The auto defrost outdoor ice merchandisers have two main components, the condenser and the air circulation fan. These add slight cost to the unit as well as higher operating costs than a cold wall model. The fans cycles the cold air from the condenser coils throughout the unit. This keeps the chamber temperature even and constant vs. a cold wall merchandiser.
(During the auto-defrost cycle, there may be moderate temperature fluctuations). Overall, this is a more popular model since it is virtually maintenance free. One should clean the coils regularly and perform routine maintenance for a very long life.