Walk-In Freezer Defrost Termination and Floor Ice
A walk-in freezer holding frozen product at 5°F uses R-448A, electric defrost, and a 30-minute defrost period four times per day. The defrost termination sensor is set to terminate at 50°F coil temperature, and the fan delay is set to 45°F. The evaporator coil is iced at the top, and ice is building on the floor near the door. The drain pan heater is warm, but the drain line has ice at the exit. The door gasket is torn along the hinge side, and the store receives deliveries three times per day. The defrost clock shows the correct time and the heaters draw rated current when forced on. The product temperature is rising, and the manager wants the defrost time extended to 60 minutes per cycle.
1. List the likely causes of the ice pattern and floor ice, in priority order.
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The top of the coil icing while the heaters draw rated current suggests defrost is not terminating properly or the termination sensor is not sensing the coil. Likely causes, in order: (1) the defrost termination sensor is loose, misplaced, or failed, so the clock runs by time and may terminate before the coil is clear; (2) the drain line is freezing because the drain heater or drain pan slope is inadequate, allowing meltwater to refreeze; (3) infiltration from the torn door gasket and three deliveries per day adds moisture that collects near the door; (4) the fan delay is set too low, so fans restart and blow cold air before the coil drains. The warm drain pan heater does not prove the drain line is clear. The coil ice at the top is important because the termination sensor is usually located where ice clears last; if that sensor is not in the correct location, defrost ends early.
2. Explain why extending defrost to 60 minutes is not the correct fix.
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Extending defrost treats the clock, not the cause. If the termination sensor is faulty, a longer defrost may eventually melt more ice, but it will also add heat to the freezer, raise product temperature, and increase moisture migration. The extra meltwater must still leave through a frozen drain, so it will refreeze on the floor or in the pan. Longer defrosts also increase energy use and compressor run time. The manager's suggestion could temporarily reduce coil ice while making the product-temperature problem worse. The correct approach is to verify termination sensor resistance and location, confirm the coil reaches the termination setpoint, clear the drain line, repair the door gasket, and then adjust defrost only after the system can terminate on temperature. If the coil never reaches 50°F, extending time will not solve the root cause.
3. What measurements would you take to confirm whether the fault is the termination sensor, the heaters, or infiltration?
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Measure the termination sensor resistance at room temperature and compare it with the manufacturer's temperature-resistance table; then measure it during defrost as the coil warms. If the resistance does not change, or the reading does not correspond to the coil temperature, replace or relocate the sensor. Confirm heater current on each heater circuit during defrost and check voltage at the heaters; rated current proves the heaters are energized but not that they are evenly distributed. Measure coil temperature at the termination sensor location and at the top of the coil. Inspect the drain pan slope and pour water through the drain to confirm it flows freely. Check the door gasket with a dollar-bill or light test and inspect door closers and traffic patterns. Record defrost duration, termination temperature, and product temperature before and after the repair. That data shows whether the fix actually restored temperature termination.