Why Is My Central Air Conditioner Not Cooling?

As the outside temperatures rise during the sweltering, humid summer months, many of us depend on central air to keep us comfortable. But things can get really heated when lowering the thermostat has no effect on cooling the air. Before you work up a sweat, you should realize that it is often one defective component responsible for a malfunctioning central air conditioning system. Understanding how the system is supposed to work is the first step in diagnosing why it is not performing as expected.

How does a central air conditioning system work?

Central air conditioning systems rely on two separate units to cool the air inside the home: a furnace, or air handler, which may be located in a basement or closet space within the house or apartment, and a condensing unit which is located outside. The temperature in the home is regulated by a wall thermostat. When the thermostat is lowered or it detects an increase in temperature, it closes a cooling circuit, allowing voltage to travel to a control board. The control board will then send 120 volts of alternating current to a circulation blower fan in the furnace or air handler and 24 volts to a contactor in the outside condensing unit. When the contactor is energized, it allows 240 volts to flow through a nearby disconnect box to a compressor and condenser fan motor located in the condensing unit.

The compressor acts as a pump, compressing refrigerant in gas form into the condensing unit’s condenser coils where the gas is condensed into a hot liquid. The coils dissipate the heat as the liquid travels through them, assisted by the spinning condenser fan motor blades. After passing through the condenser coils, the refrigerant travels to the evaporator coils mounted on the furnace or air handler. There the refrigerant expands back into a gas which makes the coils cold. The circulation blower fan draws air past the cold evaporator coils and forces the air through the venting to cool the interior of the home.

Top causes of a central air conditioner not cooling

When troubleshooting why your central air conditioner is not cooling, here are the top four potential causes you should investigate:

  • A faulty contactor will not allow voltage to be sent to the condensing unit, preventing the compressor and fan motor from running. You can visually inspect the contactor for signs of damage. You can also test the contactor with a multimeter. First touch the meter probes to the incoming contacts to confirm that the contactor is receiving 240 volts. After setting the wall thermostat to call for cooling, test the coil terminals to determine that 24 volts are present. If true, the coil should be energized and the contactor contacts should be closed sending 240 volts to the compressor and fan motor. If the 24 volt coil is open, or 240 volts is not present at the outlet terminals, the contacts are likely damaged and the contactor will need to be replaced.
  • A single-run or dual-run capacitor is used to store an electrical charge which is then released to power the compressor or fan motor. A defective capacitor will prevent the system from working. You can often tell that a capacitor has failed because it will bulge or leak. You can also use an analog Ohm meter to test the capacitor’s ability to store and release an electrical charge, and a capacitor tester (or a multimeter with a capacitor testing function) to determine if the component has the proper capacitance rating. Before handling the capacitor, you should release the potentially-stored electrical charge by placing a screwdriver blade across each set of terminals. Avoid touching the screwdriver blade when doing this.
  • There may be an incoming power problem associated with the condensing unit’s disconnect box. If the disconnect box has a circuit breaker, you should check to see if the breaker has tripped. If fuses are used, you can use a multimeter to test each fuse for continuity – a continuous electrical path present in the fuse. If either fuse tests negative for continuity, it will need to be replaced.
  • A malfunctioning condenser fan motor will prevent the system from cooling as well. The fan blades should rotate freely; if they do not, it’s likely the motor has seized. You can also use a multimeter to determine if the motor is receiving incoming power. If the capacitor is functioning properly and incoming power is present, then the motor windings are probably malfunctioning and you will need to install a new condenser motor.

Cleaning the coils is an important part of maintaining your AC system

Both the evaporator coils and the condensing unit coils should be kept clean. The condenser coils, especially, can easily become clogged by leaves or other debris which will prevent the system from functioning efficiently. We recommend using a universal coil cleaning detergent to clean both sets of coils.

  •  Turn off the power to the furnace or air handler, remove the access panels, and apply an even, coating of the coil cleaning detergent to the evaporator coils (you should wear eye and skin protection when doing this). Allow the detergent to set for at least ten minutes before replacing the panels and restoring power to the unit. When the system is running, the condensation will normally flush out any contamination, so rinsing is not required.
  • Before cleaning the condenser coils, shut off power to the unit, then, as necessary, remove the fan motor assembly, side panels, or air baffles to fully access the coils. Apply an even, coating of the detergent to the outside of the coils. Allow the detergent to set between five and ten minutes, then use a garden hose to thoroughly flush the coils with water from the inside out. Once the coils have dried, replace the fan motor assembly, panels, or baffles as required, and turn the power back on.

Find the right central air condensing unit parts with Repair Clinic

Repair Clinic carries universal coil cleaner, wall thermostats, run capacitors, contactors, condensing fan motors, and fan blades to maintain or repair your AC system. Just enter the full model number of the outside condensing unit in the Repair Clinic search bar for a complete list of compatible parts. You can then use the part category and part title filters to refine your search. Repair Clinic stocks condensing unit components for all the major heating and cooling brands including Lennox, Frigidaire, Rheem, York, Carrier, Payne, and Honeywell, but you’ll want to make sure you’re purchasing the part that is a direct fit for your specific unit.

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