"Frost Free" upright freezers are meant to function without developing the thick layer of frost so common in freezers, which is due to an automatic self-defrost feature on most units that cycle on a regular basis to prevent ice or frost from developing. But if frost is developing on the bottom of an upright freezer that is meant to be frost free, a variety of possible reasons may account for why.
Dirty Evaporator Coils
If there is frost developing in your frost-free freezer, there is a good chance that the evaporator coils are dirty and need to be cleaned. Disconnect the power to the freezer, and remove the lower access panel. Look at the evaporator coils, which look like a set of tubes, inside the freezer. If they are dirty, use a vacuum cleaner with a brush tool to clean them.
Frozen Evaporator Coils
When you are inspecting the evaporator coils, if they have a heavy layer of ice or frost on them, you will need to defrost the coils before you can clean them. Remove perishable items from the freezer and store elsewhere. Place towels under the front of the freezer and allow the frost on the coils to melt. Once the ice has melted, wipe everything down until clean. Replace the access panel, and restore power to the freezer.
Another possible reason why the freezer is developing frost is if the temperature setting is too low. Turn the freezer to one higher temperature point, and wait 24 hours for the temperature to warm. Keep the freezer door shut as much as possible during this time period.
Unfortunately, there is also a good chance that there are issues with the defrost timer or the thermostat. You can test out this theory by advancing the control to the defrost cycle and waiting. The cycle should advance automatically in approximately 45 minutes. If it does not, the defrost timer needs to be repaired or replaced. If the defrost thermostat or heater is damaged or broken, professional repair will also be necessary.