Control
Once the engine has warmed up, the coolant is kept at a more or less constant temperature by the thermostat. The temperature of the air entering the vehicle's interior can be controlled by using a valve limiting the amount of coolant that goes through the heater core. Another method is blocking off the heater core with a door, directing part (or all) of the incoming air around the heater core completely, so it does not get heated. Some cars use a combination of this system.
Simpler systems allow the driver to control the valve or door directly (usually by means of a rotary knob, or a lever). More complicated systems use electronics to control the valve or doors.
Cars with dual climate function (allowing driver and passenger to each set a different temperature) may use a heater core split in two, where different amounts of coolant flow through the heater core on either side to obtain the desired heating.
Read more about this topic: Heater Core
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