A mystery : more airflow = cooler mobo + hotter CPU

[Kuroyama]

I'm experimenting to figure out the best noise / performance for my case fans. I've got a 120mm intake in the front of the case, 92mm out in the back, and 80mm out from the PSU.

Ambient temp is ~25-26C. When I turn on the 120mm and run CPU full load I get 29 system / 47 CPU, but when I unplug the 120mm I get 35 system / 46 CPU. So the greater airflow lowers system temps significantly, as would be expected, but raises the CPU temp?!

How can this be? Yesterday when the temps were ~2-3C higher my CPU was running ~50C (with 120mm) as would be expected, so I think the CPU temp readings are reliable.

 

[mgpaulus]

There was some talk about this long ago with SS7 boards. There may be a case where depending upon where the thermistor is located, by creating more
airflow in the case, you are actually dragging more hot air off the HS, and across the thermistor. This would cause your temps to show a little warmer.

 

[Kuroyama]

I have a Shuttle AK31a.

Should I believe the 35/46 temp without 120mm fan? Obviously quieter is better, but with the mobo dropping 6C with the 120mm fan on then I can't help but believe the CPU temp should also be dropping significantly.

Past experience shows there's generally a 18-20C difference between system & CPU temps with the 120mm fan. I'm wondering, if the thermistor is improperly located then is it really only a 10-15C difference, or is the no-fan setup really more like 35/53 than the 35/46 being reported.

 

[mgpaulus]

What kind of fan do you have on the CPU? Can there be some kind of RPM selection based upon system temp, so that when your system temp goes down, the CPU fan slows down, thus allowing the CPU to actually heat up a bit? Otherwise, I'm not sure. It might be interesting to put a clear acrylic sheet in place of the door, and then run some smoke into the case, just to see how the air flows through the case.....

 

[Kuroyama]

I have an 80mm Enermax adjustable speed fan on the CPU, set on max. I don't think the speed is changing, but perhaps I'll try watching the CPU fan RPM with & without the 120mm and see if anything changes.

 

[dazedunconfused]

I have 3 120mm case fans set on a switch for high and low. I noticed that when I turn the case fans on hi the cpu fan rpm drops a little.