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Power Supply: Mounting Location And Chassis Selection

How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 1
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PSU at the Base of the Unit

Many modern PC enclosures put the power supply down under the motherboard. This installation option has many advantages, so we strongly recommend a chassis configured this way. As you can see from the diagram, a fan sucks in cool air from the floor level through the power supply's own intake, uses it to cool active components inside the PSU, and expels it at the rear of the device.

Advantages to Bottom-Mounting:

• Steady supply of cool air from the floor outside the chassis
• Direct expulsion from the case
• Lower fan speed
• Cooler operation allows for greater PSU efficiency
• Less heat stress on the components, longer life-span
• Lower center of gravity
• No hanging power cable

Disadvantages:

• Chassis must have a bottom opening
• It should also have an air filter
• Possible impact on acoustics, depending on the floor material

Despite minor drawbacks, the above configuration is preferable to some of the other installation options that follow, and you should always look for a case that accommodates it. But it's also possible to make a mistake here:

Don’t install the PSU with its opening facing up in the chassis. You should go this route only with passively-cooled "silent" PSUs, so that the warm air can rise. Otherwise, you're fighting the forces of convection, and possibly creating a situation where a screw or other loose part could fall into the power supply.

PSU at the Top of the Unit

Older PC cases manufactured according to the ATX specification put the power supply just under the case's top. Air is sucked in from inside the chassis and then expelled. Supposedly, this improves dissipation and prevents heat from building up. However, it also results in the power supply absorbing much of the waste heat generated by the graphics cards and processor. Consequently, you get inefficient operation from the PSU, whereby maximum power and efficiency are almost impossible to reach at temperatures exceeding 40°C (as they are normally based on an operating environment of around 25°C.) The longevity of components inside the power supply also suffer.

Advantages to Top-Mounting:

• Helps with cooling in some systems
• Shorter cable necessary for 12 V connections

Disadvantages:

• Higher PSU temperatures
• Inefficient and loud operation
• System ages faster

The Perfect Chassis...

...doesn’t exist. However, large, well-designed towers like the Corsair Graphite 600T come very close. There is nothing to restrict airflow inside the structure. Space, rear cable management, and lots of fans and air filters combine to create a solution that approaches what we'd consider to be ideal.

As much as possible, you should stick to enclosures that don't restrict airflow from bottom to top. If you want to include a particularly long graphics card in your configuration, employ a chassis with as much depth as possible. Otherwise, the card will split the airflow. Thick cables should always be at the rear. Also, anything that rattles around will significantly disrupt the airflow.

Display all 104 comments.
Top Comments
  • 18 Hide
    Mark Heath , November 8, 2011 4:39 AM
    Good timing as the Australian summer approaches. You guys in the US think you have it hot :S
Other Comments
  • 1 Hide
    compton , November 8, 2011 3:39 AM
    After reading the charts of PSU placement and the GPU cooling diagrams, I'm even more convinced that my Lian Li PCA05-NB is a great solution. The motherboard is upside down, so that GPUs (In my case an axial fan gpu) faces towards the top. The CPU is now at the bottom back of the case and with the rear fan acting as an intake and not exhaust, you get great CPU cooling. The PSU mounts in the bottome front as well. The great part of this design is all the heat ends up in the top. As an option, you can vent the top to release the heat rising from the GPUs, but I like the case because it has very little venting. Through unusual case design and careful component selection I have an almost silent system - but with overclocked CPU and GPUs. The front fan is the exhaust, but has a bezel over it. With a few bucks worth of acoustic dampening material I can even hear myself think sometimes. I regard low temps and low noise output to be two sides of the same coin, but I know that many seem to not mind loud systems and mainly just care about temps. It's never been easier to build a near-silent system, even with high performance gear.

    If you plan ahead of time, you can make a super quiet and cool running system. It's easier to build a cool and quiet system from the start than retroactively go back and try to make a noisy (and/or hot) system quiet with great temps.

    I'll be waiting for article 2.
  • 18 Hide
    Mark Heath , November 8, 2011 4:39 AM
    Good timing as the Australian summer approaches. You guys in the US think you have it hot :S
  • 0 Hide
    buzznut , November 8, 2011 4:55 AM
    Guten hunger YAH!
  • -4 Hide
    JOSHSKORN , November 8, 2011 4:57 AM
    I want my next PC to be able to play Crysis AND make me hot dogs and Iced Frappuccinos.

    All kidding aside...curious though, the test setup is on AMD CPUs. What about Intel CPUs? I would assume many of the concepts are similar.
  • 8 Hide
    100100 , November 8, 2011 5:02 AM
    Finally! A definitive article on how to air cool effectively! :D 
  • 7 Hide
    frostmachine , November 8, 2011 5:05 AM
    Great guide. I live in a perpetually hot n dusty place. This will come in handy.

    Would be better if there's some tips on dust management.
  • -7 Hide
    beetlejuicegr , November 8, 2011 5:45 AM
    heh i am just showing you a pic from my pc that the airflow is totally different because there is a watercooling system on the cpu, i hope that the picture is self explanatory for all.


  • -3 Hide
    amirp , November 8, 2011 5:46 AM
    Hey I have a question... for the PSU you say to not put it in upside down (ie. with it's opening facing up into the chassis..) but this is how I have mine in my ANTEC300 case since there is little room between bottom of case and the PSU if mounted right-side up. So what should I do?!!
  • 2 Hide
    BulkZerker , November 8, 2011 6:15 AM
    amirpHey I have a question... for the PSU you say to not put it in upside down (ie. with it's opening facing up into the chassis..) but this is how I have mine in my ANTEC300 case since there is little room between bottom of case and the PSU if mounted right-side up. So what should I do?!!


    PSU's don't pull that much air, normally. So unless your Unit kicks it's fan speed way up there then don't royy abotu it and do as suggested. If it doesn't work then you can always just flip it back to where it was.
  • 5 Hide
    darkrydr3 , November 8, 2011 6:20 AM
    BeetlejuiceGr, your cpu core temps are pushing 70 or 90 degrees C... thats hot!
  • 6 Hide
    MAGPC , November 8, 2011 6:44 AM
    Mark HeathGood timing as the Australian summer approaches. You guys in the US think you have it hot :S


    LOL, you both in the US and Australia think that it is hot at your home.
    Come taste the hot weather + high humidity in the Gulf countries.
  • 7 Hide
    marraco , November 8, 2011 7:07 AM
    Great article. It would be good to have also words on noise and dust (filters)
  • 1 Hide
    amk-aka-Phantom , November 8, 2011 7:54 AM
    MAGPCLOL, you both in the US and Australia think that it is hot at your home.Come taste the hot weather + high humidity in the Gulf countries.


    Seconded :D 
  • 2 Hide
    ickarumba1 , November 8, 2011 8:19 AM
    This is a decent article.

    However, the compilation of articles on Silent PC Review provide much more thorough details on air-cooling effectively. SPCR also comments on noise characteristics, with detailed comments on specific component selection. It makes this article kind of redundant.
  • -2 Hide
    v73 , November 8, 2011 8:33 AM
    So how is cooling in system with bottom PSU, downdraft cooler, top and rear exhaust and NO side fans (without any holes for silent system)?
  • 0 Hide
    Anonymous , November 8, 2011 8:42 AM
    Based on this the tower format is a heat failure as the best orientation would be for the motherboard to be laid on the floor of the case. One other aspect that wasn't mentioned and I don't know how much of an affect it would have, is Bernoulli principle that dictates moving air to have reduced pressure and temperature over surrounding static air. Tunnel structures for airflow to assist air speed inside the case?
  • 2 Hide
    jemm , November 8, 2011 8:46 AM
    Great work! As someone pointed out, dust intake is a problem when you have lots of fans pulling the air inside the case. While not all places are dusty, my place sits behind a school and there are dust everywhere. So my solution come from Organza, a thin, plain weave, sheer fabric traditionally made from silk. I fixed the fabric outside the case, and it works to filter the air.
  • 3 Hide
    ojas , November 8, 2011 10:40 AM
    great article, was going to revamp my cooling system in december anyway, this article helped a lot, and confirmed a lot of my suspicions (like the side fan).
    only prob with the side fan is that most manufacturers don't include dust filters on the side went, even if the front and psu intakes have it. you can always make/buy one though...

    MAGPCLOL, you both in the US and Australia think that it is hot at your home.Come taste the hot weather + high humidity in the Gulf countries.

    i was going to make a similar comment regarding Indian summers :D . I only see my hard drive below 30*C during winter!
  • 3 Hide
    Anonymous , November 8, 2011 11:06 AM
    You did not address or test air pressure. I spent many days testing multiple formats such as the tests above at my previous position and when I included air pressure, the whole story changed.

    Positive air pressure in a case, that has optimal channels for air movement, will cool components better than neutral, or negative pressure.

    All else equal, the positive air pressure creates more small eddies that pull more heat from the surface of components, which would otherwise be missed.

    You can test this, grab a good tower case, with a bottom mounted PSU, well groomed cabling, and make sure the case has no perforated walls (which defeats the purpose of air pressure testing). If you have 6 fans, 3 point in and 3 point out, in any configuration, it will cool less effectively than arranging the same 6 fans in a 4 in 2 out configuration.

    Try it and see if you get the same results...also any thermodynamic engineers care to join in? I am no engineer, just interested.
  • -6 Hide
    ramicio , November 8, 2011 11:30 AM
    A 60 watt light bulb emits approximately 90% of its energy through heat, so about 54 watts of heat. A computer drawing 60 watts at idle isn't going to be a particularly warm one. The power supply alone is 80%+ efficient. You just can't compare a light bulb's heat output to a computer's. Plus, a computer's heat is spread out over a higher area.

    As for the case you're working with in this article...I would not recommend it to anyone. These cases with the hard drives mounted sideways may be slightly handy, but there is basically no airflow over them. I don't see why a person with a simple tower would need quick removal. If you want server-type features, man up and buy real server hardware. Hard drives don't need much airflow to stay cool. There is no need to add anything to a hard drive to cool them, they should be properly cooled in the first place.
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