 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.In a car equipped with air conditioning, outside air, or cabin air if the recirculation flap has been set to close the external air passages, is first forced, often after being filtered by a cabin air filter, through the air conditioner's evaporator coil.This can be thought of as a heater core filled with very cold liquid that is undergoing a phase change to gas (the evaporation), a process which cools rather than heats the incoming air.