Is it recommended to use CPU_OPT header for second CPU fan?

Question

I have the Noctua NH-D15 SE-AM4 CPU cooler and ASUS ROG CROSSHAIR VI Hero motherboard and I am trying to find the best way to connect CPU cooler to the motherboard.

I was wondering if it is best to connect one fan to CPU_FAN header and second fan to CPU_OPT or is it better to connect both fans with included Y-Cable to CPU_FAN? I am not really sure what CPU_OPT is intended for. Is CPU_OPT intended to be used for a second CPU fan and would this mean both fans are controlled separately? Also, are there benefits to connecting a fan to each header so each fan has a full 1A of power available?

Here is my motherboard headers chart.

It appears both headers are Q-Fan controlled, but I am not sure what shared control A means.

Solution 1

You're going to be safer and get better behavior from the system if you put one fan on each connector instead of using a Y-cable. There are two reasons for this:

1. Many fan connectors are designed to provide a fixed amount of power, and on most motherboards, this is just enough to power one fan with a reasonable safety margin. Trying to run more than one fan from a connector runs the risk of either having both run slow (because they aren't getting enough power), or drawing too much power and causing electrical issues (including being a fire hazzard).

2. The firmware (or the OS, or the hardware sensor chip, depends on the platform) reads back the fan speed over one of the wires (even on the 3-pin fans), and uses that to regulate the fan speed (using PWM with the 4-pin fans, or by just scaling voltage with 3-pin ones). When you use a Y-cable to connect two fans to one connector, you end up messing up that tachometer signal, and run the risk of either overheating because the fans are running too slow, or being really noisy because they're running faster than needed. This isn't usually an issue for case fans (they often aren't regulated), but is a serious issue for CPU fans (they are almost always speed regulated.

Solution 2

According to the manual (p. 38), which I'd suggest you thoroughly read, CPU_FAN and CPU_OPT share their control, and both have a maximum output of 12W each.

As the each of the fans will consume around 2W, it's up to you if you want to use CPU_FAN for both via Y-cable or CPU_FAN and CPU_OPT - I'd go with the solution without Y-cable, because if one connetions fails, the other fan may still work, while if the Y-cable loses contact with CPU_FAN both fans will fail. But if you only consider the power requirements of the fans, it won't make any difference.

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Now what does "Shared Control" mean? I could not find anything in your board's manual, but on my ASUS-board, it means that CPU_FAN and CPU_OPT will share one control-value, meaning that if CPU_FAN is set to 100%, then CPU_OPT will also work at 100%. If I set CPU_FAN to 75%, CPU_OPT will also run at 75%. (The same goes for any other percentage, too, of course.)

Now on my board, I can set them to independent values not via ASUS-tools but via SpeedFan, which I use because most of my boards strangely report a wrong CPU-temp by default, while SpeedFan manages to get the correct values. So I can now get CPU-FAN to work at 100% all the time, while CPU_OPT can work between 20%-40%. _{(That's just a very stupid example - I still have both fans running at the same RPM/Temp-Curve)}

Comments

[John Smith]

Is CPU_OPT designed or intended to be used for connecting an optional second PWM CPU fan? I just wanted to make sure this header is not intended for some other purpose like water cooling or something.

[Austin Hemmelgarn]

In general, yes, it's meant for a secondary CPU fan. Most good firmware will let you configure whether or not to run it at full speed (same for the regular CPU fan), and that is what you would do for a water cooling pump.

[Jamie Hanrahan]

Re two fans to one connector - you should only connect the tachometer wire from one of the fans. That way the mobo will read the speed of that fan. You can't "mix" the tach signals together just by wiring them together, and you shouldn't.