Turn a window unit or forced-air system to the “On” and “Cool” position. Lower the thermostat or temperature setting so the air compressor activates. Let the AC unit run for about 15 minutes.
Hold an outdoor thermometer in front of the return air vent on a whole house unit or in front of the return air area on a window unit. These areas are where the air filters install to filter the internal air and recirculate it into the house after it is cooled. Hold the thermometer in place until the temperature remains constant for at least two minutes. Observe the temperature of the return air.
Hold an outdoor thermometer in front of an air-conditioning vent that distributes cool air to a room on a forced-air system or on the top air vents of a window unit. Keep the thermometer in place until the temperature remains constant without moving colder for at least two minutes. Observe the cool air temperature.
Subtract the return air temperature from the cool air temperature to find the difference between the two. The difference should be between 14-20 degrees. If the difference is less than 14, the AC is below the efficiency at which it should run and may need cleaning or the addition of Freon to cool the air in the home better.