Ice formed in a batch crystallization apparatus is melted by causing an ice sheet 20 to be cut into approximately equal size pieces by the knife-edge 32 of melter 26. The pieces of ice are induced to slide against the sides of melter by inclined rails 28. Temperature sensor 40 and controller 44 are used to operate a heater to maintain the melter at a temperature setpoint 54 for a fixed time period while the ice sheet is cut into pieces. While the ice pieces melt the heating means is maintained fully on and the product drain valve is kept open. When the ice has melted the temperature exceeds the setpoint signaling the controller that the melting phase is complete. Failure of the apparatus to freeze ice is diagnosed by the temperature exceeding the setpoint at the end of said time period. Failure of the apparatus to melt ice is diagnosed by the temperature not reaching the setpoint after a sufficient amount of time.