Sometimes ice builds up in the evaporator pans. This issue can lead to water overflow during defrosting. Typically, there are two reasons for ice build up in the pans.

1. Operating evaporators located in the penthouses create a slight vacuum in these penthouses. When one evaporator goes on defrost, the mentioned vacuum draws freezer cold air through the evaporator and defrosted water will refreeze at the bottom of the evaporator before it will drop to the pan. Over time, the bottom of the evaporator and the space between the evaporator and pan will be full of ice. To avoid this ice build up, during defrosting of one or several evaporators located in one penthouse, other evaporators should be switched off.
2. After several years of operation, the evaporator liquid solenoid often starts leaking by. During the off cycle, ice continues to build up and over time the evaporator pan can become full of ice. Liquid solenoid should be replaced when it shows sign of leaking by.

Usually, hot supply to the evaporator pan can be done in parallel to the evaporator hot gas supply or in series to the evaporator hot gas supply. I prefer the series hot gas supply. This means that all flow of hot gas goes to the pan first and then goes to the evaporator. Superheated hot gas releases the superheat in the pan and the pan becomes warmer than the evaporator and this higher temperature will minimize probability of ice build up in the pan.

If you have ice in the evaporator pan, the reason for the ice build up should be corrected first. Then the evaporator should be covered and manually defrosted before switching to normal operation.