Installing a new PC with a Venomous-X heatsink and a pair of Slip Stream 120mm PWM fans, hooked up to the motherboard (Intel DX58SO2) via a PWM fan splitter cable. First, both fans work fine at the full 1300 RPM when PWM control is disabled. If I enable it, I've noticed something odd: the fans don't spin at idle temperatures. I can see them twitch every couple of seconds. If I give them a flick with my finger, they will spin up, and the motherboard reports ~120 RPM.

I notice the official specs read as "0 (+200RPM) - 1300 RPM", and does give noise and CFM ratings of "0" at the bottom end. Does this basically mean the fans won't start turning until demand is at an equivelant of 200 RPMs? As far as I can tell they do start turning as cooling demand by the motherboard increases, but is this normal behavior?

Note that I haven't really done any software installation yet, and haven't installed the motherboard utilities which may give me further control over this.

I've read that. I personally find it hard to believe. Can you set a minimum of 500 or 600 rpm in the bios? Try that.

No, the BIOS only offers "enabled" and "disabled" for CPU fan control. Also I should note that the motherboard does NOT support voltage regulation on the CPU fan, only PWM.

You have no reason not to believe the specs. It is normal (but not usual)

I have two of the same Scythe Slipstream fans which are also on a PWM fan splitter. One is deployed as the CPU fan, the other as an exhaust. The normal idle speed is around 450 rpm under automatic control with a non-Intel motherboard. Using software I have tried setting the PWM duty cycle manually to 0%, but it does not actually stop the fans. Instead the fan speed is reported as 176 rpm. This seems to be in line with the spec quoted by Scythe, a minimum fan speed of around 200 rpm.

So what you are experiencing does not seem quite right to me. Does the same thing happen if you don't use the splitter cable and instead connect one of the fans directly to the motherboard PWM fan header?

It is normal, the starting pulses aren't overcoming the magnets in the motor.

A software solution will fix the problem by overriding the BIOS and sending its own signals to the fans; Increasing the minimum speed will cause the fans to start spinning on their own.

Well, here's some more. I installed Intel's tuning utility and it DOES let me adjust all of this.

The minimum CPU fan duty cycle allowed by the software is 15%, which results in 93 RPM (assuming I start the fan manually).
The default duty cycle was 20%, which results in 150 RPM.
The minimum duty cycle that the fan can start itself from a stopped state is 31%, or 380 RPM.
At 40% I get about 580 RPM, and 50% gives me 770 RPM.
100% shows 1430 RPM, which is a bit above specs of 1300 RPM.

I'll probably set it to somewhere between 40 and 50%.

Try setting the BIOS settings for 35 or 40 percent. If the fans spin 100% right after the power button is pushed and right before the machine turns off, they've probably been stopping before the software could take over. If the motherboard itself is configured to keep them going, your program can take over once you log into Windows with a minimum speed of 15% without worrying.

As mentioned in an earlier post, the BIOS only has "on" and "off" for speed settings. Also, when the system turns on, it doesn't actually spin the fans at full unless the system was away from power. Otherwise they start at the BIOS reduced settings (which is a bit odd as my other Intel m/b systems usually start full speed for a few seconds and then cut down).

I've noticed that even customizing using the Intel software, the system continues to boot with the BIOS default settings (of 20% CPU). About 45-60 seconds after Windows appears to be done booting, the "customized" settings take over.

Could just be growing pains. Intel's boards are usually very stable, but tend to take 6-8 months to get the minor kinks worked out.

At minimum I'm happy the board has 3 separate and functional temperature controlled zones - CPU, Intake + Outlet (2 headers), and Aux (tied to northbridge temp, which barely seems to move anywhere).

It's funny some people go through extreme measures to get their fan(s) to behave just like that (not spinning at idle, and spinning faster as the temp rises).

I'm glad you figured out a software solution to make it spin all the time.

From a noise concept, I like the idea of 0 RPM. However this is a high-end system (i7-990X), and frankly I need the air movement, even if slight.

For example, last night I "balanced" the fans. The default setting for the CPU fan was to start ramping up at 71C. The problem is, the CPU fan/heatsink is perfectly capable of keeping it at 63C under a full stress test with only a 400 RPM fan speed (I'm defaulting to ~600 RPM). However this means the components around the CPU would start getting hot due to inadequate airflow (voltage regulator) and the motherboard would crank up the case fans to compensate - eventually to full, which is no longer very silent (Slip Stream 1200 RPM). By lowering the CPU fan threshold to 55C, the CPU now maxes out at 60C, the CPU fan adjusts up to 900 RPM (still very silent) and the case fans also stay around 1000 RPM with the voltage regulator a full 10 degrees cooler than before. Subjective noise level is like half, but should I do something that REALLY taxes the system, there's still the capability to add more cooling should it be needed.

General system use is essentially silent, and it takes a couple minutes of stressing the CPU before anything starts to move.

No argument with that.

I'm running a lower power system and even though I can easily stop my CPU fan I leave it running at ~350 RPM when the system is idle for a number of reasons.

I just was amused that you had the opposite problem of the average poster on SPCR and since you'd already solved the problem I figured it wouldn't bug you too much for me to make that observation.

Very true. Silence means different things to different people with different needs. My personal systems need to be cutting edge, but as silent as possible. I absolutely do have a much smaller acceptable noise threshold than the "average" persion, but I know I can't get that kind of power with total silence. I still want near silence when idling (I do a lot of writing, and computer noise annoys me when I'm thinking), but I'm willing to accept some noise if the system is doing a lot of work such as rendering video... or gaming.

If anything, this is tougher than making a completely silent low-powered system, because with high power components you DO have to deal with unavoidable heat output. Like I found I can keep the CPU perfectly cool with low RPMs (even 350-400 RPM is enough), but that doesn't help the rest of the heat producing components around the CPU without boosting case fans to an unacceptable level (what use is a silent CPU cooler drowned out by case fans).

Right now I'm pretty happy, and I don't even have the P183's top fan installed yet (waiting on the right bearing version). Incidentally, the Antec CP-1000 is quite satisfactory from a noise level. Hopefully it lasts longer than the Phantom 500 (I've bought 3, and have had 2 fail - the 3rd is actually still going).