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Refrigerator Defrosting System Operation

Purpose of Defrost System

The refrigerator and freezer doors will be opened and closed numerous times as family members store and retrieve food and drink. Every opening and closing of the doors allows air from the room to enter.  Cold surfaces inside the freezer will cause moisture in the air to condense and form frost on the food items and cooling coils. Over time frost that is not removed will build up eventually forming solid ice. The defrost system prevents the buildup of frost and ice by periodically initiating the defrost cycle.  

Defrost System Operation

1.The defrost timer or control board initiates the defrost cycle. 

Mechanical timers initiate and terminate the cycle based on time.

Control boards initiate and terminate the cycle utilizing combinations of time, logic, and temperature sensing.

Timers and control boards are commonly located in the refrigerator section near the temperature controls behind plastic panels.  Control boards may be mounted on the back of the refrigerator. 

2.The defrost cycle blocks power to the compressor and sends power to the defrost heater.

Heaters are usually calrod heaters (look like small bake elements) or elements encased in a glass tube.

Heaters will be fastened to the bottom of the cooling coils in the freezer section. High-end refrigerators with cooling coils in the refrigerator section will have a second defrost heater.  Most refrigerators have one heater.

The heat from the heater will melt the frost and ice on the cooling coil.  The water (melted ice) runs down the cooling coils into a trough below the coils.  Water collected in the trough is routed to a condensate pan located in the compressor section where it evaporates back out into the room from whence it came.

3.The defrost termination switch (thermostat) or in some cases, a temperature sensor stops the heater from thawing the food in the freezer during the defrost cycle.

Power is routed through the defrost termination switch (thermostat) to the heater.

The defrost termination switch (thermostat) is mounted to the coil at the top.  

The defrost termination switch (thermostat) will cycle power to the heater off and on for the duration of the defrost cycle.

As the heater raises the temperature of the defrost termination switch (thermostat) the power will cycle off to the heater.

As the temperature of the defrost termination switch (thermostat) cools the power will be restored to the heater.

Some defrost systems use a temperature sensor instead of the defrost termination switch (thermostat).

 Temperature sensors and heaters connect directly to the control board.

Power to the heater is controlled by the control board.


Post time: Feb-13-2023