My ASUS has controls for heat from the CPU. Whe it is set, it will stop the
CPU fan...then turn it on when the temp get hotter I assume. What are safe
settings?
That depends a bit on the ambient case temps, which are
effected by the case airflow and room temps.
The CPU fan not only cools the CPU, but also may aid in
cooling the motherboard, especially when the board has a
passive (fanless) nothbridge heatsink. Therefore,
experimentation and touch-testing the northbridge 'sink and
power regulation area might be necessary to determine the
optimal turn-on temp.
Ignoring those issues a ballpark number for CPU might be
55C- it'll be stable at that speed, even a little (roughly a
dozen degrees at stock speed) beyond it, so that gives a
little margin for the fan to kick in and cool the 'sink
some. It could also depend on how loud your fan is (and
your "need" for lowest noise), a relatively quiet fan might
be set to a lower temp than a louder (and potentially higher
flow) fan.
In many cases, selection of an optimal fan for low noise
will even eliminate the need to turn the fan off at all.