Reversing the PSU fan - Read this first!
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Thread: Reversing the PSU fan - Read this first!

  1. #1
    Join Date
    Feb 1999
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    Reversing the PSU fan - Read this first!

    After reading several posts on how much better cooling you get when you reverse the PSU fan to suck in rather than blow out, I decided to give it a go in the hope that it would finally stabalise my 300A @ 450!

    Well, it didn't work! Infact, it made the damn thing hotter!

    This may not be the case with all systems but take note, you have been warned...

  2. #2
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    How many fans do you have total?

  3. #3
    Join Date
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    I have never seen any messages suggesting making a PS fan suck in, but it sure sounds like a bad idea. Think about it: doing so blows the hot air from the PS into the case, right onto the CPU. If the air in someone's PS is cooler than the rest of the air in their case, they obviously don't have enough cooling for the rest of their case.

    ------------------
    jjr512 "The Terminator"
    Justin J. Rebbert
    rebbertj@columbia.aim-smart.com



  4. #4
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    Actually the ATX spec insists on the PS Fan being mounted on the bottom side of the PS blowing into the case. This actually works quite well since it blows right over the CPU. Unfortunately, practically every case maker ignores that.

    Whether your fan goes in or out is not really that critical, the point is to force air to flow through the case. I like to have the PS Fan as described above, add a case fan in the front also blowing in, and finally one of the slot-type exhaust fans right next to the video card blowing out.

    No hard rules, just make sure the heat actually gets OUT of the case!

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    -=DrFoo {Sysop} Third Wave BBS (941) 355-7136=-
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  5. #5
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    Dr. I must differ with your course of treatment. The general consensus is that you should have a fan in the lower front of your case blowing in and the PS fan blowing out at a minimum, plus your CPU fan and video card fan.

  6. #6
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    If you want to use physical laws to your advantage, you need to have a fan in the bottom front of your case to pull in cool air. As the air is heated by various hot elements in the case it rises and eventually flows into the holes in the bottom of your power supply and is expelled by the fan at the rear of the supply which should blow out.


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    Mike
    www.monster-machines.com
    Mike
    Mechanical Engineer
    Dell Computer Corporation
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  7. #7
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    The old ATX 2 standard calls for the power supply fan to blow into the case.

    ATX 2.01 compliant means the fan on the power supply is set to blow air out instead of in.

  8. #8
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    dan_wilson:
    I probably should have made it clearer. The exhaust fan I use moves about twice the air of the other ordinary fans. The idea of course is to achieve some sort of balance.

    If you have just 2 fans, I agree one should blow in, the other out.

    TREB:
    Interesting, I didn't know that. Seems like a step backwards. Actually, blowing out would be fine (perhaps better) if they still put them on the bottom of the PS. I actually had a CPU one time that would work in my case (overclocked of course) but not in another identical system. The only difference was the position of the fan. As I recall the difference in CPU temp was about 5 degrees C - not a minor change in my view. We got the guy another PS with the fan in the proper place and voila, no more problems.
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    -=DrFoo {Sysop} Third Wave BBS (941) 355-7136=-
    dsl150-37.cftnet.com


    [This message has been edited by DrFoo (edited 09-17-99).]
    -=DrFoo {Sysop} Third Wave BBS (941) 355-7136=-
    http://163.125.153.98

  9. #9
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    Here's my theory:

    In the absence of extra case fans or good ventilation, ps blowing on the cpu can be a good thing. (relatively) Even warm air moving over the cpu is better than hot air stagnating. But still not good.

    On the other hand, if you have good ventilation and the benefit of an extra case fan or two, then power supply blowing hot air on the chip and into the case is no good.

  10. #10
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    Baloneyflaps:

    Makes sense to me!


    ------------------
    -=DrFoo {Sysop} Third Wave BBS (941) 355-7136=-
    http://dsl150-37.cftnet.com
    -=DrFoo {Sysop} Third Wave BBS (941) 355-7136=-
    http://163.125.153.98

  11. #11
    Sterling_Aug Guest
    You know, it can't get much easier to reverse the fan so it blows out. Take out the 4 screws and mount the fan so the label faces out.

  12. #12
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    After reading the actual ATX 2.01 spec, it doesn't seem to have changed much from what I understood before. Here's the relevant paragraph:

    With some modifications, a standard PS/2 power supply can support an ATX form-factor system. These modifications include adding a 3.3V supply rail, PS_ON, 5VSB, repositioning of fan venting locations to move air directly across the processor, and consolidating the motherboard connectors into one 20-pin header. The preferred airflow solution is to pull air through the power supply from outside the chassis and direct it onto the processor. However, other airflow solutions may be implemented to meet the specific cooling requirements. For example, one alternative solution would be to use a standard PS/2 power supply, modified with the 20-pin power connector, without repositioning the fan but using an active fan heatsink to cool the microprocessor. Although ATX power supplies may use an external fan, care must be taken in implementing external fan configurations so the fan does not violate the keepout zones necessary for ATX 2.01 compliance (see Figure 8 for detailed keepout zones).
    So, doing it the way I like is just their "recommendation", you can be compliant doing it basically any way you damn well please I guess. Some standard!




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    -=DrFoo {Sysop} Third Wave BBS (941) 355-7136=-
    dsl150-37.cftnet.com


    [This message has been edited by DrFoo (edited 09-22-99).]
    -=DrFoo {Sysop} Third Wave BBS (941) 355-7136=-
    http://163.125.153.98

  13. #13
    Join Date
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    All things being equal, nothing beats a positive pressure case with filtering. Hard to find in anything other than a rack mount case, though.

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    A life? Where can I download that?
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