Heater System A hot liquid passenger compartment heater is part of the engine's cooling system. Heated coolant flows from the engine through heater hoses and a heater control valve to a smaller heater core, or radiator, located in a hollow container on either side of the fire wall. Air is directed or blown over the hot heater core, and the heated air flows into the passenger compartment. Movable doors can be controlled to blend cool air with heated air for more or less heat.