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How Does My Home’s Central Air Conditioner Work?

In this article, we will address how central air conditioners work, as well as potential problems you may encounter.

Central air conditioners differ from system to system, but they all function on similar principles. Included among the components that make up a central air system is a condensing unit which is located outside the home. The condensing unit contains the air conditioner condenser coils, the compressor, and the condenser fan motor.

Evaporator Coils

The evaporator coils (or cooling coils) are situated inside the home and are mounted at the top of a furnace or air handler. The furnace or air handler uses a circulation blower fan to pull air through the returned vent, blow it past the evaporator coils, and force the air through the home’s venting. The room air is then drawn back through the returns and the airflow cycle continues.

Once the room air has cooled sufficiently, the wall thermostat signals the circulation blower fan and condensing unit to shut off, until the room temperature rises. This process can be broken down into three main factors:

  • temperature control
  • cooling
  • air circulation

Temperature Control

The temperature in the home is regulated by the wall thermostat. The thermostat can be powered by the furnace control board or by internal batteries. The thermostat must be in a central location in the home, positioned at eye level, and away from direct sunlight for proper temperature regulation.


When the thermostat detects an increase in temperature, it closes the cooling circuit, allowing voltage to travel to the control board. The control board uses a timed sequence to send 120 volts of alternating current to the circulation blower fan motor in the furnace, and 24 volts to the condensing unit contactor. It allows 240 volts to flow through a nearby disconnect box to the compressor and condenser fan motor when the condensing unit contactor is energized.

The disconnect box, which offers a way to shut off power to the condensing unit outside, includes a cartridge which may contain fuses. The disconnect box fuses can be tested for continuity to determine if a continuous electrical path is present if the compressor and condenser fan motor fail to work.

The compressor acts as a pump, compressing refrigerant in gas form into the condenser coils, where the gas is condensed into a hot liquid. The condenser coils dissipate the heat as the liquid travels through them, and a fan is used to assist the coils in this process.

Once the refrigerant has passed through the condenser coils, it travels to the evaporator coils located on the furnace or air handler. As the refrigerant liquid enters the coils, it expands into a gas, which makes the coils cold. The gas circulates through the coils to a suction line affixed to the compressor. The compressor converts the gas back into a liquid and the cooling cycle repeats.

The humidity in the air condenses on the cold evaporator coils and drizzles down into a set of collection trays. The trays are connected to either a condensate pump or a floor drain.

It is vital to keep the condenser clear. The heat will not be dissipated properly and the system will not function efficiently if leaves or other debris clog the coils. You can use a regular garden hose to clean the condenser by spraying the coils from the inside out.

Air Circulation

Easily the most important factor in efficient operation is air flow. To ensure proper air flow, the air filter should be checked monthly, and replaced as needed.
There are several types of filters readily available, and range from one to five inches thick. The filter can be installed into the slot in the return or in an air cleaner. Some homes will have an electronic air filter which is powered by the furnace or air handler control board.

When the circulation blower fan activates, the blower fan motor can run at multiple speeds to improve efficiency. When the furnace is heating and higher speeds (1220 CFM) are used for air conditioning, lower speeds (750 CFM) are used.

If the circulation blower fan becomes noisy during use, the set screw on the blower wheel may have loosened, causing the wheel to wobble on the motor shaft. You can fix this problem by tightening or replacing the screw, or replacing the blower wheel altogether.

The Benefit of Scheduled Maintenance

We recommend having your AC system checked out once a year to ensure most efficient operation. We’ll catch tiny problems before they become large ones, saving you money and headaches. An added bonus is you’ll save energy because your system will run more efficiently.

When it’s time to schedule routine maintenance for your home’s AC system, or your system isn’t cooling as effectively as it should, call Pickerington’s best HVAC contractor by phone at (614) 837-4026 or schedule a visit using our convenient appointment page. Ask for our AC Tune-Up and Checkout Service.