A refrigeration system equipped with a time-initiated, temperature-terminated defrost control is experiencing excessive defrost heater run times, even when the coil appears clean. The defrost termination thermostat is mounted directly on the evaporator coil. What is a likely cause for this issue?

Question: A refrigeration system equipped with a time-initiated, temperature-terminated defrost control is experiencing excessive defrost heater run times, even when the coil appears clean. The defrost termination thermostat is mounted directly on the evaporator coil. What is a likely cause for this issue?

Answer options: ✅ The defrost termination thermostat has shifted out of proper contact with the coil.

• The compressor is overloaded, causing higher suction pressures.
• The low-pressure switch is set too low, causing premature cycling.
• The system has an overcharge of refrigerant.

Correct answer: The defrost termination thermostat has shifted out of proper contact with the coil.

Explanation: If the defrost termination thermostat, which is designed to measure coil temperature and end the defrost cycle, loses good thermal contact with the coil, it will inaccurately sense a lower temperature. This will cause the defrost heaters to run for a longer duration than necessary until the timer expires, even if the coil is already thawed.