MacBook – When do the Macbook Pro fans turn on

Lets me try to help you to understand what is going on here:

Any third party software uses the already available information from your Mac as designed by Apple that uses thermal diodes attached to different points in the system. Apple has done a lots of work to correctly manage the thermal load already.

The coding and the mathematics to covert the given information correctly in to the Temperature readings outputs by third party software could be tricky, so use it with grain of slat, since it varies a lot from Hardware (diode) + EFI + CPU types from system to system. In English, the third party software's are not always calibrated for your specific system.

Note from Intel:

Intel® Processors have built-in thermal protection. If the processor gets too hot, the built-in protection shuts down the computer. If your computer is not over-clocked and is running under the design specifications, the built-protection can help prevent damage to your system.

High Temperature readings:

If you use a third party software to measure the temperature or the Extensible Firmware Interface (EFI) readings, contact the software vendor to ensure the software is validated to work with your processor.

If you still want to know more or you see weird Temperature reading, please go to Intel web site and look for your specific CPU specifications. It can not be hotter outside then inside since the CPU is the source of heat, otherwise your computer will just shut down initiated by Intel's CPU internal protection.

However, if that happens (the shutdown by the CPU), then you need to repair your cooling system (fans + heatsinks ect.)

So I just looked up at Intel for my Intel Core i5 (1.8 Ghz) and it shows Maximum temp to be 105 Degree Celsius and it has the build in thermal protection.

The short answer appears to be: yes, 12 degrees C (22 degrees F) or more cooler.

I got ahold of a custom-build Macbook Pro that has identical specifications except that it had no discrete graphics adapter. I set it up on the same table in the same room, connected to an identical monitor. I put both of them through the same sequence of operations, using yes > /dev/null & one or more times to occupy whole processor cores, running WebGL demos, plugging and unplugging monitors, etc. in lockstep. Temperatures, fan speeds, and CPU use were observed regularly using iStat menus, yielding 120 separate observations. Using the R statistical package I fit some models to these data and retained this one for the "left fin stack" temperature:

> summary(fin_fit)

Call:
lm(formula = fin_temp ~ dgpu + ext_disp + cpu_load + gpu_load,
    data = heat)

Residuals:
     Min       1Q   Median       3Q      Max
-12.1091  -2.8460  -0.2888   2.5413  12.1540

Coefficients:
            Estimate Std. Error t value Pr(>|t|)
(Intercept)  32.8460     0.7066  46.484  < 2e-16 ***
dgpu         12.2631     0.8067  15.202  < 2e-16 ***
ext_disp      2.4160     0.7836   3.083  0.00256 **
cpu_load      3.4386     0.4173   8.240 3.11e-13 ***
gpu_load     14.0506     1.2601  11.151  < 2e-16 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 4.138 on 115 degrees of freedom
Multiple R-squared:  0.821, Adjusted R-squared:  0.8148
F-statistic: 131.9 on 4 and 115 DF,  p-value: < 2.2e-16

Interpretation: This model explains about 82% of the temperature variation. The base operating temperature is about 33C, every fully occupied CPU core adds about 3.4C, a fully occupied GPU adds about 14C, each external display attached adds about 2.4C, and using a discrete GPU adds about 12C. All of these variables are very significant in explaining temperature variations, and the model as a whole is quite effective at predicting temperature.

There is an apparent jump of 10 watts or more in power consumption when using the DGPU and an external monitor. This would be coherent with the extra heat, but I did not fit a model for this response variable.

I expect the fin heat model would fit even better if I excluded all observations that happened shortly after a change of state (change in CPU or GPU usage, change in presence of external monitor). There’s some amount of thermal inertia - it takes a minute for the heat sinks to warm up or cool down.

Conclusion: On the stock Macbook with a discrete graphics adapter hard-wired to the external monitor ports, from a temperature perspective attaching an external monitor has greater impact than performing heavy computations. The fans become audible frequently under load, and the keyboard can get uncomfortably hot. On the other hand, when using the model with no discrete GPU, the fans remain quieter and temperatures more moderate, as attaching an external monitor yields only a slight increase in base temperature.

Other observations: the dual-GPU model will also enter and remain stuck in discrete GPU mode when it doesn't really need high performance graphics, which can discharge the battery very quickly and cause unnecessary heat. At one point in my testing after unplugging the external monitor, it became stuck in DGPU mode. The activity monitor reported that this was due to the Terminal application requiring accelerated graphics (!).

Coming out of sleep with a monitor attached or switching on an already-attached monitor, the monitor often seems not to be detected or used by the OS. GPU switching seems to add a lot of complexity and be susceptible to glitches.