My sister called this morning saying her car is overheating. Told her to stop.... drop and roll or just pop into convenience store and get some coolant.

She then started adding it to reservoir as I was afraid radiator was still too hot. Told her to turn heat on to max in car and she asked why...

For ages I have been doing just that when bleeding air from cooling system and today I was trying to find out why.

Having heat on in the car has nothing to do with thermostat. Thermostat is typically in the closed position until it is warmed up by hot coolant. Opens and allows coolant to flow to radiator for more cooling. What’s it got to do with heating running inside car? Anyone?

If heating is on, coolant is allowed to flow to heater core. only thing I can see is making sure there is no air in heater core. Is that it?