The automotive heater system is often not thought about during the warm months of the year. But during the winter, when that heater doesn’t work correctly, you realize it fast!

Basic Description  of How the Heater Works

So how does the heater in your car work? The internal combustion engine (Gas, Diesel, Natural gas, etc..) creates an excessive amount of heat whenever the engine is running. This is because there are literally thousands of small fiery explosions inside the engine every minute. All this excess heat usually gets dumped back into the outside air via the radiator. But the passenger heater system can take advantage of this excess heat, and warm the inside of the car with it! The water pump pushes engine coolant (antifreeze) throughout the cooling system, including the heater core. It can take a while for the engine to warm up and heat up the antifreeze, while the engine is cold, the coolant flowing through the heater core is cold, and therefore you don’t have any heat until the engine heats up.  But once that engine warms up, it creates plenty of excess heat to keep you warm even in the coldest winter months.

Diagram of Cooling System

- How Hot Coolant Flows to the Heater Core

Inside the car and behind the dashboard is a box called a Heater or Evaporator Housing. This housing is what controls the air going to your various air vents. The housing has doors inside which can redirect airflow to go through or around the heater core. The more air that flows through the heater core, the hotter the outlet air is, the more that goes around the heater core, the cooler the outlet air.

How the Air Flows through the HVAC Box

— Airflow through the evaporator housing

Most Common Causes of an Inoperative Heater System.

There are various parts of the system that can fail and cause a broken heater system. Some of the more common are mentioned below

1) Clogged Heater Core

- A heater core can become clogged, this is most often caused by lack of maintenance to the cooling system. If the antifreeze becomes rusty or gummed up, the small passages within the heater core will become clogged and will not let the warm antifreeze flow through the heater core.

2) Temperature Blend Door Problem

- A problem with the temperature blend door can cause lack of heat, if the door doesn’t move to allow airflow through the heater core, the incoming air never has the chance to become hot and will stay cold. Blend door problems can be caused by multiple things, including the blend-door actuator, a broken blend door itself, or even a faulty AC control module.

3) Heater Control Valve

- Some cars use a valve called a heater control valve. This valve opens only when the heater is turned on, and what it does is stop or allow the hot antifreeze from flowing to the heater core. If this valve becomes faulty, it can prevent hot antifreeze from reaching the heater core.

4) Bad Engine Thermostat

- The thermostat in the engine is designed to stay closed until the engine heats up, and then only open when the engine reaches a certain temperature. If the thermostat is stuck in an open position, the engine may not be able to reach it’s full operating temperature and will therefore not allow the heater core to get hot.

5) Problem with the blower motor or blower motor resistor

- In order for air to be pushed through the heater core, the blower motor has to be operative. A bad blower motor or blower motor resistor can cause lack of airflow through the heater core.

6) Mode Door Actuator Problem

- A Mode door problem will still allow hot air, but the hot air might come out of the wrong vents. Mode door issues are most often caused by vacuum problems, as the mode doors are usually controlled by vacuum controllers.