How Does Air Conditioning Work? Residential A/C Explained

Air conditioning is considered to be one of the top 25 greatest technological breakthroughs in US history; this is according to the Wall Street Journal piece on the most impactful US inventions, which is part of the Journal's USA250 series covering the world's greatest economy.

Air conditioning was invented by Willis Carrier in 1902. Exactly how does air conditioning work? This invention takes advantage of the thermodynamic characteristics of a refrigerant gas by harnessing the change of state between gas and liquid under changing pressure in a closed loop. This is the core of all heating and cooling systems.

Early air conditioning used various compounds as the mode of heat transfer, including Sulfur Dioxide (SO₂), Methyl Chloride (CH₃Cl), Ammonia (NH₃), and Carbon Dioxide (CO₂). While these compounds worked, they were known to be either toxic or flammable, or required operation at extremely high pressures. These refrigerants were the first generation of gases used in cooling, and became obsolete with the invention of Freon in 1928. Freon is a second generation refrigerant gas, and is considered much safer than the first generation gases; this is because Freon is stable, non-toxic as well as non-flammable.

The use of R-12 also brought about larger degree of safety, efficiency and operation due to running at pressures that were well within comfortable engineering limits. This turned air conditioning into a common, safe and reliable feature of household comfort.

R-12 was phased out of production in the early 1990s, due to its ozone-depleting nature. Today, most residential AC units use R-410A (Puron), an ozone-friendly refrigerant. Emerging high-efficiency gases such as R-454 and R-32 have entered the market and will become more common as new units designed to use those refrigerants are purchased and installed in residential or commercial structures.

How Does Air Conditioning Work?

Using a typical residential home for this example, a central air conditioning system works by transferring the heat from within the house to outside of the house, where it is released into the atmosphere. As heat is removed, the interior of the house cools. And as the temperature drops, condensation occurs, which is also expelled to the outside of the house. This results in cool, dry air.

This is accomplished by placing specially-designed components both inside and outside the house, and connecting them in a specific order to circulate a refrigerant gas with compatible thermodynamic qualities through a closed loop. When the components of an air conditioning system are properly selected and installed along with its supporting parts, they should efficiently cool a house.

The 4 Key Components Of An Air Conditioning System

An air conditioner's closed loop is made up of 4 components, which are connected in compressor > condenser > expansion valve > evaporator order. The exiting gas from the evaporator becomes the input to the compressor, maintaining the loop.

The four major components of an air conditioning system is comprised of the compressor, condenser, expansion valve and evaporator. These form a closed loop system for refrigerant gas to circulate (direction depicted by arrows)

Outdoors (Head or Discharge Side)

Compressor

An air conditioner's compressor is basically as specialized pumping mechanism, which is designed to compress the lower-temp/low-pressure refrigerant gas it receives from the evaporator into a much smaller space at a much higher pressure, changing its state into a high temperature pressurized vapor. This compressed vapor is then sent to the Condenser.

Condenser

When the high-temperature pressurized gas is received by the condenser, it travels through the coils and radiates the heat it carries into the environment. The condenser sheds its heat to the ambient air, assisted by the condenser fan.

Indoors (Suction Side or Low Side)

The Expansion Valve

The expansion valve is a device that takes high pressure liquid refrigerant and meters its expansion into a low pressure gas. It is located inside the structure within the evaporator coil casing, just before the refrigerant enters the evaporator coil. The are many types of expansion valves in use today, and their design vary widely based on their application, such as refrigeration, Home AC, heat pump systems, and automotive AC systems. The 5 most commonly used types of expansion valves in residential HVAC use today are:

1. Thermostatic Expansion Valve (TXV) - A type of valve widely used in residential and commercial HVAC systems; it uses a thermal sensor to adjust the valve's operation.
2. Electronic expansion valve (EEV) - Provides the most accurate electronic metering of refrigerant, giving the most precise temperature control in residential systems. Due to its complexity, it is usually the more expensive than other types of expansion valves.
3. Capillary tube - Having no moving parts, this coil-like device acts as an expansion valve by slowing and cooling refrigerant. It is the simplest type of expansion valve, with the lowest rate of failure.
4. Piston-Orifice type - This type of valve is almost as simple as the capillary tube version. It only uses 2 moving parts: a piston and a spring with a known rate of tension. This meters the flow of refrigerant to the evaporator.
5. Automatic Expansion Valve (AEV) - The AEV is a simpler than the EEV or TVX types, in that it does not have an external temperature sensor; instead, it responds to evaporator pressure to function. This type of valve is useful in designs that do not experience large temperature variations.

Evaporator Coil

An air conditioning evaporator coil is created from a long aluminum or copper tube that is then wound in a uniform way to allow it to take up a minimal space. This coil has evenly spaced metal fins connecting the coil tubes externally, defining its shape while giving support to the structure. The furnace unit's blower fan pushes warm indoor air across the coil, causing the coil to absorb heat and transfer it to the gas traveling through the coil. The warm that loses heat in this way becomes the dry cool air that circulates throughout the house. The exiting gas is then pulled in by the compressor for conversion into a high pressure liquid and pushed out into the outdoor condenser, where the heat is released.

Additional Parts Of An Air Conditioning System

All air conditioning systems use the four components above, but there are a few more key parts that an air conditioner can't run without:

• The Receiver-Dryer - Inline between the condenser and expansion valve, this container that houses a desiccant material. This material pulls errant moisture and small particulate (if any) from the refrigerant before it reaches the expansion-valve and the evaporator. Any debris or moisture is captured and stored within the desiccant material.
• The Thermostat - A central device in a house that the resident uses to selected what temperature they would like the AC (or heat) to maintain. The thermostat uses sensors to detect ambient temperature, and cycles the system to run until the desired interior temperature is reached. When that temperature is reached, the thermostat will signal the system to temporarily shut down. When the temperature moves away from the selected setting, the thermostat signals the system to restart and cool the structure. This cycle continues as needed to keep the inside of the building at the desired temperature.
• Furnace blower - This is an electric fan that blows air through the ductwork of an HVAC system inside of a home. It is usually part of the furnace, and is used to distribute cool dry air when the AC is being used, or hot air when heat is being used.
• Compressor/Condenser fan - This electric fan sits in the same casing as the condenser and the compressor unit. It is designed to pull air through the condenser coils to aid in heat transfer.

Additional parts of an air conditioning system include the copper tubing that forms the connections between the components, as well as the galvanized steel ductwork within the home that form the conduit to circulate cooled air².

Certification Needed To Handle AC Refrigerants

Air conditioning components are designed to handle the pressures generated within the closed loop using a specific refrigerant gas. Each type of refrigerant has a unique vaporization point. The vaporization point of the refrigerant used directly affects the pressures that the system will operate under.

R-410A accounts for the vast majority of residential AC refrigerant in use today. R-410A turns from a liquid into a vapor at −55.3°F (−48.5°C) at 1 atmosphere, which is 14.7 psi at sea level. This approximate operating pressures for R-410A depends on ambient temperatures, and location in the closed loop:

• The low-side (suction) pressure for R-410A usually ranges from 105 to 145 PSIG.
• The high-side (head/discharge) pressure typically ranges from 200 to 450 PSIG.

The high-side maximum pressure for R-410A is over 30 times atmospheric pressure1. Because of this high pressure, working with refrigerants requires following procedures carefully, wearing protective equipment, and using the right tools to measure operating pressures. Therefore, it is imperative that an HVAC technician is properly trained and certified to handle all of the refrigerants in the field. This requires specialized knowledge, training and earning the proper credentials.

There is a variation in what refrigerants an HVAC technician might encounter on the job, and it wouldn't be too surprising if a tech encounters a much older unit that is running the now-banned R-12 or the discontinued R-22 refrigerant. With R-410A widely used, and 2 newer refrigerants (R-454 and R-32) on the market, the technician needs to know the profiles of every gas they might encounter in the field and select the proper procedures and equipment to handle that specific gas.

HVAC Technicians Need To Earn The EPA Section 608 Certification

The Environmental Protection Agency (EPA) certifies technicians depending on what kind of air condition system they work on. For Motor Vehicle AC (MVAC) systems, technicians need to earn the EPA Section 609 certification. For technicians who install and service residential and commercial climate control systems, there are 4 different EPA Section 608 certifications, depending on the type of system the technician is works on. There are excellent study guides available for technicians seeking their EPA certification, with two of the most used guides available through ESCO and the Environmental Protection Agency.

¹ R-410A's maximum high-side operating pressure is 450 pounds per square inch. Dividing that pressure by standard atmospheric pressure at sea level (14.7 pounds per square inch) represents a high-side pressure of 30.61 times standard atmosphere.

² "What are the Components of a Residential HVAC System?" found at https://www.acdirect.com/blog/what-are-the-components-of-a-residential-hvac-system/#:~:text=are%20copper%20tubes. Retrieved on September 4, 2026.