Originally Posted by
feral
Just correcting a possible bit of mis-information here.
Once the thermostats are all open the coolant is in circulation continuously throughout the vehicle. This includes the heater core, radiator, fuel cooler and all pipe work. This occurs even if its 50 deg outside and your air con was flat out. If fact, even with the thermostats closed the hottest coolant available is first fed through the heater core.
The heater temperature is controlled by flaps in the heater not by the blocking or stopping the coolant flow through the heater core.