Originally Posted by thejordan
Thermostats open when hitting a certain temperature (150-180F for most cars), and fans kick on and off as well, which all act to cool the engine, especially at idle. Running the heater acts as an additional source for heat removal so the thermostat doesn't have to open as often.
Agree. With a cold engine. the thermostat is closed until the coolant warms up in the engine enough to open the thermostat. then all that hot coolant flows out to the cold radiator, and the engine is cooled by the incoming cold coolant.
So it would be natural in cold weather for the heat to fluctuate some untill the system reaches a steady state.