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Your car's heating and air conditioning systems control the temperature of the vehicle's cabin. So whether it’s Fourth of July in Death Valley or Christmas in Green Bay, at least you’ll be comfortable in your car—provided these systems are working properly.

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The AC system operates by compressing and expanding refrigerant to transition it between liquid and gaseous forms. Liquid absorbs heat when it becomes a gas; gas gives off heat when it becomes a liquid. Using these thermodynamic properties, heat is removed from the passenger compartment and released to the outside air while cool air enters the passenger compartment.

Cars use two different types of refrigerant.

• R12 (Freon) was invented in 1928 and discontinued in the early 1990s after being found to have significant ozone layer–depletion properties.
• R134a is a more environmentally friendly option to R12 used in modern cars and to recharge AC systems in older cars.

The AC system consists of 5 components.

• The compressor is the heart of the system. The compressor is a belt-driven pump fastened to the engine. Refrigerant is compressed and then turned into a hot gas before traveling to the condenser.
• The condenser is where heat dissipation occurs. It uses the air passing through to absorb heat, so the refrigerant can cool and become a liquid.
• The dryer functions as a refrigerant filter, removing moisture and contaminants.
• The orifice tube/expansion valve is a filter that regulates how much refrigerant is released into the evaporator. In older cars, the refrigerant flow is measured and regulated by an expansion valve. As the refrigerant passes through either device, it expands and becomes partly vaporized, rapidly cooling it off.
• The evaporator removes heat from the inside the car. Refrigerant enters the bottom of the evaporator as a liquid. The warm air passing through the evaporator fins causes the refrigerant to boil. As it boils, it absorbs large amounts of heat. This heat is carried off with the refrigerant to the outside of the vehicle, leaving cooled air. Fans blow the cooled air into the passenger compartment.

If your car is overheating, turn off the AC, open all the windows, and blast the heater. This is because your car's heating system is a smaller version of the engine’s cooling system. Hot engine coolant circulates through the heater core, which is basically a miniradiator. A fan (blower motor) that sits in front of the heater core moves the air past it. As this air passes over the core, it heats up and passes through the heater vents into the passenger compartment. Although dissipating this hot air might not keep you cool and comfortable on a hot day, it will help prevent your car from overheating, so you can visit your mechanic before serious engine damage occurs.