How an Air Conditioner Works | The Refrigeration Cycle

April 10, 2024

When those hot California summers hit, you’ll likely need an air conditioner to keep you cool. Whether you’re considering investing in a new unit or trying to better understand how your current unit operates, you’ve come to the right place. To understand how an air conditioner works, you’ll need to understand the HVAC refrigeration cycle. Keep reading to learn about your air conditioner's cycle and key components.

How Does an Air Conditioner Work?

There are two laws of physics that we should review before explaining the inner workings of your air conditioning system.

Combined gas law

The first is the relationship between pressure and temperature, known as the combined gas law since it combines Boyle’s Law, Charles’s Law, and Gay-Lussac’s Law:

  • Boyle’s Law states that the pressure-volume product is constant.

  • Charles’s Law shows that the volume is proportional to the absolute temperature.

  • Gay-Lussac’s Law says that the pressure is proportional to the absolute temperature.

To keep it simple, the combined gas law says that whenever you heat a gas, pressure also increases. And vice versa, whenever you pressurize a gas, heat increases.

If the pressure increases, so does its temperature. This is why a tire heats up as you pump it with air.

If pressure decreases, so does its temperature. This is why an aerosol can get colder as you depress the nozzle and release pressure.

An air conditioner uses this combined law by pressurizing and depressurizing the refrigerant to increase or decrease its temperature.

2nd law of thermodynamics

The second law of physics you need to know is the 2nd Law of Thermodynamics.

You may remember from your high school physics class that the second law of thermodynamics states that heat flows from hotter to colder bodies naturally. You can only transfer heat from a colder body to a warmer body through some kind of external work.

Air Conditioning 101: The Fundamentals

Air conditioners transfer heat from the indoors to the outdoors.

Although you may think that air conditioners create cold air, they actually extract heat from the indoor air and send it outside.

When heat is removed from the indoor air, the air is cooled down. It’s best to think of the air conditioning process as heat flowing from the indoors to the outdoors.

The Refrigeration Cycle

An air conditioner works using a thermodynamic cycle called the refrigeration cycle. A simple explanation of the HVAC refrigeration cycle is that it pipes refrigerant through a cycle of components to move heat away from the area you’re trying to cool.

The refrigerant — also known as coolant — is the fluid used in the refrigeration cycle. It absorbs heat from the inside of your home and then pumps it outside. The key properties of a good refrigerant include:

  • A high critical temperature

  • Non-flammable or explosive

  • A low boiling point

  • Non-corrosive

  • Easily liquefies

  • Non-toxic

Most air conditioners are air-source, split systems. This means that there is one unit inside and one unit outside, which is why it is called a split system.

The air-source part refers to the place where the thermal energy is dumped, the outside air. Other potential places where the heat can be transferred are water or the ground, known as water-source or ground-source systems.

The inside unit is normally inside the house somewhere, in the attic, basement, closet, or crawl space. The outside unit is normally located on the side or back of the building.

Other kinds of air conditioning systems, such as ground-source and water-source, follow the refrigeration cycle, but some of the specifics, such as location and parts may differ.

The key components

The key refrigeration cycle components involve the compressor, condenser, expansion device, and evaporator. 

The compressor’s function in refrigeration is to raise the pressure of the gas. Gas enters at a low temperature and pressure and exits at a high temperature and pressure.  

The condenser — also known as the condenser coil — is responsible for condensation in the HVAC system. It receives the high-temperature, high-pressure gas (vaporized refrigerant) and removes the heat until it condenses into liquid. After it’s condensed, the refrigerant is sent to the expansion device as a high-pressure, low-temperature liquid.   

The expansion device reduces the pressure of the refrigerant after it exits the condenser, causing a two-phase mixture in a process called “flashing.” The two-phase mixture then moves onto the evaporator.

The evaporator is responsible for evaporation in HVAC and is the final stage of the cycle. It absorbs heat from your space into the low-temperature, low-pressure refrigerant and then releases cool air. 

The process

Here are the basic steps of the refrigeration cycle (the same process that your refrigerator uses to keep food cold):

Air flows over the indoor coils, which contain extremely cold refrigerant

When air flows over the cold coils, heat from the air gets transferred to the refrigerant inside the coils. After the air flows over the coils, it gets cold, normally dropping around 20 degrees.

This process follows the 2nd law of thermodynamics, which says that heat naturally (spontaneously) flows from a warmer body to a cooler body.

After the refrigerant absorbs the heat, its state changes from a liquid to a vapor. This warmer refrigerant gas then gets transferred to the compressor (step 2 in the refrigeration cycle).

Warmer, vaporized refrigerant gets compressed (pressurized) to a hot temperature

Even though the refrigerant has absorbed heat from the indoor air, it is still fairly cool. The still cool but warmer vaporized gas enters the compressor (located in the outside unit) to increase its pressure and temperature.

We increase the temperature of the refrigerant because it needs to be warmer than the outdoor air. Remember the 2nd law of thermodynamics again—heat flows from warmer to cooler bodies.

If the refrigerant is 120 degrees and the outdoor air is 90 degrees, the outdoor air is cooler, which means the heat from the refrigerant will flow in the direction we want — outside. If the temperature outside is 120 degrees, the compressor will have to work extra hard to increase the temperature of the refrigerant to a higher temperature.

After the refrigerant’s temperature is increased above that of the outdoor air’s temperature, it then flows into another set of coils, known as the condenser coils (also located outside).

Very hot refrigerant flows into condenser coils where it loses heat to the outdoor air

Since the refrigerant has been compressed (pressurized), it is now hotter than the Southern California outdoor air. A condenser fan blows hot outdoor air over the even hotter outdoor condenser coils.

As outdoor air flows over the outdoor coils, heat is removed from the refrigerant and released into the outdoor air. Again, this is due to the 2nd law of thermodynamics.

After the refrigerant loses thermal energy to the outdoor air, it condenses back into a liquid and gets pumped back inside.

The still-warm refrigerant from the outdoor unit needs to get cold

When the refrigerant leaves your outdoor condenser unit, its temperature is still pretty high. The refrigerant’s temperature will need to drop significantly before it can absorb more heat from the indoor air.

The metering device, usually a thermostatic expansion valve, is a special device that depressurizes the refrigerant, causing a drop in temperature. It does this by expanding the refrigerant into a larger volume.

The refrigerant needs to be colder than the indoor air in order to absorb heat. Once the refrigerant gets cooled down, it flows back into the evaporator coils. The evaporator coil is located near the blower fan and is where the refrigerant absorbs heat. After this step, the refrigeration cycle begins again.  


Can I run my air conditioner 24/7?

Yes, you can run your air conditioner all day, as there are no safety hazards to continuous use. However, you will see rising energy bills and faster wear of your unit. The more you use your unit, the shorter its lifespan will be. You will likely see the average 15-20-year lifespan drop as the machine starts to fail early due to prolonged use.

How much energy does an AC unit use?

The amount of energy an air conditioner uses depends largely on the type and size of the unit. A window unit uses the least amount of energy, averaging between 900 and 1440 watts per hour. A portable unit follows with between 2900 and 4100 watts per hour, and a central air conditioner will use the most amount of energy, consuming between 3000 and 3500 watts per hour.

Contact Service Champions for Your Air Conditioner Needs

Even with your new understanding of air conditioners, you may not be ready to take on the job if your unit fails. Service Champions are here to help with all of your A/C maintenance, repair, and installation needs.

We’re dedicated to providing top-quality heating, cooling, and air quality services to the residents of the East Bay, South Bay, and Sacramento areas, including Pleasanton, San Jose, Concord, and more.

Our online scheduling and 24/7 availability make it easy to contact us if you run into trouble. Our experienced technicians have helped over 200,000 clients with their HVAC needs and received thousands of five-star reviews. Rest easy knowing we’ll get the job done right the first time. We offer financing options and easy payment plans that fit your budget.

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