Defrost System Service
In-depth service procedures for electric, hot gas, and off-cycle defrost systems including timer programming, heater testing, termination thermostat calibration, and troubleshooting defrost failures.
- Program and troubleshoot mechanical and electronic defrost timers for commercial refrigeration
- Test electric defrost heaters for continuity, proper wattage, and correct installation
- Service hot gas defrost systems including solenoid valves, check valves, and pressure regulators
- Diagnose defrost termination and fan delay failures that cause ice buildup or product warming
Lesson 1
Electric Defrost Systems
How Electric Defrost Works
Electric defrost is the most common defrost method for commercial freezers. Resistance heating elements are mounted in or on the evaporator coil, and during defrost the compressor and evaporator fans shut off while the heaters melt accumulated frost. A termination thermostat mounted on the coil ends the defrost cycle when the coil reaches a safe temperature.
Defrost Heater Testing
Electric defrost heaters are resistance elements rated in watts per foot or total watts. To test:
- Disconnect power and isolate the heater circuit
- Continuity test - Use an ohmmeter across the heater leads. Infinite resistance (OL) means the heater is open - replace it
- Resistance check - Calculate expected resistance from the heater rating: Ohms = Volts squared / Watts. For a 1,500-watt heater on 240V: R = 240^2 / 1,500 = 38.4 ohms. Measured resistance should be within 10% of calculated.
- Ground fault test - Measure resistance from each heater lead to ground (heater frame/sheath). Should read infinite (OL). Any measurable resistance indicates the heater element is grounded and must be replaced.
Defrost Termination Thermostat
The defrost termination thermostat (also called a defrost limit or klixon) is a temperature-sensing switch mounted on the evaporator coil. It opens at a preset temperature (typically 50-55 degrees F) to end the defrost cycle and prevent the box from warming excessively.
Testing the termination thermostat:
Place the thermostat in a cup of warm water and verify it opens at its rated temperature. If it does not open, the heaters will continue running after the coil is clear, overheating the box and potentially damaging product. If it opens too early (below 45 degrees F), the coil may not fully defrost, leading to progressive ice buildup over multiple cycles.
Progressive Ice Buildup
If each defrost cycle does not completely clear the coil, ice accumulates progressively. After several days, the evaporator becomes an ice block. This is the most common result of a defrost termination thermostat opening too early, a failed heater element, or insufficient defrost duration. When you find a completely iced evaporator, always test the termination thermostat and all heaters before returning the system to service.
Test electric defrost heaters for continuity and correct resistance (Ohms = Volts squared / Watts), and verify the termination thermostat opens at 50-55 degrees F - a thermostat that opens too early causes progressive ice buildup over multiple defrost cycles.