Step by step guide on how an automotive heater system works, this information
pertains to all internal combustion vehicles and hybrids.
- 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.
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.
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.
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.
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.
Air Deflection Door (plenum removed)
Step 6 - Electrical blend door motors are used
to move a specific door which controls air vent outputs such as defrost, mid and
Vacuum Control Lines
- The heater core itself looks much like
a small radiator, which acts much in the same way, by running hot coolant
through the core while forcing air over it which delivers the heated air.
- When engine coolant is warmed its forced
through the heater core via the water pump, if the engine coolant level is low the
heater will not operate.
Step 9 - A blower fan motor is used to push air
throughout the ventilation system, which is controlled by the driver via the temperature
control knob or climate control computer.
Step 10 - A fuse is used to protect the heater
system which is located in the power distribution center and can be checked by using
Blower Fan Motor
Step 11 - Warm air is pushed through the upper
vents which are located at the front of the dashboard near the windshield. These
vents are used to remove moisture from the inside of the windshield which helps improve
Heater System Fuse
Step 12 - The mid level vents are primarily used
for passenger comfort, and have the ability to control and direct the airflow.
Step 13 - Heater floor vents are used to provide
warm air to the lower portion of the passenger compartment.
Mid Level Vent
Many aspects of the heater system can malfunction which require
air cabin filter is used to clean impurities from the air before it enters the ventilation
system, when this filter becomes plugged it can slow the airflow in which case
An engine coolant leak can develop which hinders the performance of the heater system,
these leaks must be
proper heater operation will return. When a heater core fails, a coolant residue
may be observed upon the passenger side floorboard, this will typically be accompanied
by a sweet, pungent odor. Most heater core inlet hoses can be identified by the
diameter sizes of 1/2", 5/8" and 3/4".
Heater system controls are adjusted by one of two ways, either a manual system
is used which is adjusted by the driver, or a climate control computer responds
to a setting, which is then automatically adjusted. Some manufactures have separated
the heater system into passenger and drivers side controls which offer independent
Some vehicles use a heater control valve which is used to stop the coolant
from flowing into the heater core when the heater system is not being used. If
an audible bubbling noise is heard from the heater core area (like a fish tank)
its is due to air being trapped inside the heater core, in time, the air should dissipate
to silence the noise.
Seat heaters use an electronic element, not engine coolant to achieve the heating
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Article first published 2016-02-04