Car heating system: how it works

March 15, 2020
How the heating system works in a car with an internal combustion engine: The heating system is connected to the engine cooling system and uses heat from the engine to warm up the cabin.
Car heating system diagramCar heating system diagram.
There is a heat exchanger called a heater core built inside the vehicle HVAC system (see the diagram). The heater core is a small 'radiator' that is connected to the engine cooling system.

The cooling system of an engine is filled with liquid coolant that circulates between the engine and the main vehicle radiator. The part called a water pump creates the flow of coolant within the system. The thermostat valve blocks the circulation through the main radiator when the engine is cold; it opens when the engine reaches the specified temperature. See the photo of the water pump and thermostat. Read more about the water pump.

As the engine is warming up, hot coolant starts flowing through the heat exchanger (heater core).
HVAC system of a car diagram Air flow in the HVAC system of a car.
The blower motor of the HVAC system creates the air flow. The air flows through the A/C condencer and then through the heater core; see the illustration. Various ducts and doors inside the HVAC system distribute the warm air to the vents. The door that controls the mix of warm and cold air is referred to as a the temperature blend door. Doors that direct the air flow to different vents are called mode doors.

If the air blowing from the vents is not hot enough, the first thing to check is the coolant level in the overflow bottle. If it's low, the coolant must be topped up and the cooling system must be checked for leaks.
Low coolant level Normally, the coolant level should be between Min and Max marks in the coolant expansion bottle.
If you want to top up the coolant yourself, don't open the cooling system cap when the engine is hot as the coolant is under pressure. Check the precautions and instructions on how to do it in the owner's manual. Dealership mechanics typically top up the coolant in the overflow bottle during regular oil changes.

The coolant density also needs to be measured. If the density is not within specs or the coolant appears dirty in the expansion bottle, your mechanic may recommend flushing the cooling system, as the heater core could be partially clogged up. This problem is not uncommon in older high-mileage cars. Flushing a cooling system in a car costs from $140 to $250.

A thermostat that is stuck open can also cause long warm up times, however, in most cars, the Check Engine light comes on if the thermostat is stuck open.
Coolant tester Checking coolant density.
Another symptom of a stuck-open thermostat is when the engine temperature drops below normal when driving on a highway.

In some cars, if the temperature blend door inside the HVAC system is not functioning correctly, it can also cause a lack of heat from the vents. Often, an electrical actuator or parts of the cable mechanism in the manual HVAC system wear out, causing the blend door not to close off the cold air completely. In a system with manual heater controls, it may feel like the hot/cold knob is jamming and not moving all the way to "Hot".

If there is a lack of air flow from the vents even at full blower motor speed, the cabin filter must be checked. A dirty cabin filter can restrict the air flow in the HVAC system. Normally, once the engine is fully warmed up, the air from the vents should be very warm at any speed or when the car is stopped at a red light.