How Your Car’s Interior Heating System Works

Maintaining a comfortable level of interior heat is crucial to safe and comfortable driving in the winter. In order to blow hot air at passengers, a car needs to route air to the cabin interior, and to heat up that air.

Air gets into the interior when the heat is on in the same way AC works when your car is blowing cold air: a duct is placed in a high-pressure area, generally near the front of the car, where lots of air can flow in. This airflow is routed to an area where it can be heated up, and then splits into smaller ducts to reach the various vents in your car.

Your Car’s Heating System

Luckily, internal combustion engines have no shortage of heat, so the air that is brought in through the duct can pass by a heater core. The heater core is essentially a small radiator that is filled with hot coolant from the car’s engine cooling system. The airflow passes over this heater core, allowing for heat exchange that cools the core while heating up the air before it reaches the cabin interior.

Automotive Cooling System Diagram

This might seem very straightforward at first, and it certainly is efficient, but the system gets more complicated when it comes to regulating temperature. Without some mechanism to do this, your car’s interior temperature would depend on how hard the engine was working. The two most common ways that cars regulate temperature are by using a water-valve or air-blending. The valve method is more common on older cars, and has a valve that allows more or less hot coolant into the heater core depending on the desired interior temperature. Air-blending systems keep the heater core at full operation, but keep the temperature within a desired range by using flaps that can let in cold outside air to mix with the heated air.

Why is My Car Heater Not Working?

Having your car blow cold air in the winter is terrible, and in some climates in can be downright unsafe. Common causes of a car heater that’s not working include:

  • Low Coolant: If there’s not enough coolant in the car, the heater core might not be able to get enough hot coolant in it to heat up the cabin air. Check this one first, because it is comparatively easy to top off the coolant. Also, air pockets in the coolant system can cause your heater core to not get a proper supply of coolant. This happens often when coolant is flushed and replaced without properly bleeding the system, which needs to be done on certain vehicles to remove these air pockets.
  • Bad Blower: This one’s also pretty easy to diagnose. If your car gets hot, but won’t blow that hot air, you may need a new blower fan motor. To make sure, check the fuse, blower motor resistor, and wiring first including the ground circuit.
  • Faulty Thermostat: If your car’s temperature gauge stays on C well into operation of the engine, you might have a non-working thermostat. Some cars won’t divert hot coolant to the heater core until the engine has heated up. For a lot of reasons, it’s important to replace the thermostat ASAP if it is not working.
  • Clogged or Restricted Heater Core: Coolant that is not changed frequently enough gets contaminated, and this contamination can cause corrosion or clogs in the heater core. Make sure to check every other possible cause first, installing a new heater core can be a time-consuming DIY job or an expensive professional one.
  • Stuck Blend Door / Actuator: Inside the dash, you have a blend door that shuts off the airflow to the AC side of the system and opens the heat-side, or vice-versa. There are also a series of actuators that help open and close the blend door, or various directions that the air flows (like defrost as opposed to the floor or vents) The blend door can become stuck, or actuators fail, causing air not to flow through the heater core properly. 

If something is wrong with your car’s interior heating system, head to AutoZone. Our store associates can help you find the part that fits your vehicle, and we can even help you find a Preferred Repair Shop in your area.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

FREE Loan-A-Tool® program requires returnable deposit. Please note that the tool that you receive after placing an online order may be in a used but operable condition due to the nature of the Loan-A-Tool® program.

Related Posts