If you do not see ice build-up on the larger copper tubing (covered with a black, spongee insulation) when you run your system, then you probably have a clogged drain line.

A clogged drain line is usually caused by algae build-up inside the drain line. And yes, there is
something you can do to prevent this condition. Algae is a living plant and will grow in your drain
line until it clogs the line.

The air handler provides a cool, damp environment for development of molds and mildew and if
left untreated these growths can spread into your ductwork. If only moderate to light buildup is
present then there are chemical disinfectants specifically designed for use in air handlers that will
kill the existing mold and mildew and control new growth.......

These disinfectants are safe and very effective and are applied by simply spraying into the filter
intake and by placing "Algae Strips" directly in the drain pan. If the coil has mold or mildew
present then it also should be treated. Make sure that the face of the cooling or evaporator coil
is clean so that air can pass through freely.

And, don't forget! Dirt is the biggest enemy of your air conditioner, and a good air filter is the best friend! Giving your air conditioning system an efficient, high MERV rating air filter is the best thing you can do for your air conditioner. A good air filter will extend the life of your air conditioner because the important parts, like the blower assembly, the cooling coil, and other inner parts will stay cleaner, operate more efficiently and last longer.

And, as a side benefit of having a good air filter, your family and you will enjoy breathing much healthier indoor air. Air that will be free of dust mites, mold, mildew, dust, pet dander, etc. etc.