Some air conditioning (A/C) systems operate on the reheat principle. Reheating means that all of the air passing through the system is first chilled to an almost freezing temperature. The air is then reheated to a more comfortable temperature before being discharged into the passenger compartment.

Dehumidified air is reheated passing through the heater core in the air conditioning module. The heater is warmed by engine coolant passing through the tubes of the heater core. The hot engine coolant transfers heat to the heater core fins. The fins then heat the air. The relative position of the air temperature valve in the air conditioning module determines how much of the total airflow passes through the heater. The amount of reheated air determines the warmth of the total airflow discharged into the passenger compartment.