Why is my CPU cooler fan delayed at startup?

Question

I've just finished building a new computer with one of those new-fangled Haswell CPUs; in particular, the Intel Core i5-4570.

I plug in the power, turn it on, and watch as everything starts spinning up right away... except the stock CPU cooler fan. Instead, in the first 5 seconds or so the CPU cooler fan simply twitches a couple of times, after which it starts up with a loud whir for 2 or 3 seconds, before spinning down to normal speed and quietness.

Being a first-time PC builder, naturally I got worried that something had gone horribly wrong with my CPU cooler, especially seeing how the fan blades were twitching in the first few seconds. But apparently, this is normal, and something that many motherboards (including my ASRock H87M Pro4) do by default as part of their fan control procedures, so I'll assume that it's safe. I'm not overclocking anyway.

But what's the purpose of this behavior? Why doesn't the CPU cooler just spin up normally like all the other fans (case, GPU and PSU)? Also, if I can change it so that the CPU cooler fan does spin up normally, should I do that or is it better to leave it as it is with this seemingly unusual behavior?

Answer 1

I actually bugged our Intel rep. about similar behaviour (full speed, then slowing down) in the past. :)

Basically until the SMBus kicks in, the MB has no way of knowing the temperature(s) and so cranks it full blast to ensure it's cooling as much as possible. That way if the BIOS/POST/SMBus (etc.) fail for some reason then the fans will be left on full so that, if nothing else, the system will be doing it's best to correct possible overheat issues which may be causing the POST/SMBus problem in the first place.

My understanding as to why they wait/stutter a bit at first (which IS normal from my experience with the last couple/few generations of machines - but not 5 "real" seconds as you suggest, more like 1 or 2 maybe) is because it's getting initial power, but it's purposefully being stopped/limited for the first bit to ensure that the rest of the system/MB gets proper current to fully energize (fans suck a lot of juice when spinning up and could prevent other parts from getting their required power during the initial power-on). Once that happens (or some time passes -- I'm not sure how it's actually checked/decided) then the full amperage required by the fan is supplied and the fan spins up.