Defrosting the Evaporator Coils

When you open the refrigerator or freezer door, all of the cold air rolls out and is replaced by air with moisture in it. If it is hot and humid in the summer you can see the cold air condense the moist air as it goes into the refrigerator. As soon as you close the door the moist air is drawn across the evaporator coils and freezes in the form of white frost on the coils. The frost will build up over a period of two or three days and would completely fill the back of the freezer section with frost. The evaporator fan could no longer carry the freezing cold air away to other parts of the refrigerator and temperatures would start to rise. The defrost system in a self-defrost refrigerator keeps this frost cleared out.

One of the key components of the defrost system is the defrost timer. The timer is located inside the refrigerator near the thermostat for a refrigerator that is equipped for a crescent mold icemaker. If your machine is an older model with a plastic twist tray in the icemaker, the defrost timer is integrated into the icemaker. The timer has a switch to remove power from the compressor and divert it to the defrost heater. The switch lever is riding on a cam driven by a synchronous timer motor. The timer motor is designed to switch into the defrost mode every 8 hours. Sometimes the timer is wired to switch to defrost after 8 hours of compressor run time.

The defrost heater is positioned under and up both sides of the evaporator coils. It looks like an oven element that operates on 120 volts AC. When the heater is on you can hear the melting frost hiss when it hits the hot heater element.

If the defrost heater element were allowed to operate during the entire 15 to 20-minute defrost cycle, it would melt the frost and thaw half of the food in the freezer section! A defrost thermostat is wired in series with the heater element to limit the temperature in the evaporator coil section to less than 55 degrees F during the defrost cycle. The heater is on most of the time at the start of the cycle and when the frost is melted the heater is off most of the time until the cycle is over. There is no insulation between the evaporator coil and the back metal plate in the freezer section of a Sears or Whirlpool refrigerator. Therefore the food resting against the back panel of the freezer section may thaw a little during the defrost cycle. Food in plastic wrapping with air spaces can start to thaw allowing the moisture in the food to evaporate into the air space. When the defrost cycle is over and the compressor comes back on, the moisture will condense on the inside of the plastic wrapper and freeze. After dozens of defrost cycles, the moisture will be drawn out of the food and deposited on the inside of the food container. This is called "freezer burn". Fish and other seafood is best when it is frozen in water. Food stored in containers with air spaces is best stored for long periods of time in a manual-defrost freezer.

When a component of the defrost system malfunctions, frost will completely fill the evaporator coil section and frost will appear on the outside of the rear panel of the freezer section. The icemaker, if you have one, will stop making ice and the ice in the bottom of the bin will start to melt.