult to design around a top mounted PSU).

If that doesn't answer your question with enough depth, I've got no idea what will. Sure, it complicates things more, but it's detailed 

slm is correct in what he said but there are other things that can effect this ? and that is if there is a metal partition between the PSU mounted on the bottom of the case and room for case fans down there as well. As slm said I too prefer the top mounted PSU cases since heat rises I want the heat out of the case as quickly as possible and will gang bang the fans at the hottest point in the case for this reason and even tolerate a little more noise than most to keep the system cool since I do OC cool is better than heat.

Lets not forget everyone's favorite of dropping screws into the fan on bottom mounted psu's with top fans.

Thanks all for responding, your points were exactly why I was somewhat concerned about buying a case with a bottom mount for a power supply; I was surprised to see so many nice cases like that, and thought I must be missing some great thing about bottom mounts.  Given that the system I'm building will already have a lot of heat right above a bottom mounted power supply from two graphics cards, and that I'm very prone to dropping screws, I'll go with the one case I found with a top mount!  

A bit late, but don't forget the inevitability of the ATX/EPS 12v connector always coming up a few inches short on bottom mounted PSU's on main boards with the ATX/EPS 12v connector all the way at the top of the board requiring and extension cable to be bought before you can even use it.

 

I'll be the voice of dissent here.  To begin with, I would note that top-rated case makers favor the bottom mount.  CoolerMaster, Lian-Li, Silverstone, Antec, Corsair...  

You will generall find bottom mounts in higher-end cases that imply additional fan placements, and usually come with extra fans.  In this is their marked difference from mainstream cases, where PSU fan is usually the only exhaust and thus needs to be placed in the most critical position - near top.  In a fairly low-power system, where CPU is not exactly cooking, this does not pump a lot of heat through PSU.  But sitting above a high-end chip can severly shorten PSU's lifespan and cripple its efficiency.   Besides, what do you think moves hot air out the case faster - an unobstructed fan or a vented steel box stuffed with electrical components?

Now, videocards.   You might recall that their coolers are on their bottom side.   And while dual-slot solutions exhaust some via backplate, there's quite a bit of hot air generated around those coolers.   A bottom PSU is in ideal position to suck that air away and out the back.   With a top PSU....  some of that hot air goes right back into the card coolers (bad), or into CPU area (also bad). 

Oh,unless top-placed PSU is modular, the unused power rails have to be bundled up somehow - and they often end up blocking the most effexctive exhaust fan location - case top.  

To address the mentioned 'drawbacks':   A too-short 12V rail is generally not an issue with PSUs designed for hi-end rigs.   If you are really worried, just add an extension to your original order.   But reading some PSU reviews should get you an idea about its rail length.   Dropped screws...  it's generally best to lay your PC on its side for assembly to begin with.    If you are prone to dropping screws, and plan to do the assembly upright, think of the damage potential of dropping a PSU as you maneuver it into top position.   Oh, and invest into a magnetic screwdriver.   As to heat 'emanating' from PSU - well, if you talk about *convection*, PSU fan more than takes care of that.   You are not exactly fighting a fireplace-magnitude draft here.   However, radiated heat from PSU doesn't really care about up or down.   Not that you'll get a ton of heat coming from an 80+ PSU (You ARE using an 80+ PSU, right?)

GmsCool:

A bit late, but don't forget the inevitability of the ATX/EPS 12v connector always coming up a few inches short on bottom mounted PSU's on main boards with the ATX/EPS 12v connector all the way at the top of the board requiring and extension cable to be bought before you can even use it.

Some cases come with the extension cable.  I have 2 of the 8pin extension cables from the 2 HAF 932's I have (using 1 of them).

sjr:

GmsCool:

A bit late, but don't forget the inevitability of the ATX/EPS 12v connector always coming up a few inches short on bottom mounted PSU's on main boards with the ATX/EPS 12v connector all the way at the top of the board requiring and extension cable to be bought before you can even use it.

Some cases come with the extension cable.  I have 2 of the 8pin extension cables from the 2 HAF 932's I have (using 1 of them).

That was certainly nice of them. The CM 690 cases I've used didn't, and of course it was about 1/2" short from reaching, lol.

 

1.  Air taken into the upside down PSU on the bottom is less warm than air taken in at top of the case at top of case.

2.  May need to mount PSU upside down...make sure case allows for that.

3.  If case allows air draw from case bottom -- and therefor right side up mount -- PSU will be pulling in air at room temp and exhausting it back out of the case without ever effecting the air inside the case.

4.  Air drawn into the case nearer the floor typically has more dust within it. 

5.  Bottom mount may improve wiring harness deployment for improved air flow.

6.  Rear I/o and expansion slots ride up higher off of the floor.

7.  Floor mount usually has more room for lengthier PSU's. 

8.  Unrestricted top case fan option available in bottom mount PSU. 

I have the 690, and yes, the 12V rail of my 700W OCZ would not reach if I tried to go through either pre-installed clips or under the mobo tray.   A solution that did work beautifully was to run this cable between PCI slots and the backplate - PCI cards have this notch on the bottom seemingly made just for this - and then between rear ports and voltage regulator heat sinks.   A few drops of silicone along the way assure the cable stays put.