Step by step guide on how an automotive heater system works, this information pertains to all internal combustion vehicles and hybrids.

Step 1 - All cars are equipped with a passenger compartment heater which is designed to utilize heat from the engine which is transferred into heated air which is then blown into the passenger compartment.

Heater Setting

Step 2 - Once the engine has run long enough to warm up, the heated coolant is transferred into the heater core which is located inside the passenger compartment.

Heater Hoses

Step 3 - The heater and air conditioner evaporator core are housed in the same unit called a heater box or plenum. This unit is where several air deflection doors and ducts are located which divert air to various vents.

Heater Plenum

Step 4 - In this illustration the heater box has been removed to show the air flow diverter door, this door swings from one opening to the next, mixing or stopping the air flow from one vent system to another.

Air Deflection Door (plenum removed)

Step 5 - These air control doors are actuated by cable, vacuum or electrical servo motors which can be controlled using a manual lever, or automatically from a computer.

Vacuum Control Lines


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2015-01-09)