Frost on Outdoor Coil:
Most likely to occur on one of the rare very cold days here in florida. In this case your Heat Pump is heating the indoor air which means the outdoor coil has to get cold. In a properly functioning system the outside unit will detect the frost and run a defrost cycle. Defrost cycles start with a loud whoosh and end blowing off humid smoky air like your breath in winter.
Frost on Outdoor Piping:
Are you finding frost on the piping between the outdoor unit and your home? This type of freezing up starts on the indoor coil and propagates outside. If the outdoor unit is to blame causes can be a refrigerant leak in your sealed system or an internal refrigerant flow restriction. Also see frost on coil for possible causes related to Air Handlers.
Frost on Indoor Coils:
Do you see frost or ice on your indoor coil? One common cause is lack of air flowing over the coil. Failure of the blower motor to pull air stops that needed airflow. A badly plugged filter or a coil impacted with dirt can also stop the correct airflow. Also see frost on outdoor piping for possible causes related to Condensing Units.