Product review (2 stars): Heat Sentry turns on at around 100 degrees (F)
I purchased the QP4 based on its low speed noise level.  The unit is very quiet at the low speed, but it's impossible to use.  As soon as I turn on the stove or even boil water the Heat Sentry feature ramps the speed of the range hood to medium or even high.  I contacted Broan Technical Support and they they were very friendly and sent me a new thermistor.  The tech said that the Heat Sentry would activate at around 150 degrees (F).  I received the new thermistor and characterized the resistance change as a function of temperature.  The new thermistor and the original thermistor read the same. At room temperature the resistance is around 10 k Ohms.  At 150 degrees F the resistance was about 2.33 k Ohms.

I purchased a 10 k Ohm potentiometer and connected it to the Heat Sentry circuit.  I Ran the range hood in low speed and adjusted the potentiometer until the  Heat Sentry activated in medium speed.  This occurred at 6.42 k Ohm.  I then continued to adjust the potentiometer until the Heat Sentry switched to high speed.  This occurred at 4.78 k Ohms. Based on the above measurements it appears that the Heat Sentry circuit activated around  100 degrees (F) and switched to high speed around 110 degrees (F).  I contacted Broan Customer Support again with this new information and was told that they would send me a new Heat Sentry circuit board. I will wait to see if that fixes the issue.

As to the range hood itself, it looks good and the halogen lights are very bright (and hot).  The suction is definitively stronger on the left side (where the blower is located).  At this point I am not happy with the unit and wish that I had gone with my second choice (one of the Kobe units).

Update:  I installed the new Heat Sentry circuit board and there was no change in the activation point.

Broan provided the following:  The board looks for temps equivalent to 113 F, 122 F, 131 F and 140 F.  If the hood was off and the temp reach 113 F, it would start on low, 122 F it would ramp up Medium.  131 F- High, and 140F - Boost.