How Does An Air Conditioner Work? AC Units Explained

When your home gets too warm to be comfortable, you head to a switch on a wall and turn on the air conditioner. After a few minutes, your home is cool and cozy. But do you know how your air conditioner works?

This article will dive into all things AC and explain the different types of systems, system parts, and give you an understanding of how the entire process works. This knowledge will help you with troubleshooting, installation, and repair and can even save you money on your energy bills.

The History of Air Conditioning

To understand modern air conditioning, you should know where it all came from. We can thank the engineers and pioneers of the late 1800s and early 1900s for the home climate control systems we have in our homes today.

While ice was used to cool refrigerators and keep food fresh, little was done to cool homes and businesses. It wasn’t until 1902 when a paper and textile company in Buffalo hired an engineer named Willis Carrier to devise a system to prevent the humidity from wrinkling their papers.

While he was successful, adding or removing humidity by heating or cooling water, he soon broke from the paper company. He started his own business with half a dozen other engineers. The company Carrier Engineering Corporation is still around today, cooling millions of homes.

From there, auditoriums and theaters began being cooled by rudimentary prototypes until the systems were near-perfected. In the 1930s, engineers working at companies like General Electric and General Motors made the systems smaller, more portable and introduced chlorofluorocarbon refrigerants.

With synthetic cooling materials, better designs, and the ability to control humidity and temperature, these units started making their way into homes.

Today we still use a lot of the same technology, parts, and cooling principles created at the turn of the 20th century.

Basic Components of an Air Conditioning Unit

Understanding the main components of your air conditioner system will help you in troubleshooting and repairs. Each part has a specific job to do, and if it fails, the result can impact the entire system.


The evaporator is a series of copper coils and fins. As the refrigerant enters the evaporator, it loses pressure, turns from a liquid to a gas, and becomes much cooler than the air around the coils. This causes the heat to become absorbed from the air, resulting in cooler air that is sent through to the home.


The condenser is the opposite of the evaporator. In a split AC system, this is the portion mounted outside.

It has smaller coils that condense the refrigerant. This condensing results in heat loss that is vented to the outside.

As the refrigerant cools, it is sent through the compressor.


The compressor is generally mounted inside the condenser unit and takes the cooled refrigerant gas, compressing it into a high-pressure liquid.

The higher compression causes the liquid to heat up and is then pumped back towards the evaporator unit.


There are generally two fans in the system. The main fan is on the condenser unit.

It helps pull the heat from the refrigerant and expels it into the air outside. It helps keep the compressor cooler and pulls heat from the coils of the condenser.

Inside, the fan blower is used to collect the warmer air from the home and blow it over the colder coils of the evaporator.

The high powered blower pushes the colder air through the vents into the various rooms of the home.

Expansion Valve

The expansion valve is located on the high pressure line leading from the condenser unit. It removes the extreme pressure of the refrigerant from the compressor and allows the liquid to return to a low-pressure gas state once it reaches the evaporator.


The thermostat is the main control system of the unit.

Here you can set the temperature for the home, which then controls when the condenser, compressor, and fans operate or shut off.

There are many types of thermostats, including manual, digital, and wireless smart models.

Related Article Analog Vs Digital Thermostat


The refrigerant is the cooling material running through the system. The most common type is R-410a for residential and commercial applications. Older systems use R-22, which is now phased out completely and no longer produced due to its harmful effects on the environment. Refrigerant is sometimes referred to (inappropriately) as coolant.


The capacitor is a small device located in the main electrical panel of your condenser unit. It has a single job: start the system. It stores and uses energy to provide a high voltage jolt required to start the fans, motors, compressors, and blowers.


AC Filters are used to clean the air before it is taken from the home into the evaporator to be cooled.

This helps keep the internal system clean of dust and debris and removes contaminants, allergens, and even smoke or odors from the air inside your home.

Sometimes, the filters becomes too dirty and a replacement is in order to get cold air coming in again.

Some air conditioners come with a permanent filter in which case, cleaning it is something you should do.

Understanding How Central Air Conditioners Work

Your central air conditioner system is a large and relatively complex piece of machinery. However, once you understand all the working pieces and how they all fit together, it becomes much more straightforward.

Because the central AC system works in a loop, we could technically start anywhere, but let’s begin at the thermostat. The thermostat is the main point of control that you use to set the temperature, timers, run the fan, turn the system on or off, and check on your settings.

Most systems will be a total unit, meaning they have heating, ventilation, and cooling capabilities (also known as HVAC). For this article, we are mainly focused on the cooling aspect. When your home becomes too hot or humid, you can turn the thermostat to turn the system on, set it to cooling mode, and set a temperature.

The Temperature Rises Above Set Levels

When the thermometer inside the thermostat (or wireless sensors in other rooms) rises above the set temperature, the thermostat turns the system on. This sends a signal to the capacitor.

The capacitor has stored energy and sends a high voltage shock to the system (usually between 400 and 600 volts). This jolt causes the compressor to kick on, the fans and blowers to begin turning, and the refrigerant begins moving through the system.

At the evaporator, low-pressure gas draws heat from the air brought in from the blower. This air is the same air in your home, suctioned in through the intake vent(s) in your home. The warm air is blown over the evaporator coils, which removes the heat from the air and stores it in the refrigerant running through the copper lines.

Hot Refrigerant Moves to the Condenser

The now hot refrigerant gas travels through the low-pressure line out to the condenser unit. As it travels through the condenser coils, the condenser fan pulls ambient air from around the unit in and over the coils. This causes a heat exchange, which rapidly cools the refrigerant.

The heat exchange causes the gas to return to a liquid state as it enters the compressor. The compressor is the heart of the system, pumping the refrigerant to and from the evaporator.

As its name implies, the compressor compresses the liquid refrigerant to a high pressure, which causes it to heat up, and this near-instant heating forces the liquid through the smaller, high pressure line to the thermal expansion valve.

High Pressure Liquid Passes The Expansion Valve and Filter

The expansion valve has a single job. It releases some of the pressure of the liquid refrigerant, cooling it slightly, allowing it to expands somewhat as it reaches the refrigerant filter. This filter removes any oils and impurities that may be present from the compressor.

The clean, cool liquid refrigerant then makes its way to the evaporator coils, where it once again draws in heat from the air, forms in a hot gas, and returns to the condenser. The air passing over the evaporator’s coils is supercooled, reaching temperatures around 40 degrees (Fahrenheit).

Cold Air Enters Your Room

The cold air is then pushed through the vents and out into the various rooms of your home, resulting in a cooler temperature throughout the house. Once the temperature drops below the set temp on the thermostat, the thermostat will stop the system by removing the electrical input to the capacitor and contactor at the condenser.

The thermostat will then sit idle until the temperature rises, once again, above the set temperature. The thermostat then sends the power back to the capacitor, the system turns on, and the entire cycle repeats.

Other Types of AC Units & How Each Works

The central AC system is the basis that all other systems tend to model. Each different version makes different changes to certain components, location of the parts, or the order of operations. Let’s take a look at the different air conditioner types now to get a better understanding of how they work.

Mini Split System Air Conditioners

Mini split systems, as the name implies, are like central systems, only smaller and split in half. The external half, a heat pump system, cools or heats as needed and sends the cool air into the home through ducts (in a ducted system), or the process is all internal in a ductless system.


A ductless system uses a heat pump outside the home. The heat pump works much like a condenser unit but can also run backward. When in cooling mode, the heat is removed from the home and pushed outside. In heating mode, the warmth is pulled from the outside and pushed into the home.

Inside, you have the copper refrigerant lines running from the heat pump to the evaporator and fan unit. These are wall mounted units that works like the evaporator in a central system. They blow air over the coils and directly into the room the interior unit is mounted in.

They are among the most energy efficient air conditioners on the market. However, they only work in rooms that have an interior evaporator unit mounted. Multiple rooms require multiple interior units. The external heat pumps can also only support a certain number of internal units.


Ducted mini-split systems work much the same way as ductless, with the exception that they are connected to the various interior fan units through ducting.

The external part is still a heat pump, a reversible component that moves heat in both directions, based on need. However, instead of individual evaporator units inside, you have a single evaporator and multiple fan units. The system heats or cools as needed based on the individual settings of the fan units mounted in each room.

You do save money with these models as well since you can heat or cool a room, multiple rooms, or the entire home. However, they require more installation and must be positioned in specific areas of the room, or they may not work.

Small Portable Air Conditioner

Portable air conditioners take all the components of a central AC system and put them in a compact model that can be moved from room to room, permanently mounted or installed.

Window AC

Window units are compact air conditioners that sit inside your window frame. The top of the window is used to hold the unit in place as it sits on the lip of the window sill. These units have a small compressor and a fan. They pull air in from the outside, cool it off in the same way as a central AC, just on a smaller scale.

For most window ACs, they can cool spaces of a single room or two, up to about 800 square feet (usually much less, though). These models aren’t known for their overall efficiency ratings but cost much less to use than a central AC system.

However, because they are designed for a single room each, you would need multiple units to cool an entire home.


Wall-mounted AC units take the space of a mini-split system and the compact, all-inclusive design of a window unit.

These wall AC models are mounted on the wall near the ceiling and use remote controls or mobile apps for control. They have a small compressor unit and evaporator coils that cool the air taken in from the sides or bottom.

The cooled air is pushed out through the adjustable front louvers to cool a single room. They are generally more efficient than a window or through the wall design, but offer smaller capacities for cooling areas.

Through the Wall

Through the wall ACs are identical to window units in almost every way. They, too, take all of the central AC components and put them in a compact form. Designed to cool a single room or open space, the difference here is that windows aren’t used.

Instead, a hole is made through the wall from the inside to the outside. A cage is installed to hold the AC, which then slides in place, and vents as well as condensates outside, reducing clean up and improving airflow.

For DIYers, they can go with build a window AC support frame but there is always the option to just buy an AC window bracket if your unit did not come with one.

Evaporative Coolers

Evaporative coolers, also known as swamp coolers, are a different type of cooling system. Instead of using refrigerant and removing heat, they add cool air to the space through the use of water or ice (or both).

A large fan blows air over cold water or ice, which lowers the temperature of the air. That cooler air is then blown into the room or space. The benefit of a swamp cooler is that you can use them indoors or outside.

Larger units can even sit in the yard or garage and cool quite a bit of space, in direct sunlight or on a covered patio. However, they require a lot of upkeep and add quite a bit of humidity to the area. If using indoors, you need to get an indoor rated model with collection trays for the absorbed moisture.

Boat Air Conditioners

Boat air conditioners work a lot like mini-split systems, utilizing a heat pump to reverse flow depending on needs. The air is blown over coils to chill and get released into the cabin, cooling the space.

Boats can use refrigerant-based systems or utilize pumps that pull in and remove seawater for water cooling effects instead of refrigerant. Water-cooled systems are also found in solar ACs, but on a boat, the reserve water is seawater, making it much cheaper to run and more comfortable to cleanout.

Related Article Best Boat Marine AC

Tractor/RV/Large Vehicles

The AC system in an RV or another vehicle has fewer parts and relies on the battery and engine power of the auto to power the system. It still uses refrigerants but has a different formula than residential systems. Usually, you will use R-134a refrigerant, though there are other options based on the system’s age and type.

You will also have a small compressor and the refrigerant lines that run through a coil to cool and remove heat. Instead of using an evaporator unit, though, the coils are located near the radiator to use the air from driving to the engine fan to cool the coils.

Under the dash, you will find the evaporator itself, along with the ducting that leads to the vents where the cool air is pushed into the cab. Instead of higher voltage, the compressor is run from a belt-drive, and the removed heat is exhausted into the engine compartment where driving and the engine fan remove it from the engine bay.


Solar power is used to run window and through the wall systems as well as some mini-split and even central AC systems. These systems can be refrigerant or water-cooled, depending on the setup and needs of the home.

Instead of using electricity as the primary power source, solar panels are mounted (usually on the roof) to provide the energy needed to run the system. In many cases, the correct number of solar panels in your grid dedicated to the AC will be sufficient.

However, it is possible to run a hybrid system that uses solar energy to run the system but electricity to power the thermostat and supply the voltage required to start a larger system.

People Also Ask (FAQ)

How does a DC inverter air conditioner work?

The compressors in all ACs use an electro-motor. In a typical installation, the electricity supplied is AC, causing the compressor to need massive jolts of voltage to start. In a DC motor, the compressor uses a variable speed to control the temperature and output of the compressor. In theory, these motors never completely shut down, meaning they don’t need the large voltage jolt to get running.

They also require less energy overall, run cooler, and last longer. Because there is less voltage needed to start, run, and utilize the compressors in an inverter system, they are much more energy efficient to use.

How does a water-cooled air conditioner work?

Instead of relying on refrigerant to transfer heat from the air, water is used. A pump will pull water through coils instead of refrigerant. The water conducts heat the same as a refrigerant (though in most cases at a lower quantity). The water pump is an additional part of the system, moving the water through the evaporator and condenser.

However, because the water doesn’t need to be compressed as much, smaller compressors are used, if any at all. While the system has less electrical needs to run, they can save a bit of money and tend to be more reliable. However, they also need to run longer to remove as much heat as a standard refrigerant-based system.

Do all AC brands use the same technology?

Not all AC brands use the same technology, though most do. With the emergence of DC inverter technology, there is a wider difference in the market right now. However, all companies that produce air conditioners use the same technology for the most part.

This will include the compressors, condensers, coils and copper tubing, etc. There will be some slight variances in many brands. However, many brands are often controlled and owned by a single parent company that uses the same systems, manufacturing plants, and even assembly lines to produce the different brands.

How should I determine the right size of air conditioner to buy?

Choosing an AC that is too small for the space will result in inefficient cooling and often much higher electric bills. Likewise, running a system too large can result in more energy use than needed for the space.

To get the right size, you will want to measure the area in square feet (length times width). The general guideline, once you have this measurement, is to require 20 BTUs of air conditioner rating per square foot. For example, to efficiently cool a 1400 square feet room, you would need a system with at least 28,000 BTUs.

If your measurement falls between BTU ratings, always go with the one slightly higher in rating to be more efficient.

How much are the running costs of having air conditioners installed in my house?

The running costs of air conditioners will depend upon a lot of different factors. You will need to know the wattage draw of the system, how much you pay for a kilowatt-hour of electrical use, the home’s size being cooled, how often you run the system, the ambient average temperature, and a host of others.

This also considers the type of air conditioner, it’s energy efficiency, and anything else you do to help make the system run less and be more efficient when it does run.

To get an average, you can take national averages for the things you don’t know and input your specific numbers for things you do know. For example, the national average is 13 cents per kilowatt-hour (kWh), a portable air conditioner draws 1500 watts on average, and if you run the system for 8 hours a day, 7 days a week for 3 months, the average will be 672 hours.

Using the base formula of kWh per time used, you will use 1.5 kWh (1500 watts/1000) for 672 hours, or 1008kwh at 13 cents per kWh, for a three month total cost of $131.40.

Are there effective ways of lowering cooling costs?

You can make the air conditioner more efficient using a few different techniques. The first and easiest is to use oscillating fans in the occupied rooms to help distribute the cooler air more efficiently and quickly. This makes the air conditioner need to run less often.

You can also monitor the humidity levels in the home. Using a dehumidifier to help bring the levels down will remove the necessity for the evaporator unit to do the same job. This makes a marked increase in functionality and lowers operational costs. It also helps the air conditioner last much longer.

How can I tell if my air conditioner is working well?

The efficiency and effects of the AC will dwindle over time. You can ensure that the air conditioner is working correctly (and tell that it is) by performing a few checks, cleanings, and maintenance steps.

The airflow should be steady, cold, and powerful at the vents. If you notice lower airflow, there could be a clog somewhere.

Check the air filter. These need to be replaced every 30 to 90 days, and a clogged intake air filter will drastically cut down on the airflow, causing the system to run longer and more often.

Make sure the vents are free of dust, debris, and blockages. Don’t stack items, furniture, or heat sources near vents (lamps, refrigerator, televisions, etc.). Properly working air conditioners shouldn’t run very long. There is also a rule of thumb to whether the vents should be left open or closed during summer and winter.

The air conditioner should come on within a single degree of your thermostat setting. It will also shut off between 0.5 and 1.5 degrees lower than the initial setting. For example, if your thermostat is set to 71, the system should turn on when the actual temperature is between 71.2 and 72 degrees and shut off again between 69.5 and 70.5 degrees.

With a 2 to 3 degree shift, a properly working AC should only run a few minutes at a time ( between 5 and 10 minutes is average).


Air conditioners are one of the most common and useful home climate control items in the country. In certain areas, they are even required. However, knowing that you have an air conditioner versus understanding the type of air conditioner and how it works are drastically different.

Hopefully, this article has explained what air conditioners are, how they work, and will help you find, choose, and install the best type for your home, business, or vehicle.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.