WiremanWireman · Walk-in cooler or freezer defrost problems: troubleshooting for electricians · 2023 NEC ·
Walk-in cooler or freezer defrost problems
Iced coils, drain line heaters, the defrost timer and the termination control.
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How it works
A freezer evaporator runs below freezing, so ice builds on the coil and has to be melted off. With electric defrost, a timer or controller periodically shuts off the refrigeration and evaporator fans and energizes heaters on the coil and the drain pan, plus a heater on the drain line. A termination thermostat or sensor ends the defrost when the coil is warm, with a time limit as a backup, and a fan delay keeps the fans off until the coil is cold again. Coolers above freezing usually use off-cycle defrost: the compressor stops and the fans melt the ice. Refrigerant work goes to a refrigeration tech.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What do you see?
- Evaporator coil iced solidIs it a freezer with electric defrost, or a cooler?
- Freezer with electric defrostTest: Force a defrost and clamp the heatersExpect: The heaters draw close to their rated watts divided by the voltage, and ice starts to melt.
- Heaters draw current, ice meltsDefrost schedule or duration
- Timer switches, heaters draw 0 ATest: Test the defrost heatersExpect: Each heater near R = V² ÷ W from its rating, and OL to the frame.
- A heater reads OLOpen defrost heater
- A heater reads to the frameDefrost heater shorted to the frame
- Heaters test goodHeater circuit not getting power
- Timer won’t switch into defrostFailed defrost timer or controller
- Cooler (off-cycle defrost)Cooler coil icing: airflow, fans or charge
- Freezer with electric defrostTest: Force a defrost and clamp the heatersExpect: The heaters draw close to their rated watts divided by the voltage, and ice starts to melt.
- Drain line or floor iced, water leakingTest: Test the drain line heaterExpect: A steady reading for its rating (not OL), and heater along the whole drain line in the cold space.
- Open or missingFailed or short drain line heater
- Heater is goodDrain line pitch or trap
- Box warms up during or after defrostTest: Check the termination controlExpect: It changes state between cold and warm the way the diagram shows.
- Doesn’t change stateFailed termination thermostat or sensor
- Works correctlyDefrost schedule or duration
- Ice on fan blades or the fan guardFans starting before the coil refreezes
- Evaporator coil iced solidIs it a freezer with electric defrost, or a cooler?