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Icing can be caused by a number of factors. Units that have not been cleaned over a long period of time may have coils and fins clogged by dust, dirt or debris. Clogged coils and fins restrict air flow through the unit, which can cause the compressor to work even harder – so hard, that it may reduce the coil temperature below the normal range in an attempt to make up for the lost cooling power from the blocked coils.

Another potential cause is running a unit with a very cold set point (i.e. turning the target temperature down very low) on low fan speed for an extended period of time. This can cause excess condensate to build upon the coils, further lowering the temperature of the coil and leading to icing.

This problem generally has one of two causes: the unit was not installed properly with the outside lower than the inside to allow for adequate exterior drainage, or there may be a clog in the drain pan where condensate collects during operation.

If you feel that neither of these conditions exists, check for air leaking in from the outside anywhere around the unit. Air leaks will cause condensation to form on the outside of the unit, and this water will drain into your room. If there are any air leaks, resealing them should address the problem.