Most home central air conditioning systems consist of two major components: the indoor unit and the outdoor unit. The presence of two separate units is an important part of what makes your central air system work. Of course, if you're like most homeowners, you aren't too concerned about the details of your system as long as it functions properly and keeps your house cool. Understanding the basics of how the system works can be valuable when something goes wrong, however, and will put you into a better position to discuss your issues with HVAC contractors.
The Basics of Air Conditioning
Before you can understand why your central air system is split between two units, it is important to have a fundamental understanding of how all air conditioning systems work. The core purpose of any air conditioning system is to remove heat from the air and to push that cooler air into your home. This is accomplished through the use of a fluid known as refrigerant. Refrigerants are chemicals which are particularly good at absorbing heat and moisture so that it can be transferred from the inside of your home to the outside environment. As the refrigerant absorbs and releases heat, it undergoes a phase change from liquid to gas so that it can be easily moved through the system.
The Role of Your Outdoor Unit
The outdoor unit contains the condenser coils and compressor, as well as a fan to blow air across the condenser. This unit must be located outdoors (or in a location with access to fresh air) so that the condenser fan has access to ambient air. The compressor takes the cool refrigerant in gaseous form and compresses it into a very hot, high-pressure vapor state. The relatively cool outdoor air is then blown over the refrigerant as it passes through the condenser, cooling it into a liquid state. The cool refrigerant then travels inside of your house to the indoor unit.
The Role of Your Indoor Unit
The refrigerant passing into your house is now cold, but that cold refrigerant still needs to absorb heat from inside your house. This is the function of your indoor unit, which contains the evaporator. While the outdoor unit is responsible for compressing the unit and ultimately converting it into a liquid state, the evaporator in the indoor unit allows the refrigerant to absorb heat and return to a gaseous state. Indoor air is blown over the evaporator as refrigerant passes through it, and this has the effect of pulling heat out of the air and cooling it. This cool air is then distributed throughout the house via your AC ducts.
Putting it All Together
Now that you understand the basics of split system air conditioning, you can begin to see how everything fits together and allows your central AC system to cool your home. While it is easy to think of an air conditioning system as something that generates cool air, behind the scenes the system's real role is to provide a method to transport heat from inside of your home to the atmosphere outside. In doing so, it creates a source of cool air that can be used to keep your house at a comfortable temperature.