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Air conditioning and heating are essential for just about any vehicle. They keep you hot when the climate gets too cold and keep you cool when the sun’s too harsh. Regulating the temperature in the cabin is something all drivers should be able to do, so why not learn more about the systems and components that make that possible?

Air Conditioning and Heating: What’s the Difference?

Most vehicles have an air conditioner and a heater. Both are responsible for altering the temperature, but how exactly do they differ?

There are three ways heat is transferred: conduction (contact), convection (carrying heat away), and radiation (as in warmth from a fire, a heater, or the sun).

To put it simply, the air conditioner blows cool air to lower the temperature inside the cabin. It keeps passengers cool, lowering humidity along with the temperature, bringing comfort and peace of mind on hot and humid days.

On the other hand, the heater does the opposite, increasing the cabin’s temperature to keep it nice and warm. When the weather gets too cold, turning on the heater is the perfect way to combat the discomfort brought about by freezing temperatures. The dehumidifying element of the HVAC system will dry the air before heating it, preventing sudden windshield fogging when the defroster is engaged.

With the air conditioner and the heater, heating and cooling your vehicle becomes infinitely simpler. You can adjust their settings on the fly with ease, giving you total control over your cabin’s temperature.

HVAC: Heating, Ventilation, and Air Conditioning

The air conditioner and the heater are both parts of the HVAC system. This system regulates the temperature in the cabin and improves the vehicle’s overall humidity, purity, and air quality. The last two are when there’s a cabin air filter, which isn’t always the case.

Drivers and passengers alike can breathe inside cars without getting sick thanks to the HVAC system’s cabin air filters (when the vehicle has one) — all while enjoying cool or warm temperatures.

If you’re curious about what HVAC stands for and what the important components of the HVAC system are, read on.

Heating System and Components

The H in HVAC stands for “heating”, referring to the heating system. The heater is responsible for generating hot air. It accomplishes this by using the heat generated by the engine’s cooling system and using the ventilation system to distribute it.

Here are some of the core components of the heating system.

Heater Core

image of car heater core
A car’s heater core | Image Source: Richard McCuistian

The heater core plays a crucial role in transferring heat into the cabin. Hot engine coolant flows through the heater core, then the blower motor distributes it. Think of the core as a small radiator that absorbs and releases heat.

When the engine is first started and the thermostat is closed, all the coolant flows through the heater core to provide cabin heat as soon as possible. On vehicles with heater control valves, many of those will simply divert the coolant so that it travels to the control valve and then back to the engine rather than passing through the heater core.

Heater cores can get clogged by external debris like leaves and dust that impedes airflow through the heater core.

scope inspection of the heater core housing
In the illustration shown, an airflow problem led the technician to do a scope inspection of the heater core housing, where he discovered that the problem was caused by external debris.  | Image Source: Richard McCuistian

Likewise, the internal passages in the heater core can clog with rust and scale, preventing adequate coolant flow through the heater core.

External debris may need to be cleaned out by a professional, because on most vehicles the heater core is difficult to access. Internal clogging requires flushing the cooling system and the heater core’s coolant passages.

, A Comprehensive Overview of Air Conditioning and Heating in Vehicles

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Most heater cores have an inlet pipe that either has an internal restrictor or is smaller than the outlet, and hooking the hoses up backwards can cause the heater core to burst, so be careful about this.

Heater Control Valve

While the heater core absorbs and releases heat, the control valve controls the flow of coolant in the heater. Note, however, that not every vehicle has a heater control valve. Some of them always circulate coolant through the heater core. But if you want to know how a heater control valve works, it’s simple.

When the valve opens, coolant goes through it and enters the heater core. When the valve closes, it blocks coolant and seals off the heater core. Some heater control valves divert the coolant back to the engine, bypassing the heater core (see illustration).

heater control valve types
The heater control valve on the left blocks coolant flow to the core while the heater control valve on the right bypasses the heater core but lets coolant keep flowing. | Image Source: Richard McCuistian

The heater control valve controls coolant flow through the heater core, but the blend door typically regulates the temperature of the heat leaving the vents.

illustration showing compressor usually runs in heat or defrost mode by default
Notice in the diagram that when the blend door is on full warm (up position in the picture), the air still passes through the evaporator, which, when the compressor is running, will dehumidify the air. Usually, the compressor runs in heat or defrost mode by default. | Image Source: Richard McCuistian

Ventilation System and Components

The V in HVAC refers to “ventilation.” Unlike the air conditioner and the heater, the ventilation system doesn’t generate cool air or hot air. Instead, it distributes the emissions of the air conditioner or heater into the passenger cabin with the help of fans.

The blower motor is one of the most important parts of the ventilation system, which is made up of components that perform different purposes:

generic blower schematic illustration
Note: The following section makes reference to the various parts of this generic blower schematic illustration. Vehicles all differ slightly on the way they’re wired, but this will provide a basic frame of reference for the sections below. | Image Source: Richard McCuistian

Blower Motor Relay

The blower motor is directly responsible for turning on the fans that distribute air from the heater or cooler. In tandem with the fan control switch, the blower motor relay manages how much voltage the blower motor uses at any given time.

More power means more speed, accelerating the distribution of hot air or cold air in the vehicle. The switch positions control blower speed by way of a stepped resistor as you see in the diagram. Note that the blower relay in the schematic feeds power to the blower all the time and the switch provides ground to the motor through the blower resistor’s steps.

If the blower motor relay is damaged, there’s a good chance your blower motor will stop working outright. Some vehicles have the blower relay built into the blower resistor, but others have a separate blower relay that can be replaced.

Blower Motor Resistor

Unlike the blower motor relay, the blower motor resistor doesn’t control the voltage that the blower motor uses. Instead, the resistor controls the speed of your vehicle’s blower motor.

blower resistor mounted in the air stream inside the plenum
You can see in the schematic how the blower resistor drops varying amounts of voltage as different speeds are selected because this is what a resistor does; it drops voltage and limits current flow. However, it gets hot in the process, so it’ll always be mounted in the air stream inside the plenum as shown in this photo | Image Source: Richard McCuistian

If you want to know how to test a blower motor resistor to see if it’s malfunctioning, check the blower fan. If the air conditioner or the heater is on but no air blows out of the vents, there’s a good chance something’s wrong with the blower motor resistor.

, A Comprehensive Overview of Air Conditioning and Heating in Vehicles

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Look carefully at the wiring connections on the blower motor and the resistor because they like to melt and oxidize. This means that a new resistor and pigtail will be in order if the resistor is where the connection has failed. This is fairly common on many platforms.

Similarly, if the fan only works at certain settings, the resistor might be damaged. Blower motor resistor damage is commonly caused by corrosion, overheating, and wear and tear.

wiring connections on the blower motor and the resistor tend to melt
 Wiring connections on the blower motor and the resistor tend to melt and oxidize. | Image Source: Richard McCuistian

Air Conditioning System and Components

Last but not least, the A/C in HVAC refers to “air conditioning.” The air conditioning system performs the opposite role of the heater, cooling your passenger cabin instead of heating it.

Air conditioners have many different components, each playing a unique role in distributing cool air all over your vehicle. Listed below are some of the core components of an air conditioning system.


Refrigerant is a special fluid with properties that let it transform into a gas at relatively low temperatures. It can also revert from gas to liquid when the temperature and pressure are reduced. The principles involved are the latent heat of vaporization (evaporating refrigerant absorbs heat) and the latent heat of condensation (condensing refrigerant gives off heat). These diagrams illustrate the most common two types of R134 A/C systems.

two most common types of r134 ac systems illustration
Diagrams illustrating the two most common types of R134 A/C systems. | Image Source: Richard McCuistian

When the compressor is running and the system is working right, evaporating refrigerant absorbs heat as air passes through the heat exchanger (evaporator in the dash) on its way to the register inside the passenger compartment.

Because refrigerant plays a crucial role in your air conditioning and the refrigerant system is a closed system, it’s always best to be on alert for any sign of leaks. Note that due to the molecular bleed-through of the refrigerant passing through the hoses over a long period of time, it’s possible for a vehicle to lose refrigerant even without a leak.


Another one of the most essential parts of the air conditioner is the accumulator, which is also known as the receiver or the drier. Systems with a fixed orifice type refrigerant system will have an accumulator in the low pressure side between the evaporator and the compressor. Meanwhile, systems with an expansion valve will have a receiver-drier either mounted in the side of the condenser or in the liquid line between the condenser and the evaporator.

ac accumulator and drier diagram
AC accumulator and drier diagram | Image Source: Richard McCuistian

The accumulator and drier are both responsible for absorbing moisture to prevent water in the refrigerant from entering the compressor. It also filters any refrigerant in the air conditioner and contains a reservoir for any excess refrigerant. The accumulator naturally contains a reservoir of refrigerant and oil, which is “misted” into the suction line to keep the compressor lubricated.


The compressor pumps refrigerant throughout your vehicle’s air conditioning system. The refrigerant turns into high-pressure gas when it goes through the compressor.

You can tell if you have a bad A/C compressor if the air conditioning doesn’t work, if the refrigerant leaks, or if the drive belt breaks or starts to screech. Fortunately, you can replace malfunctioning compressors.

Typically, the A/C compressor is located at the front of the engine. If you want to access it to conduct repairs or replacements, make sure to recover all refrigerant first.

different types of ac compressor
Different types of AC compressor | Image Source: Richard McCuistian


If you’re wondering how the unused refrigerant in the air conditioner remains in liquid form, it’s because of the condenser. As you can see in the diagrams, the condenser is the heat exchanger where the refrigerant discharges the heat it picked up in the evaporator using the latent heat of condensation principle. That means that as the refrigerant turns from a high pressure gas into a liquid, it gives off heat.


Another crucial part of the air conditioner is the evaporator. Cold low-pressure refrigerant enters the evaporator and is pushed through the vents by a blower fan, cooling the cabin and its passengers. The evaporator is a heat exchanger that is mounted inside the vehicle, and uses the latent heat of vaporization principle to cool the air. As the refrigerant evaporates (at -22 F), it absorbs the heat from the air passing through the evaporator’s fins.

Thermal Expansion Valve and the Fixed Orifice

For liquid refrigerant to begin the evaporation process in the inside heat exchanger (evaporator), it must pass through an orifice, sort of like the way water passes through the small holes in a lawn sprinkler. The thermal expansion valve has a variable orifice based on suction flow temperature. The fixed orifice does the same job but the orifice size doesn’t change. Typically, a low-pressure cutout switch cycles the compressor on fixed orifice systems to prevent the evaporator from freezing up.

two types of expansion valves and fixed orifice
Two types of expansion valves and fixed orifice | Image Source: Richard McCuistian

HVAC Troubleshooting: Common Problems

The HVAC system occasionally experiences issues, typically when parts break or malfunction or mold and other foreign contaminants build up and clog the condenser. Here are some of the most common problems drivers experience with their vehicle’s air conditioning, heaters, and ventilation.

Why Doesn’t My Car’s A/C Cool the Cabin?

If you’ve got problems like your car A/C not cooling, then there’s a possibility there’s something wrong with some parts of the HVAC system. Usually, the problem will be low refrigerant, but there can be issues with one or more components.

Why Does My Car Overheat When the A/C Is On?

Typically, your car overheats when the A/C is on because the air conditioner puts a lot of extra strain on the engine. Because the A/C condenser gets hotter when the air conditioner is active, it also heats up the engine by proximity.

Under normal circumstances, the engine can function without overheating even if the A/C is on. However, if either the engine or the A/C is damaged, the likelihood of your engine overheating increases.

In short, it’s important to turn off your A/C if the engine overheats to reduce the load on it.

, A Comprehensive Overview of Air Conditioning and Heating in Vehicles

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: If the condenser fan is operating, overheating when the A/C is on won’t typically happen. But sometimes there will be debris (like plant trash) trapped between the condenser and the radiator (which is behind the condenser on most vehicles), and you won’t be able to see the clogging unless you unbolt the radiator and tilt it back to look.

Why Are There Strange Smells Coming From My Car’s A/C and Heater?

Some reasons why your car A/C smells bad include a dirty cabin filter, mold and bacteria inside your A/C system, and a clogged condensate drain line.

You can deal with mold by putting the vehicle’s A/C on high blow and “norm” (not “recirc”) and then spray disinfectant in the cowl vents at the base of the windshield. This will enable the disinfectant to coat the evaporator and will usually take care of a mold smell.

The best way to prevent foul odors in your vehicle is to change the cabin air filter regularly if the vehicle has one.

Why Is My A/C Switch Not Responding?

One of the most common signs your A/C switch has developed issues is if changing the settings doesn’t do anything. Air door actuators fail so that they don’t redirect the air, and sometimes they’ll make clicking noises. Other bad A/C control switch symptoms include the A/C compressor failing to turn on and the A/C blowing hot air instead of cold air. This is likely to be an electrical problem. However, on many platforms, if the A/C refrigerant is low, the compressor won’t engage, and this is the most common scenario.

Why Is There A Rattling Noise Coming From the A/C?

Your air conditioner isn’t meant to rattle. If you’re hearing rattling noises coming from the vents, you might need to replace the compressor, which also requires flushing the system to remove debris from the failed compressor and replacing the accumulator or drier and the expansion valve or the orifice. If the noise only happens when the blower is at higher speeds, there may be a problem with the blower motor.

A malfunctioning serpentine belt tensioner can make all manner of rattling noises. Note that the belt can look good and still make noise due to a failed damper within the tensioner body.

Where to Buy New HVAC System Components for Your Vehicle

Replacing HVAC system components has never been easier with The best part? You won’t even have to leave your house.

Visit and use the vehicle selector tool to view compatible HVAC system parts for your A/C, heater, and ventilation system.

Our parts are sourced from some of the most trusted names in the industry, and our warehouses are strategically located all over the US. They’re also on hand and ready to ship, so you can receive your order in a matter of days.

Don’t hold off on getting new parts for your vehicle’s HVAC system. Check out our selection of air conditioning and heating components now at

About The Authors
Richard McCuistian, ASE Certified Master Automobile Technician
Reviewed By Richard McCuistian, ASE Certified Master Automobile Technician

Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

CarParts Research Team
Written By Research Team

Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

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.

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