Turn the fan switch to "auto" and set the thermostat below the room temperature (76° to 78° is recommended). The fan and condensing unit should come on, unless there is a time delay, which may slow its start.

Let the system run for eight to ten minutes to balance the temperature in the ductwork. Check the temperature at the supply and return registers. The temperature at the supply register should be 14° to 20° Fahrenheit cooler than at the return ducts or current room temperature.

* If the temperature differential is more than 20°, it indicates restricted air. This points to three possible problems: 1) a dirty filter, 2) improper ductwork, or 3) a fan that is sized wrong, not working properly, or moving too slowly.

* If it is less than 14° differential, the possible causes are 1) refrigerant loss, 2) a dirty coil, 3) a laboring compressor, 4) an oversized fan, or 5) a deficient return air system.

There are two copper lines going from the condensing unit on the exterior to the air handler on the interior. The smaller line is the liquid or high-pressure line and will be warm, not hot, to the touch when it is operating properly. The larger line covered with insulation is the suction or low pressure line and will be cold to the touch. The suction line should be "sweating" or condensing, the way a glass of iced tea "sweats." It should not be forming "ice."