I can see where it might make a noticeable difference if you have a fresh air heater. You could have air crossing the core any time the vehicle moves. My '38 isn't a fresh air heater so to get the heat into the passenger compartment I have to turn the fan on. There may be a bit of convection heating with the fan off but not enough for me to notice it.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.