A commercial freezer defrost system uses an adjustable time clock set for 4 defrosts per day, each 30 minutes long. The defrost termination switch is set to 8°C. During a service call, the technician observes that the coil is still covered in a thin layer of ice, but the defrost element is off and the fans have restarted. The box temperature has risen to -10°C from its setpoint of -18°C. What is the most likely cause of this issue?

Question: A commercial freezer defrost system uses an adjustable time clock set for 4 defrosts per day, each 30 minutes long. The defrost termination switch is set to 8°C. During a service call, the technician observes that the coil is still covered in a thin layer of ice, but the defrost element is off and the fans have restarted. The box temperature has risen to -10°C from its setpoint of -18°C. What is the most likely cause of this issue?

Answer options: ✅ Defrost termination switch is opening prematurely.

• Defrost heater elements are open-circuited.
• The time clock is not advancing correctly.
• The defrost drain line is plugged.

Correct answer: Defrost termination switch is opening prematurely.

Explanation: If the coils are still iced but the defrost cycle has terminated, it indicates that the defrost termination switch is sensing an 'end of defrost' condition prematurely, preventing complete defrosting. This could be due to a faulty DTT or improper placement, causing the defrost to end before all ice has melted, leading to ice buildup and elevated box temperatures.