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On many computers, when the computer is turned on (even before POST), the fan spins up to high speed momentarily, then settles down to normal operating speed. This appears to be normal, but why is this done, and what purpose does this serve?

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I don't understand these answers. The fan running at high is ridiculous and there is no need for this. Most computers at boot up run the CPU and other fans at a normal speed period. No concern with hurting the CPU, because if the CPU would heat up then the speed could be increased steadily. There should be a fix to the fast CPU at boot up. Check the BIOS. –  user147530 Jul 22 '12 at 1:31
    
@KevinSmith, > No concern with hurting the CPU, because if the CPU would heat up then the speed could be increased steadily. What if the system is messed up? How exactly would it “steadily increase the fan speed” then? > There should be a fix to the fast CPU at boot up. Yes, there should be, but isn’t. –  Synetech Jul 22 '12 at 1:55

4 Answers 4

up vote 20 down vote accepted

Because the power is turned on to the fan, before the BIOS loads any real time controllers that will base the speed of the fan on the temperature of the processor. This also keeps the processor from getting excessively hot if you were to try the alternative... which would be to keep the fan off until those controllers were loaded and basing the fan speed on processor temp. More of a safeguard than anything. The processor is starting to work the moment you turn the computer on, but the BIOS still needs time to load.

Here is another alternative. What if the BIOS didn't load for some reason... a stick of Ram gone bad, for example. Would you really want your processor sitting there with no fan, waiting for the BIOS to turn on the fan after it loaded the appropriate controllers? I know I wouldn't.

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+1: It's a fail-safe in case of POST failure. It's better to have the chassis fans on high in case the POST failed due to an over-heat condition that the not-yet-ready sensors can't detect. –  Ƭᴇcʜιᴇ007 May 23 '12 at 4:18
    
And a test of this for example.. is to turn off the fan quietener / fan calmer thingy in the bios, and then the fan goes full speed regardless of cpu temp. Slower speed gives less noise and more life for the fan. –  barlop Sep 23 at 8:12

Fans are made to operate at a certain speed with a certain voltage (generally 12v). When the computer starts, all fan voltages are at their default setting of 12v, which result in 100% speed. There's no programming in this - simply the act of being powered on.

Later on, either the BIOS or a software utility is able to either reducing the DC voltage or use a pulsing PWM method in order to reduce the effective fan speed.

Now, of course, motherboard makers could have altered the circuity logic so that fans start at another voltage (50%, or even off until needed), but Bon Gart hit the nail on the head: there's a potential for disaster if the BIOS fails to boot properly while the computer remains powered.

But even if they wanted to, unlike self-contained devices the motherboard have no idea what sort of fans you may have so there's no "good guesses" to be done:

  • some fan models are relatively slow at 100%, while others need their voltage reduced to 50% for an equivalent result
  • some fans -especially the large ones- might not even start with 50% voltage, while they can actually be slowed to do 50% after the initial "push", complicating things further.
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+1 for mentioning that fans should receive full voltage for proper spin-up. IMO that's the correct answer. –  sawdust May 23 '12 at 5:24

Because until a modern OS that knows to idle the CPU is booted, the CPU is running hot (I explained why at that question). If the motherboard and BIOS support fan-speed–regulation, then once the POST has completed and the BIOS starts its work, it will lower the speed if needed; otherwise, the fan remains at high speed.

If you connect your system to a power-meter like the Kill-a-Watt, you can observe this numerically as the system will draw significantly more power while in the BIOS editor or DOS, paused POST, or even the OS boot-menu. However, when a power-aware OS loads, the power usage drops (in fact, even running idle.com in DOS will drop it to the same amount). The specific difference will vary, but 30-50W is not uncommon.

Another way you can see this in action is with a virtual machine. If you pause the VM at the POST or enter the BIOS configuration tool, you will see the CPU load on the host being high (100% on a single-core processor, 50% on a dual-core/threaded, etc.) If you boot into DOS in the VM, the host’s CPU load remains high until you execute idle.com, at which point, it drops to ~0%. It also drops when you boot the guest into Windows or other modern OS.

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Think about this scenario. When your car starts up it rev's at a higher rate then running at a normal idle speed. Like most common systems this is called power on self test (POST). The computer hardware will check the sensors and verify is running at the speed attended. If not then you might see a error, somewhere.

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