Reach-in Freezer Defrost Termination Failure with R-290
A two-door reach-in freezer using R-290 (propane) with a 1/3 hp hermetic compressor and an electric defrost system. The defrost timer initiates every 6 hours for a 30-minute maximum. The defrost termination thermostat is a bimetal mounted on the evaporator coil, set to terminate at 55°F coil temperature. Store reports product softening near the top of the cabinet, and ice is visible on the evaporator fan guards. The evaporator is heavily frosted at the end of the defrost cycle, and the drain pan has standing water. Technician measures defrost heater resistance at 18 ohms (nameplate 20 ohms), line voltage 208 V, and coil temperature at termination of 38°F. The termination thermostat is located 2 in. from the heater. Ambient 75°F, 55% RH.
1. What is the likely cause of the incomplete defrost, and why is the heater probably not the main fault?
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The defrost termination thermostat is ending the cycle too early. The coil is only 38°F when the heater is de-energized, well below the 55°F termination setpoint. Because the bimetal is mounted only 2 in. from the heater, it is sensing radiant heat from the heater rather than the coil temperature; it opens before the coil has cleared. The heater resistance (18 Ω vs. 20 Ω nameplate) is close enough that the heater is not the primary fault. Standing water in the drain pan suggests the drain may also be restricted, but the immediate cause of ice on the fan guards is incomplete defrost. Check the thermostat location, calibration, and defrost time. Verify by moving the sensor to the recommended location, or test the bimetal in ice water and compare to a thermometer.
2. How would you verify and correct the repair, including drain and timer checks?
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Temporarily relocate the termination thermostat to the coil area recommended by the manufacturer, away from direct radiant view of the heater, and run a manual defrost. Monitor coil temperature with a thermocouple; the heater should stay energized until the coil reaches 55°F at the termination point. If the thermostat still opens at 38°F, replace it. Also check the defrost timer for the correct initiation interval and maximum duration, and confirm the drain line is clear and trapped. After the repair, confirm that ice clears from the coil and fan guards and that the drain pan empties. Do not simply increase the defrost time; that can overheat the cabinet and waste energy.