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  • Your car’s A/C system uses refrigerant to remove heat from the cabin.
  • The heater uses hot engine coolant flowing through its core to warm your car’s interior.
  • The air management system is the primary connection between the cooling and heating systems.
  • Overall, the heater is more important than the A/C system because of its link to the engine via the coolant.

Most people understand that their home’s heating and air conditioning (A/C) are basically two distinct systems (usually) tied together by ductwork. There’s the A/C unit, which does the cooling, and the furnace, which does the heating.

Together, the two units—and everything that goes along with them—are known as the heating, ventilation, and air conditioning (HVAC) system. 

Your car’s HVAC system is very similar in that the A/C and heater are mostly separate from one another. The A/C uses refrigerant, while the heater uses hot engine coolant. Select parts of the air management system tie both sides of the climate control together. 

To clarify further, we’re going to discuss how both the heating and A/C systems work, and what they have in common. 

How Your Car’s A/C Works

Your car’s A/C system uses refrigerant to remove heat from the cabin. In the past, R12 refrigerant (also known as Freon) was the industry standard. Today, however, most vehicles use R134a. There are also a few applications that employ a new type of refrigerant called R1234yf. 

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diagram of how car ac airconditioning system works
A typical automotive A/C system

A/C operation is more complex than you might think. Everything starts with the compressor—an engine-driven device that pressurizes the refrigerant and forces it through the system. 

This is how the A/C refrigerant cycle works: 

  1. The refrigerant leaves the compressor as a high-pressure vapor. 
  2. Next, the refrigerant travels to a radiator-type device called the condenser. The condenser dissipates some of the system heat into the atmosphere, causing the refrigerant to change from a vapor to a liquid. 
  3. From there, the refrigerant enters a metering device (either an orifice tube or an expansion valve). The metering device reduces system pressure and regulates the flow of refrigerant into another radiator-type component, called the evaporator
  4. The blower motor blows warm air across the evaporator, causing the refrigerant to boil, evaporate, and absorb heat. Then the air, which is now cool from transferring heat to the refrigerant, is directed into the cabin. 
  5. After leaving the evaporator, the refrigerant heads back to the compressor. 

All A/C systems also have either an accumulator/drier or a receiver/drier to remove moisture from the A/C system. 

Here’s a video describing how the A/C system works:

How Your Car’s Heater Works

To warm your car’s interior, the heater uses hot engine coolant flowing through a device called the heater core

Here’s how it works:

  1. The engine thermostat controls the temperature of the coolant. 
  2. The water pump causes the hot coolant to circulate from the engine to the heater core. 
  3. When the driver turns on the heater, the blower motor forces air across the heater core, transferring warmth from the coolant into the cabin. 
    Note: Some vehicles also have a heater control valve that controls coolant flow into the heater core. 
  4. The coolant then leaves the heater core and returns to the engine. 
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heater core
A heater core

Because the heater core is part of the coolant circuit, it’s also part of the engine cooling system. As such, for the heater to work properly, the rest of the cooling system must be in good condition. Issues, such as a low coolant level, stuck-open thermostat, or faulty water pump, can prevent the heater from working as it should. 

Here is a video that demonstrates how the entire cooling system, including the heater core, works:

What the Cooling & Heating Systems Have in Common 

By now you understand how your car’s heater and A/C are separate from one another in most ways—what links the two together is the air management system. 

Basically, the air management system includes everything that controls the hot and cold airflow into the cabin. A molded plastic housing (often referred to as a plenum) is the heart of the system. 

The housing usually contains the A/C evaporator, heater core, blower motor, and most of the air distribution doors. A molded plastic distribution section and inlet section connect to either end of the housing. 

Airflow through the system is controlled by three or more doors, including the air inlet door, blend door, and mode door.

As you might guess:

  • The air inlet door is used to select outside or recirculation air. 
  • The blend door adjusts the air temperature by routing air through the evaporator core, heater core, or both. As was mentioned, on some vehicles, a heater control valve is also used to regulate temperature. 
  • The mode door controls the air discharge location (floor, upper body, etc.).
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The doors may be operated by cables, engine vacuum, or electric actuators, depending on the year/make/model of the vehicle. 

And let’s not forget the blower motor; it drives the fan (squirrel cage) that circulates the air inside the passenger compartment. 

Which is More Important: The Heater or the A/C System? 

If your car’s A/C system stops working, you’re going to be extremely uncomfortable in hot weather. The good news is, a failure within the system is unlikely to cause damage to other parts of your vehicle.

The same thing cannot be said for the heater, though. Because the heater relies on engine coolant, it’s possible for a failure within the system, such as a leaking heater core, to cause engine damage. 

In other words: a failure of either system can cause discomfort in extreme temperatures. But an issue with the heater is far more pressing than a problem with the A/C system.

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About The Author
Written By Automotive Subject Matter Expert at

Mia Bevacqua has over 14 years of experience in the auto industry and holds a bachelor’s degree in Advanced Automotive Systems. Certifications include ASE Master Automobile Technician, Master Medium/Heavy Truck Technician, L1, L2, L3, and L4 Advanced Level Specialist. Mia loves fixer-upper oddballs, like her 1987 Cavalier Z-24 and 1998 Astro Van AWD.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication. Holiday Campaign
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Randy Verburg

How can you start a air cooled VW motor out of the car if the starter isn’t on the motor when you pull it out back !!

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