Question Understanding case fans


Aug 1, 2006
I just built my new system and everything is running well. The last piece I'm trying to setup are my fan curves. Below are the fans that I have.

SYS 1: 140 mm rated at 42.6 CFM @ 1200 RPM, 3 pin DC ,top exhaust
SYS 2: 120 mm fan rated at max 65.51 CFM @ 2000 RPM, 4 pin PWM, rear exhaust
"Pump" fan : 2 x 140 mm rated at 41 CFM 1000 RPM, 3 pin DC power, front intakes
CPU Fan: 120 mm rated at 66.17 CFM @ 1550RPM PWM intaking from front of case to the rear.

What I don't understand are that I have the 2power connectors for the two front intake fans connected together so that only one 3 pin plug goes to the "Pump" fan, I can only use this space because it's the only one that can be reached. Since this is DC and 12V when I adjust the voltage does it get split between the two fans that power is being supplied to so that at most each fan gets 6 V or does each fan get 12V?

I have a general idea of calculating the intaking and exhaust CFM but what about the CFM of the cooler? I know it capacity but how much of it can I judge to be intaking or exhausting??

My plan was to then set a speed of about 20-30% of RPM from 0-50 degrees so that the intake slightly exceeds the exhaust and then ramp up accordingly as temperatures go up every 10 degrees to about 80 when all fan are running 100%.

The GPU has fans but I think I will let it run itself.
Last edited:

Fan connectors on the motherboard: I assume you have 4: Sys fan 1, sys fan 2, CPU, and Pump.

Only 1 of the 4 is a 4 pin PWM connector?

Is that correct? If not, clarify.

Your CPU heatsink fan would connect to the CPU connector, leaving 3 unused fan connectors on the motherboard.

Is that correct?

How many case fans are you trying to use?

I think you can get extension cables for fans if existing cables don't have enough reach, but it might be simpler to get one new fan that had a long enough cable by itself??

I would NOT get bogged down in CFM ratings.

There is NO substitute for experimentation. I'd devote an hour or two with various curves under a good load and see what I learned. You need to get a grip on how many of your various fans can be controlled from the BIOS.


Aug 1, 2006
Yes, I have 4 system fans and 2 are PWM, the CPU and SYS 2. Yes, CPU is connected to CPU fan and the other connected as indicated. I have a total of 4 case fans + 1 CPU fan. Getting extensions does not solve anything because I am still using all of the fan headers on the board. Is the general idea of the fan curve I have correct?
You have 3 connections on the motherboard that can be used for case fans:

sys fan 1, sys fan 2, pump

I'd connect 1 case fan to each. 3 case fans total.

Can you do that without extensions?

Then evaluate and play with fan settings however you want.

Use PWM fans and PWM 4 pin fan connection points wherever possible.

It's a trade-off between observed temps and noise.

You can't tell in advance what type of fan curve might work out. You need to experiment.

You may be more or less sensitive to noise than the next person.

You may think 70 degrees is OK to tolerate long term or you may think 60 degrees is time to panic.

You may find that a 4th fan does virtually no good.

I'm fairly noise-conscious. My CPU fan cannot spin above 1300 and vary rarely is above 800. My case fans never spin above about 550. I've got a good air cooler and don't generate much heat. I don't even have a video card. I don't much care about temps below 70.

BUT, that is just my PC and my experience. You've got to actually evaluate your own hardware in conjunction with your own sensitivity to temps and noise.

You are on the right track. Just play with the hardware you have and don't come to any conclusions right now.


You have 4 case ventilation fans to connect to 3 headers, and are doing that by using a Splitter for the two front intake fans on the PUMP_FAN header. That is quite all right. There are details for you to consider.

I expect ALL of your mobo fan headers have 4 pins, whereas you have both 3- and 4-pin fans. If you go into BIOS Setup and examine the details of each screen for configuration of those three headers, you should find that each has an option for the MODE setting - that is, the method of sending signals to the fans for speed control. A 3-pin fan's speed can be controlled ONLY by varying the Voltage sent out on Pin #2 of the header, so for those fans you must set the headers to use Voltage Control Mode (aka DC Mode). For your 4-pin fans, set their headers to PWM Mode, which sends a constant +12 VDC power supply from Pin #2 but also sends the PWM speed control signal out on Pin #4. To begin, I suggest each header's PROFILE be set to "Standard" or "Normal", a setting that will have the fan speeds changed automatically according to the temperature measured by a sensor on the mobo. (The CPU cooler is guided by a different sensor inside the CPU chip.)

The CPU cooler fan draws air from inside the case and discharges also inside the case, so it does NOT contribute to overall balance of intake and exhaust. Ignore its impact for that calculation.

For the two 3-pin intake fans on one header using a Splitter, they are connected in parallel to the power supply lines 1 and 2. They are identical fans and receive identical VOLTAGE supply, so they will both operate identically, each drawing an identical CURRENT (in Amps) from the supply on Pin #2 to run at identical speeds. However, your BIOS will only be able to tell you the speed of ONE of those fans, and you just presume the other is the same.

For air flow through the case, what goes in MUST come out. What can vary, though, is the air pressure inside the case compared to outside. So if your intake fans CAN push more air than the exhaust ones, the pressure inside the case is slightly higher - called a positive pressure - even though the actual air flow rate in and out is the same. Positive pressure means that, at any small cracks or holes, air will leak OUT of the case and prevent intake of air at that point. Thus if you use dust filters at all the INTAKE fans you can minimize dust intake for the case. This is the setup I prefer. In your situation, though, the MAX air flow rating for the two intakes is 84 CFM, and the max exhaust capacity is 108 CFM. So at max speed for all fans (and at comparably-reduced speeds) the net result will be negative pressure pulling unfiltered air into the case at leak points. You can change this by a suitable change to the confgguration of one or both headers for the two exhaust fans. At those headers you also will have a PROFILE setting option to change the "fan curve" of fan speed versus measured temperature to your own curve rather than the pre-set one. Usually this is a "Custom" or "Manual" option you adjust by moving a few points on a graph for that header. You can simply set that to run the fan slower than "standard". Thus you can reduce the speed of either or both exhuast fans so that their actual air flow over the entire temperture range is slightly lower than the intake fans and the internal pressure becomes slightly positive undere all operating conditions. A CAUTION for you, though, about the MINIMUM speed you set. If you set it too low for the lowest temperture, the fan may actually stall under those conditions and fail to re-start. So don't set the fan min speed too low.

How to do that? You use a smoke tracer technique. You need a small source of smoke - a smouldering cigarette or an incense stick. Get your system running under idle (low load) conditions. and move the smoke source around the outside of the case near small holes or cracks, and observe the smoke flow. If it goes INTO the case, you are sucking air in due to negative pressure. If it blows AWAY from the case, you have positive pressure. If the smoke moves VERY quickly the pressure difference is rather high. Note your results. Now try to get the system moderately busy and do this again. Repeat for a VERY heavy workload (a complex game? or a "stress test"?). I assume from your setup that you will find negative pressure (smoke going into the case). With these results, go into BIOS Setup and set the exhaust fan headers to use a custom fan curve using lower speeds for all the temperature range. Re-do the tracer tests to see which way the smoke moves, and how fast. Repeat this process several times until you can get small positive pressure (smoke flows away from the cracks slowly) under all load conditions.
Last edited:


Nov 28, 2019
To your other question no one answered... 12 doesnt get split into 6v. Thats not how eelctricity works.. 12v will be 12v to both fans.. 120v will be 120v to anything split.. The issues is power draw will lower voltage for length/thickness of wire and amperage usage.. but thats a whole nother topic when it comes to electricity..

Fan headers usually have a max amp usage each.. i think 1.2 or 2 amps ? This is why RGB fans. Tend to get power through USB pins on thr board and only use the fan headers for controllers through the PWM


I did point out that fans on a Splitter are connected in PARALLEL to the power supply lines, but maybe that does not make it clear that this circuit design (as opposed to serial connection) does not "split" the Voltage.

Fan MOTORS and LIGHTS are powered and controlled by separate sources in all current multi-colour lighting systems - plain RGB or ARGB. For this purpose most such fan units have two separate cables, but some makers combine them into one cable, still with separate power sources. The reason for that is simple: motors and lights require very different methods of controlling them. So the set of signals to these different devices needs to be quite different.

A fan MOTOR requires a power supply of 12 VDC plus Ground, and some means of controlling the fan speed. Almost all fans also include a wire to feed the fan speed signal (a series of 5 VDC pulses generated by the fan) back to the header. Fan motor speed control requires either a means to reduce the Voltage supplied (for older 3-pin fans) or a PWM control signal on a separate wire for the fan to use.

The LIGHTS in a fan frame require a power supply and a set of signals to control the display patterns and brightnesses. For plain RGB (4-pin) light systems the power is fed as a single fixed +12 VDC line plus three separate "Ground" leads (one for each of the three LED colours used). Each "Ground" line's resistance to full true Ground is varied by the lighting Controller to achieve varied brightness of that LED colour along the entire strip of LED's. At any one moment, the entire length of the light strip is one colour and brightness.

For Addressable RGB (3-pin light systems) the power for all LED's is a common +5 VDC and Ground line pair, plus a digital packet signal control line. Along the light strip the LED's all are grouped into Nodes. Each Node consists of one LED each of three colours plus a control chip. The chips in all the nodes have unique digital addresses. Each "listens" to the Control Line for an instruction packet with its individual address and then does what it has been told with its own group of three LED's. Thus each Node group along the strip can be different in brightness and colour from all others at any one moment.

As you can see, the differences in power supply and control details for fan MOTORS and LIGHTS require very different signals and hence Controllers., and hence separate signal lines. The signals for LIGHTS cannot be accomplished by a simple USB connection scheme; they require dedicated unique Controllers with direct connections to the light strips. The CONTROLLER for a light set may be built into a mobo with a lighting header, OR may be a third-party box. Such a box normally has its own power supply connection from the PSU. The user interface for that separate Controller box may be a hard-wired manual pushbutton box, a hand-held remote control box, or a software tool that uses a connection from a mobo USB header to the Controller to communicate instructions. The USB connection is NOT the means to power and control the light strips directly.