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I was poking around with an old desktop I own when I noticed it has a CPU fan, but not a system fan. What's the difference between the two? Can I use the computer safely without a system fan?

Here's a photo of the BIOS information on my fans:

enter image description here

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As an aside, most fans use 3 wires, the yellow of which sends the fan speed information. Older fans often didnt have this, which means sometimes you can have a fan present that the bios knows nothing about. – Sirex Oct 31 '10 at 20:14
Most newer processor fans are 4 wire. 3 wire is still the most common outside of the processor fan. The system fan may still be running just the TACH isn't hooked to the expected header. – hanleyp Nov 1 '10 at 2:55
up vote 2 down vote accepted

The difference between the CPU fan and the system fan is that the former is the one attached to the heatsink of the processor, and the latter is generally on one of the sides of the case to exhaust hot air. The BIOS determines which is which, though, by the mere fact that one is plugged to the cpu_fan plug of the mainboard and the other is on the sys_fan, so it could be reversed without much problem if one would have any occult reason to do so (or just a broken plug).

You can safely use the computer without a sys_fan as long as the cpu and the other components run at temperatures inside their safety ranges, which is quite probable that they do, even without system fan, as long as the computer isn't under heavy load or naturally power hungry.

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Even though the BIOS isn't detecting a system fan doesn't mean there isn't one. Not all fans, especially on older systems, are designed to actually transmit data to and from the BIOS, so it's possible you have a model that just turns on when there's power supplied to it, and shuts down when there's not.

The only real downside to having a dumb fan rather than a smart fan, other than the disconcerting "0 RPM" message, is efficiency. A dumb fan will be on all the time, whether or not your system actually needs it. With smart fans, the BIOS can turn it on full during heavy use, shut it down when the system's idle, and warn you when it's not working properly.

Even if it's missing entirely, it's not 100% necessary unless your system has a tendency to overheat without one. Older computers in general don't generate near as much heat as modern systems, so their cooling requirements are far less imposing than anything you'd find today.

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The only real downside to having a dumb fan [...] is efficiency. And noise! – jj_ May 6 '15 at 13:23

Yes it is, if airflow within your system chassis is not overwhelming.

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the stock CPU fan is not enough for cooling System. even it isn't enough for CPU itself. – Azfar Niaz Oct 31 '10 at 11:43

Unless your PC is stuffed away in a poorly ventilated cabinet or the chassis is chock full of goodies, it's doubtful that you'll need additional cooling. Checkout your PSU, it probably has a fan blowing air out of the chassis.

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Fun fact: older computers, in addition to being slower than modern computers, use less power at the high end (this doesn't necessarily apply to mobile CPUs, or CPUs clocked low for efficiency).

Since the power put into a computer is pretty much all converted to heat, it means older computers run cooler / need less air flow. My 486 didn't even require a CPU fan, getting by with just a power supply fan. This is why your newer computer requires a system fan, but your older computer does not. It simply doesn't need as much cooling.

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