In my column in the June 6 issue ofThe NEWS, I discussed the problem of frozen evaporator coils in walk-in freezers and specifically focused on defrost cycle termination. In this column, I want to review a couple of problems you can encounter when dealing with defrost systems. They relate to the system either being overdefrosted or underdefrosted.


Overdefrosting is when the defrost heaters stay energized too long, causing the box temperature to rise too high during defrost. The product may begin to melt and then refreeze. This problem can be identified by monitoring the box temperature during defrost or by examining the product in the case. Ice crystals forming on frozen product may be signs that the product began to melt and then refroze.

Overdefrosting is normally caused by a malfunction in terminating the defrost cycle:

  • On time-time defrost systems, either the defrost time is set too long or the time clock is defective.

  • On time-pressure systems, either the terminating pressure control is set too high, the pressure control is defective, or the solenoid coil in the defrost timer is incorrectly wired or defective.

  • On time-temperature systems, either the temperature control is set too high, the control is defective, or the solenoid in the defrost timer is defective.


    Systems that are underdefrosting will result in a frozen evaporator. The frost that normally develops on the evaporator coil will continue to build until the entire surface of the evaporator is iced over. This can be caused by a defective or incorrectly set time clock that does not initiate a defrost cycle, or it may be caused by an open heater element or a defective defrost termination switch that continually terminates defrost on each initiation of a new cycle.

    A technician will need to defrost the coil when the evaporator ices over. Sometimes this can be done by initiating a defrost cycle. This can be done on a mechanical defrost timer by rotating the inner knob clockwise.

    Many times, however, this cannot be done. The technician will have to manually defrost the coil. Extreme care must be taken when doing this. Do not use any sharp objects to chip away the ice from the coil - this could easily puncture the coil and cause a refrigerant leak to develop in the evaporator. Using water is the best method, but is not always practical since the water will need to be drained away. If water cannot be used easily, a heat gun usually works well. Defrosting a coil manually is time-consuming. Do not rush this procedure. Making a careless mistake can be costly.

    Resolving defrosting issues on walk-in freezers is usually not difficult. Understanding how the defrost system operates will enable any technician to correctly troubleshoot and repair the various problems.

    Joe Marchese is owner of Coldtronics of Pittsburgh. He can be reached at 412-734-4433,, or

    Publication date: 07/03/2006