 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 (or re-heated if the air conditioning compressor is active).