Two of the three Molex-connected 80mm case fans that came with my case are dead or dying, so it's time to replace them. However, I'm confused by my options. What I'd like to do is replace them with some reliable fans with onboard speed control. Fortunately my mobo (Gigabyte GA-P35-DS3L rev 2.0) has three fan headers besides the CPU fan. However, two have 3 pins and one has 4 pins. Also, two are labeled "SYS_FAN" and one is labeled "PWR_FAN", or something similar. Do I have onboard speed control with the three pin headers, or just the four pin header? What's the difference between "SYS_FAN" and "PWR_FAN"?
The same type of splitter can be used to have the last fan share with the cpu fan, if you only run 2 on the sys fan header.
I run 2 140's on system fan(as well as 2 high speed Antec 120mm's on the cpu header) without issues, so maybe 3 x 80mm is fine.
There is no onboard control, and as far as I can tell, no way to rig the fan to respond to voltage or PWM control. The fan does have an onboard thermostat that controls the fan speed, as well as a three pin connector to provide RPM data back to the motherboard. I would, however, be giving up the ability to manually control fan speed. Does anyone have any experience with this model, or fans with onboard thermostats? The thermostat appears to be integrated into the fan, rather than an external sensor you could place on a board or a drive.
nukemaster is right about the PWR_FAN mobo header - its intended use is only as an input from SOME PSU's that have a signal line to feed the PSU's internal fan speed back to the mobo for monitoring - it does NOT control this fan's speed. However, on some mobo's the header also does have 12 VDC on pins 1 and 2 to provide full power (no control) to a standard 3-pin fan. The only advantage of this (instead of connecting the fan to a Molex 4-pin supply) is that you get to see the fan's speed, but it is mislabeled as your PSU fan.
Both of the SYS_FAN headers probably can provide speed-controlled operation of case fans, but they are designed differently. A 3-pin fan is controlled by varying the +12 VDC line's voltage (pin 2). A 4-pin fan's header supplies +12VDC all the time on pin 2, and then the PWM signal on pin 4. A special circuit within the fan case uses that PWM signal to control just how much of the power from the +12 VDC line is actually fed through the motor, thus achieving fan speed control. So if you connect a 3-pin fan to this header, it always runs at full speed because the pin 2 line is always at 12 VDC. Well .... almost always. SOME mobo 4-pin fan headers are able to detect which fan type is connected to a 4-pin header, and will revert to behaving just like a 3-pin header is that is the fan you connect. Your mobo actually can do this for the 4-pin CPU_FAN header (see manual p. 46 under "CPU Smart Fan"), but it is not clear whether the 4-pin SYS_FAN header can do this, also.
Exact details of how each of the two SYS_FAN headers can control case fan speeds are not clear in the manual, but may be clearer if you install and run the application called EasyTune5 on the CD that came with your mobo. It allows you to tweak a number of items on the mobo, likely including all controls for the case fans on the SYS_FAN pinouts. Assuming that both can actually do case fan speed control, you could follow nukemaster's advice and connect UP TO 2 identical fans to each SYS_FAN output. BUT you would have to get 3-pin fan(s) to the 3-pin pinout near the back edge, and 4-pin fan(s) to connect to the 4-pin one near the front. In each case, connect identical lines in parallel together EXCEPT do NOT connect to the mobo any more than ONE fan speed signal line (pin 3) to each pinout. Leave other fan speed lines unconnected.