Freezer Evaporator Ice Build-Up After Defrost
A reach-in freezer uses R-290, electric defrost, and a 6-hour defrost timer with a 20-minute fail-safe. The termination thermostat is set to open at 55°F and close at 30°F. The evaporator fan is delayed until the coil cools. The owner reports the box is at 15°F and the evaporator is heavily iced at the bottom; the drain pan is also iced. The compressor runs continuously and suction pressure is 8 psig. When the technician forces a defrost, the timer motor advances and the defrost contact closes, but the heater ammeter reads 0 A. The heater resistance measured cold is within specification. The termination thermostat has continuity at 20°F. The drain line has a standing water trap and passes through the freezer wall.
1. Explain why the defrost cycle is not clearing the evaporator, and list the possible causes consistent with 0 A heater current.
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The evaporator is iced because the defrost cycle is not delivering heat. The timer is advancing and the defrost contact is closing, so the timer motor is working, but 0 A at the heater means no current is flowing through the heater circuit. A good cold heater resistance does not prove the circuit is complete under load.
Possible causes include a defective defrost contactor or timer contact that does not carry current, an open termination thermostat or safety limit under actual conditions, a blown fuse or tripped defrost limit, a broken wire or connection, or a heater that opens when hot. The iced drain pan also suggests that defrost water is not draining, but the primary problem is the lack of heater current. If the heater never gets hot, ice accumulates until airflow and capacity are lost.
2. Describe safe electrical tests for the heater circuit on an R-290 freezer.
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R-290 is flammable, so use appropriate safe procedures: lock out and tag out the unit, avoid live work where possible, use a meter rated for the circuit, and do not create sparks or arcs near refrigerant. Verify that the circuit is de-energized before checking resistance. Confirm the heater resistance against the manufacturer’s specification and check each wire and connection for continuity.
Then re-energize only as required for voltage testing. Measure voltage at the heater terminals during a forced defrost. If voltage is present at the heater but current is zero, the heater is open under load. If voltage is missing, work backward through the termination thermostat, defrost limit, timer contact, and contactor. Check voltage drop across contacts; a contact can show voltage but fail under load. Do not bypass any safety limit for testing.
3. Recommend repairs and defrost settings to prevent recurrence.
Read the worked answer
Repair the open circuit in the heater branch: replace the defective contactor, timer, termination thermostat, limit, or heater as indicated by the tests. Clear the drain pan and drain line, and verify the trap is correct and not blocked by ice. Confirm that the drain line slopes properly and does not terminate in a location that allows ice to form.
Set the defrost termination to open at the manufacturer’s specified coil temperature, typically around 55°F for this type of freezer, and verify the fail-safe does not exceed the manufacturer’s limit. Check the fan-delay control so the fan does not blow water into the box before the coil drains. After repair, force a full defrost and verify heater amperage, termination temperature, drain flow, and that the evaporator is clear before returning the unit to service.