I had been letting Firestorm run even when I wasn't around or awake as a virtual presence made some sense even then. The situation changed several weeks ago and I stopped keeping a near-constant presence via Firestorm. One morning recently I got home and found belgian's CPU fan alarm was buzzing. The system had not failed. It was just that Winter is approaching.
I had the side of the case off for something or other and hadn't put it back into place yet. It was a very cool morning, and Firestorm wasn't running. Firestorm seems to keep one core (or one core's worth of CPU...) fairly busy. Thus the CPU got cool enough that the motherboard controlled fan wasn't needed (there is a second, uncontrolled fan, set to slow). The motherboard BIOS, however, doesn't have a very smart alarm. It makes noise if the fan isn't running - no matter if the reason it isn't running is that the system itself switched it off as unneeded.
So I put the cover back in place (after giving the idled fan a quick manual spin to hear the alarm shut off for the bit it was spinning) and fired up Firestorm to burn some cycles and warm things up. As if I hadn't already known it, this all showed that the CPU cooler really does the job.