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I would like to buy new computer case. Last time I bought a computer was in 2008 and many things have changed up to day. Many new computer cases have power unit placed down, on bottom. I'm thinking about buying some of those cases, but i'm not sure about something - if power unit is placed on the bottom it can't take away hot air from the case and pump it out right?

All my PC parts are silent - CPU (E8200, placed below 12cm Nochtua fan of power unit) has heat-pipe cooler with Nochtua fan spinning at only 800rpms, GPU has cooler powered by 7V instead 12 and that's why i don't want to HAVE TO place another fan to pump out hot air instead of PU placed on top. That might make some noise.

So i ask someone more experienced: if i buy some computer case with PU placed down, do i HAVE TO place some fan to pump out hot air?

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The life of a PSU might be extended if it draws its air from the bottom of the case instead of the hotter air at the top. Beware that a PSU that uses a 120mm fan draws in & heats up air but might recirculate 20-40% of that air back into the case instead of expelling it out for good. BTW it's power supply unit, not "power unit" which could encompass a unit that consumes power. –  sawdust Sep 8 '12 at 19:50

2 Answers 2

The original ATX designs used the PSU as part of the overall cooling system - they sucked in outside air, and blew it into the case. While the air was slightly hotter than ambient it was still better than not having it.

From the ATX spec

Adequate venting should be provided in the system to allow for unimpeded and well-directed airflow to cool key components such as the processor. One recommendation that is implicit in the ATX specification is the placement of the power supply. The power supply should be placed in close proximity to the processor if the power supply is expected to cool the processor properly (but be sure to observe the component height keepouts over the PC board). Chassis venting should be placed strategically to allow for proper cooling of other components such as peripherals and add-in cards. A system fan should be considered to allow for proper cooling of all system components.

Modern gaming boxes tend to have significantly better cooling systems (and invariably have bottom mounted PSUs. and very large fans) - they don't rely on the PSU fan for cooling, and as such it makes sense not to have the PSU in proximity to the processor. You can still mount the bottom mounted PSU to blow air up (hot air rises, and the hot air will exaust through a top vent or fan), or down( PSU simply blows the air out, the other fans can handle cooling the rest of the system anyhow). I've got two relatively similar systems in an older top mount, and a modern bottom mount case, and the ambient temperatures on the modern case is about 2-3 degrees lower.

In most cases, with bigger cases, venting straight out the bottom makes sense, and is what seems to be recommended. Your rear case fan can handle additional processor hot air exhaust duties.

That said, keep in mind that some older PSUs have shorter cables that'll barely reach the motherboard on a modern gaming case - in my case I have it venting in cause its the only way I can use that darned old power supply.

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@Keitari Completely off topic, but Boxen can be used as a plural of "box" when it comes to computers :) –  slhck Sep 9 '12 at 6:45
    
"heat rises..." - No, hot air tends to rise because it's less dense, but heat does not have any propensity to travel upwards. Heat will disperse by taking the paths of least thermal resistance. –  sawdust Sep 11 '12 at 0:52

depends on the type of system. ATX cases per the atx spec have the power supply at the top sucking hot air away from the cpu memory. while mini itx cases may not even need a fan. server boards are designed with memory parell with the pcie plugs to allow front to back air flow needed in rack cases.

if you are looking for more cooling capibility when using a near silent system you may need to get some large fans preferably pwn controlled with decent cfm even at lower speeds to keep it below the audible range. also using higher quality fans may cut down alot on noise. Good bearings and better blade shape can make a large difference.

With that said if the case was designed with a power supply at the bottom then it may well do a good job as is because its airflow was designed to work that way. also using heat pipes and or prefabbed "water cooling" units to move where the fans are placed may allow gills and fans out side the case to cool it. also you may want to put some rubber bumpers around any fans you add to keep them quieter. the pc I recently built for my wife came with rubber pins to hold the fan to the cpu cooler. that whole system is so quiet that I hear the dvd drive working over the fan noise. I got a video card that doesn't need fans to operate and a normal power supply. I never thought about trying to keep it quiet even though it is what we use to play videos from to the surround sound.

also some power supplies have 2 fans with the push pull idea so that they dont re circulate hot air as much. look up reviews for all the parts you are interested in and see what they say about noise/heat capibilitys. current pc technologies are getting much better so that you can easily build a silent system. pwm systemboards make it nice that way as they automatically run the system as quietly as possible untill you hit it with heavy work loads aka gaming/video rentering etc. Some people have manual fan controls so they can crank it while playing games and keep it silent all other times.

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