June 18, 2024

What Does Central Air Include?

Central air conditioning is the most widely used cooling system in the United States. As you evaluate replacing your old unit or adding a new one to your home, it’s important to know what it needs and what’s included in a standard installation. Use this guide to explore what central AC is, what’s included with the system, and what else you may need.

What Is Central Air Conditioning?

Central air conditioning is an AC system where air is cooled and distributed around your home. Central air conditioning is also often called a split system because part of it is outside and the other part is inside. In this type of system, the indoor unit delivers cool air, and then a central fan circulates it around your home through your ductwork.

In non-central systems, such as window or portable air conditioners, the entire system is in a compact unit. There are also ductless mini-split systems that have a similar outside unit to a central system but have multiple indoor air handlers, each with its own evaporator coil. They can be a good option for homeowners without existing ductwork.

What is Included With a Standard System?

When you install a new unit in your home, what’s included may vary from one HVAC provider to another. Some have various installation packages or varying levels of installation service. What’s included may also vary based on whether you’re bundling a furnace installation simultaneously with your air conditioner. The following are the components that are included with all central air installations.

Evaporator Coil

The evaporator coil is the primary component where the circulating air cools before returning to your home. At the evaporator coil, the refrigerant expands, which reduces its temperature. The circulating air then moves over the coil, and the refrigerant absorbs heat from the air, providing a cooling effect.


The compressor is responsible for regulating the high-pressure side of the air conditioning system. Once the refrigerant absorbs heat from your home, it flows to the compressor. Here, it compresses the refrigerant, which is in gas form at this stage of the process, into a smaller space, increasing the pressure, which makes it superheated. This pressure and superheating allow the unit to vent the heat to the air outside and then cycle the liquid refrigerant back inside to absorb more heat.

Condensing Coil

The condensing coil is in the outside condensing unit. While the indoor coil gets cold, the condensing coil gets hot to vent the heat the refrigerant absorbs from the indoor unit. If you look at the condensing coil, you may see thin strips of metal called fins. These help transfer heat from the refrigerant to the air outside and move it away from the unit. If these fins get damaged, it prevents air from flowing over them properly and dissipating heat. Hiring a professional is important to prevent fins from sustaining damage during the installation process.

Thermostatic Expansion Valve

Before the refrigerant flows back into the evaporator coil inside, it passes through the thermostatic expansion valve. The valve restricts the amount of refrigerant flowing into the evaporator coil. The system can adjust the coil’s temperature by controlling how much refrigerant flows into it. This is critical to ensure the coil is cold enough to promote heat transfer from the air to the refrigerant but not so cold as to cause the coil to freeze.

Electrical Components

The system needs several electrical components in the condensing unit to run effectively. The first is the compressing capacitor. The compressor requires a large surge of power to start, and the capacitor stores the charge to provide that surge.

The second is the contactor, which acts as a switch to start and stop the condensing unit. When the thermostat inside calls for a cooling cycle, it signals the contactor to close, which allows current to flow from the supply power to the condenser and fan motor.

Refrigerant and Refrigerant Lines

The system depends on refrigerant to transfer heat from the air inside to the air outside. The system also needs lines to move the refrigerant throughout the system. The system cannot provide effective cooling without the proper refrigerant charge. Leaks in the refrigerant lines compromise the system’s ability to cool your home.

Condensing Fan and Motor

To vent the heat the system absorbs from inside your home, it must move air over the condensing coil. The condenser fan draws air from around the unit and then vents it through the top and sides.

What Else You May Need

Aside from these features, there are additional components you’ll need. You likely have all these if you already have a central HVAC system. However, an experienced technician should evaluate them to ensure they are in satisfactory condition and don’t need replacing.

Air Handler

A central air conditioner can’t cool your home without a way to move air over the evaporator coil and then through the ductwork. The air handler typically houses the circulating fan, motor, and evaporator coil. If you have a forced-air furnace, it likely uses the same air handler.


The ductwork runs throughout your home. The supply ducts carry conditioned air from the air handler to all the registers. Most systems have a single return duct where the air from your home is returned to the air handler for conditioning. The return vent is much larger than the supply registers and is usually on a wall or the ceiling. Your installation technician should inspect your ducts to ensure they are still in sufficiently good condition or recommend sealing or replacing them if necessary.


The thermostat is the brain of your system, telling it when to initiate and terminate cooling cycles. They do not last forever but are usually reliable for 10 to 15 years. Some installation companies will replace the thermostat as part of a new furnace or AC installation. Consult your technician to determine if it’s included and if you should consider an upgrade, such as a smart thermostat.

Air Filter

Every HVAC system needs clean air filters to prevent the buildup of contaminants in sensitive areas of the system that could restrict airflow or cause excess wear on the system. During installation, chat with your technician about both the size and rating your system can handle. Air filters use a minimum efficiency reporting value (MERV) rating. A higher MERV rating will remove more contaminants from the air. Most residential systems can handle up to a MERV 13. Ask our technician or consult the manual to determine the highest rating your system can handle.

Since 1978, residential and commercial property owners around St. Louis have trusted Scott-Lee Heating Company to keep their homes and buildings comfortable and safe. Our NATE-certified team installs, repairs, and maintains heating and cooling systems. We are also indoor air quality experts and can install whole-home air purification systems. Call to schedule a consultation with one of our expert installation technicians to explore the best cooling options for your home.