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Question Are Chassis fans supposed to increase speed under CPU load?"

rv_el

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Oct 20, 2008
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My build is a Be Quiet! Dark Base Pro Rev 2 that comes with a PWM board on it.
I have the 3 case fans tied to that PWM and that is plugged in to Cha_1 on my Asus x299 Deluxe II board.

In bios I believe CHA_1 is set to PWM. Other options are VA and Auto. Default was Auto. I tried setting back to that it makes it so the fans run faster by default (idle) and its hard to say if they get faster on load. If so its very slight.
When I max-cpu i.e. through CPU-Z or rendering in a program such as 3dsmax the fans remain at the same speed.

A few more details that might help

  • The Be Quiet! case has a slider on the front. Left is supposed to be PWM controlled from mobo which is what i have set. And it stays "slow" it seems. If i slide it to the right (manual mode) the fans speed up immediately (at idle) and then at load seem to be the same speed (no change).
  • I do not have the case side on right now. the side is open. So may be its not seeing a reason to speed up? or are the fans suppsoed to speed up no matter what during load?
  • The h100i fans speed up and the read out on the CPU temp hits about 55 maximum. So the machine is running fine and not rebooting or anything like that. Idle is in the high 30's
 

USAFRet

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Mar 16, 2013
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Fan speed increases in response to heat, not CPU load.
The two can be tangentially related, but if you're only seeing 55C under load, it's not getting warm enough to speed things up. The system sees no need for more airflow.
 

Karadjgne

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Cha_1 is a chassis temp sensor. That can be motherboard itself or just a general case temp. Most normally, with a decent case with decent airflow, even with heavy usage temps inside the case might never reach 40ish °C. That's only @ 16-20°C above a air-conditioned room temp of 73°F (23°C).

So if the case really isn't getting hot inside, the case fans will just keep chugging along as normal. The only way to change that (pwm/auto/VA is HOW a fan is changed, not WHY it's changed) is change the fan curve. In bios, you'll have a fan setting corresponding to cha_1. Normally default is 70°C = 100% duty cycle. That's the temp at which the fan runs max. Minimum is whatever the fan needs to run. If you change that to 50°C =100%, that raises the fan curve dramatically, instead of 30°= 30%, 40°= 50%, it'll be 30°= 40%, 40°= 70% of the fans maximum rpm.

Definitely hear/see the change.

PWM is 12v constant, with the 4th wire is a pulsed switch on/off. VA is voltage change, usually 5v or 7v minimum, 12v maximum. Voltage fans respond to voltage changes to determine speed, at 5v a voltage fan will run @ 40% of max, at 12v max. Pwm fans are always 12v, the longer the 'on' pulse, the faster it spins, before shutting off, then on, etc..

Your mobo can control either type of fan. Pwm is for setting the headers to pwm signal type. Auto leaves it upto the mobo to determine what's in each header, VA sets the headers to voltage type.

You can use either pwm or auto, makes no difference, won't change anything with a pwm hub/fans. Setting it to VA, and you'll have issues, those are not voltage fans.
 
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