Although not part of the A/C system, the heater core is another heat exchanger. In a vehicle, the heater core is located in the same case as the evaporator because automotive A/C systems are known as series reheat systems. That is, all the air entering the vehicle first passes over the A/C evaporator, and then is directed over the heater core should heat be desired. In cases where cooling is desired, the air is defleted past the heater core, directly into the vehicle.

For maximum temperature control and passenger comfort, the blend door works to mix both cold and hot air so that a more even temperature and comfort level can be maintained.

In order to provide heat, the heater core uses engine coolant. Being a heat exchanger, the heater core causes the heat in the engine coolant to be transferred to the incoming air.

Heater cores typically fail by leaking or by plugging. When internal corrosion restricts the flow of engine coolant enough, the level of heat produced by the heater will be diminished. Flushing a heater is typically not a reliable solution as the corrosion that it holds is not easily removed. If it is removed, in most cases it exposes leaks.

Heaters also leak. The tell tale sign of a leak is when you see a mist coming from the vents and a film being left on windows of the vehicle. The only guaranteed solution for leaking heaters is to replace them. Using stop leak products will typically stop the leak, but in doing so, restrict the flow of engine coolant enough tha the level of heat is no longer adequate.