For the Scythe Kotetsu fan, the CPU PWM fan header gave a minimum of 25% speed, which is likely to be the absolute minimum. Switching this to DC raised the min to 50%.
Thank you Mike.
25% minimum for a PWM fan sounds rather high.THIS
thread indicates that the ASUS X99 Deluxe will drop a PWM fan to a minimum of 14% in BIOS.
It would be interesting to know if the 25% you speak of is a motherboard or fan limitation.
In case you find the time to dig a little deeper - keep in mind that Asus QFAN-tune might have a defect when it comes to determine a (3-pin) fans start-up voltage. Source
My guess is that this defect, if present, will affect 4-pin PWM fan tuning in a similar fashion.
Disclaimer, i dont own a Asus Z97/H97, but i do a Z87, so take my comments with a grain of salt.
Personally i find Asus bios fan control to be lacking, specially in terms of allowing the user more control, i find the presets to be relatively good, and probably work for most but that not good enough for me. I find the manual settings very high, at least on the Z87, my lowest was 40%, in past gens was 25% maybe they change it back, but its sad when you see MSI and AsRock give much control to users on pure bios, on MSI you can reach 12.5% as long as the fan can drop there, Asrock allows any %pwm to what i have seens (don't own an AsRock motherboard), and increments on 1% with multiple breakpoints, kinda like a little fanXpert graph on pure bios, something that MSI also implemented on their Z97/H97 on pure bios, but with the restriction of 12.5% as increments.
But not all is great on MSI and Asrock, for example, the weakness of MSI bios fan control is that their 3pin voltage controlled headers (SYS_FANS) can only be dropped to 50%, but instead of the 12.5% increments here is 10% ... but it wont go lower than 50% no matter what, still pretty decent as you can set it up with the graph or setup min/max temps/%, but its no where near what Asus FanXpert will do with their CHA_FANs. AsRock in the other hand that gives you extremely good control with the 1% increments and any PWM % (to what i remember), has its weakness on CHA_FANs, where its only setup as presets.
Personally i feel where Asus excels is FanXpert, where probably its the best, it overrides all bios settings on fan control, and allows fans to drop as low as they can.... but for bios, i wouldn't chose asus.