Question Fans are all at different speeds

May 2, 2020
My chassis fans are all at different speed, the difference in speed is very slight, less than 100 rpm difference difference. Is this a big problem for me? I'm very worried. 1150-1200 for example


A difference like that means nothing. Fans are never instructred to run at a particular speed. The signals they get are either a VOLTAGE (3-pin) or a % of full speed (4-pin). Each fan delivers a slightly different speed for the same control signal.


Fans are cheap. Realistically. Mass produced. Each magnet used is not identical in ability, different resistance in the coils (extremely slight) but all of that adds up to being a ±% for each fan. A fan rated at 1800rpm is really rated at 1800±% rpm. So you could see anything from @1750-1850rpm on average. With voltage or pwm signal being consistent between 2 fans, that means at any given setting you could easily see upto @ 100rpm differences, even if they were identical fans on a splitter. You could have 1800rpm fans on a splitter read 1750rpm. Swap the fan leads (splitters only read 1 fan) and suddenly that header with no other change is now reading 1850rpm.

Its not something to worry about unless there's a serious difference, like 1800rpm fan at 100% duty cycle reading 200rpm.

It's one of the reasons I prefer fans grouped in splitters, so all 3 fans on intake are 1 header, both top exhausts are 1 header etc. That way I assume all 3 intakes are the same speed ±, all top exhausts are the same etc. You'll know if individual fans have an issue, big difference in temps or visible rotation differences etc.

It also means just 1 control temp, motherboard temp sensors are at variable points around the board, it's easier to turn up/down all the fans in a group with just 1 curve, and not have different sensors controlling adjacent fans. Also frees up a lot of headers.

With an aio, you only need 3 headers. Cpu_fan for the pump or fans, sys_fan1 for pump or fans, sys_fan2 for fans. Depending on the AIO.
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