How An Air Conditioner Works


When it’s a hot summer day outside, but a brisk 68 degrees in your home, you might find yourself wondering, “How do air conditioners work?” To sum it up, air conditioners remove heat and humidity from the air inside and place it outside to ultimately cool down your home.

To complete this process, a lot of different parts of the air conditioner need to work together. And let’s not forget that there are multiple types of systems that all fulfill the same goal of cooling your home. Let’s dive into the specifics, so you can understand how your air conditioner really works and discover which system is right for you.


Generally speaking, there are two categories of air conditioners: central cooling systems and ductless split systems. While both work to cool your home, these systems function in vastly different ways.


Central air conditioners are designed to cool your entire home. To do this, they distribute cool air through the ducts in your home, so the air reaches every single room. Central air conditioners have one outdoor portion and one indoor portion that are connected by copper tubes, unless it’s a packaged unit which is located completely outside the home.

Because central cooling systems cool entire homes, the units take up more space than ductless systems (we’ll get to those in a second!). The smallest possible size for a central air conditioner is 1.5 tons. These kinds of units can reach up to 22 SEER, or Seasonal Energy Efficiency Ratio, which is a measure of how energy efficient an air conditioner is during the spring and summer months. Minimum SEER ratings in the United States are at least 14, according to the Department of Energy, so while central AC can cool your entire home, they’re not always the most energy-efficient option and can come with high energy costs.


Ductless systems are meant to cool just one room or portion of your home, rather than the entire house. Ductless systems are inherently different from central cooling systems because the air flows through indoor units into individual rooms instead of ducts throughout the house. Ductless systems also have an indoor and outdoor component, but unlike central cooling systems, this is not a 1:1 ratio. Instead, ductless systems can have up to five indoor units for every one outdoor unit. This means up to five units can be installed throughout a home to cool up to five rooms without the presence of ducts.

When the ductless outdoor to indoor ratio is 1:1, it’s called a mini-split. When the ratio is any more than 1:1, it’s referred to as a multi-split. Because these units are designed to cool smaller spaces, they come in a small size, with the smallest being .75 tons. Ductless split systems can reach up to 33 SEER, making them a much more energy-efficient option than central cooling systems. Using them will likely save you money on your energy bill despite their higher initial costs.


Understanding the different types of air conditioners will help you determine which is right for your home.


As we mentioned above, central heating and cooling systems, like central air conditioners, work by feeding air via ductwork through your home. Central air conditioners have both an indoor unit and an outdoor unit and are one of the most common types of air conditioners. These units are great for homes with a basement or crawlspace because the indoor portion can be tucked away in an unused space.


Packaged air conditioners are another type of central air conditioner. Packaged systems are unique because they provide air to the entire home, but don’t feature a 1:1 indoor to outdoor unit split. Instead, packaged systems are two in one and are just located outside the home. These units are great for homeowners who want central air conditioning but have nowhere to store a large indoor unit in their home.


Dual fuel systems are a one-stop shop for all your heating and cooling needs. A dual fuel system doesn’t use an air conditioner, but instead is made up of a heat pump and a furnace, with the heat pump doing the cooling during the spring and summer months. A heat pump can both heat and cool, so in a dual fuel system, it can switch between functions depending on which is needed. Dual fuel systems function best in mild to extremely cold climates, so if you live in a place with changing temperatures, a dual fuel system could be the right fit for you.


Ductless mini-splits are perfect for those who need to cool just one room. In the case of a ductless mini-split, there is one outdoor unit connected to one indoor unit, and all of the cooled air will be pumped into the single room where the indoor unit is located. Ductless mini-splits work great for people living in their own bedroom with other roommates in the rest of the home, like college students.

Ductless units can be mounted on the floor or the wall. Wall-mounted air conditioners are normally installed higher up, so it takes longer to feel the effects of the cool air, but they’re better at evenly distributing air. Floor-mounted ACs make you feel the cool air quicker and they look more discreet, but airflow can be compromised.


A ductless multi-split works the same as a ductless mini-split, except the outdoor to indoor unit ratio can get up to 1:5, meaning up to five rooms can be cooled without ducts. Ductless multi-splits are expensive to install initially because they require purchasing up to five individual ductless units. But, they are more energy-efficient and cost less in utility bills compared to central cooling. Ductless multi-splits are ideal for people who live in homes without ductwork or those looking to save on their energy bills in the long run.


Window air conditioners are true to their name — they’re air conditioners installed in the window of your home. These air conditioners work in a similar way to ductless AC units in that they cool a single room in your home. But, they’re installed in a similar way to central AC units in that they’re typically half inside and half outside. Window air conditioners can be noisy and unappealing to look at, but they’re inexpensive to install and maintain.


Now that you know about all the different types of air conditioners, let’s look at the major parts that make them up.


All air conditioners are controlled by a thermostat that sends signals to the HVAC system when it’s time to start cooling. There are two different types of thermostats: electronic and electromechanical. Electronic thermostats have sensors that read the temperature while electromechanical thermostats have metal strips and mercury that tip off the air conditioner when it’s time to cool.


Refrigerant is an essential part of an air conditioner — without it the entire system wouldn’t be able to function. This chemical cooling compound flows throughout the air conditioner, absorbing and releasing heat at different stages to cool down your home. The cooling process kicks off when a fan blows heated air from inside your home into the air conditioner, and the refrigerant absorbs that heat inside the evaporator coil.


The evaporator coil is the place for absorbing heat. When the fan blows hot air from your home over the cold evaporator coils, the cool liquid refrigerant inside absorbs the heat from the air completely, then continues on through the air conditioner.


After the refrigerant absorbs heat in the evaporator coil, it goes to the compressor. The compressor increases the pressure of the now-hot refrigerant, which increases its temperature even more so that it’s hotter than the temperature outside. While in the compressor, the refrigerant turns into a gas. Once the gaseous refrigerant is hotter than the outdoor climate, it can dispense heat outside.


The condenser coil receives that hot, pressurized gaseous refrigerant from the compressor. The condenser coil is designed to release the heat the refrigerant is carrying outside. This cools the refrigerant down and turns it back into a liquid, ready to absorb more heat from your home.


When the refrigerant leaves the condenser coil, even though it releases most of its heat outside, it’s still too hot to re-enter the evaporator coil. So, the expansion valve works by decreasing the pressure of the refrigerant and cooling it back down even more. The expansion valve sends the cold refrigerant back into the evaporator coil, where it picks up more heat from the air inside your home and the process repeats itself.



Central air conditioning works by moving around air in your home until it reaches your desired temperature. Central air systems pull hot air from your home, work to cool it down by releasing its heat outside, then distribute the newly-cooled air through a system of ducts in your home. That way, every room is reaching the ideal temperature.


Ductless air conditioners work to cool just one room at a time. Unlike central air conditioning, which cools your entire home via a system of ducts, ductless air conditioning directly releases cool air into a single room. As we mentioned earlier, ductless systems require one outdoor unit and at least one indoor unit. If you want to cool more than just one room but still use ductless air conditioning, you can install up to five indoor units that will all work at the same time together with the singular outdoor unit.


No. Air conditioners take heat and humidity out of the air inside your home, then distribute that heat and humidity outside while returning the newly cooled air back to the house. Air conditioners never take air from outside and put it in your home. Instead, they work by cooling down the air that’s already in your home.


Air conditioners can improve air quality. After heat and humidity are absorbed from the air inside your home, the air is filtered for dust, lint, and debris. The heat gets moved outside, but the air that remains and goes back into your home is cleaner than before, resulting in better air quality.

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