Refrigerator Defrost - Refrigerator Automatic Defrost Systems

How A Refrigerator Defrost System Works

On automatic defrost refrigerator or freezers, 100% of the air inside the unit is circulated through the evaporator. The evaporator is extremely cold, as much as -35 degrees Fahrenheit. As the air passes through the evaporator, the heat is removed and when the air exits the evaporator, it is extremely cold. In addition to removing the heat, because the air is full of moisture, also known as humidity, that moisture is removed from the air. The moisture is left behind on the evaporator in the form of frost. If this frost is not removed on a regular bases, it will block the evaporator from allowing air to pass through it. This will diminish refrigeration, especially in the fresh food section.

That's because almost all refrigerators today only have one evaporator. That evaporator is located in the freezer. After the air passes through the evaporator it is divided up by a damper in the airflow system. A significant portion of that air is forced into the fresh food section, and the rest is circulated through the freezer. As you can imagine, if the evaporator is blocked with frost, no air can pass through it to enter the airflow system. Therefore the temperature in the fresh food section and the freezer will go up.

Refrigerator manufacturers have built in a system to remove this frost from the evaporator. That system is called the automatic defrost system. It's called automatic because the user of the refrigerator does not have to do anything to clear the frost from the evaporator. A defrost system consists of a defrost heater, a defrost terminator or thermostat, sometimes a defrost thermistor, and a timer or electronic control. Refrigerators with timers are much easier for the inexperienced repair person to diagnose problems in the defrost system.

The Defrost Heater

Most defrost heaters are basically made of the same material as the bake element in your oven. That is to say that there is a metal tube with an insulated resistance wire inside. At the factory, this tube is formed to fit snug against the evaporator. That's right, in most cases the evaporator heater is physically attached to the evaporator. Other refrigerator manufacturers, use a sealed glass tube with a resistance wire inside. The glass tube does not touch the evaporator, but is in close proximity to the evaporator and radiates heat when energized. Whether the defrost heater is attached to the evaporator, or in close proximity to the evaporator, it is designed to do the same thing, melt the frost off the evaporator. Doing so on a regular basis, prevents the evaporator from becoming blocked with frost.

The Defrost Terminator

A defrost terminator is usually a bimetallic disc that is sensitive to heat at a very specific temperature. It is designed to open the circuit to the defrost heater when the evaporator is cleared of frost. It is also designed to prevent the defrost heater from coming on if there is no frost on the evaporator. For example, if you move the refrigerator to an new location and it has had an opportunity to completely warm-up, there is no sense in the defrost heater coming on right after you first plug it in. If the refrigerator uses a timer in the defrost system, when it was unplugged it may have been just two minutes from going into defrost. If the refrigerator did not have a defrost terminator or thermostat, when the refrigerator got to the new location and was plugged in, within two minutes the defrost heater would come on and radiate heat from the defrost heater into the freezer unnecessarily. If the defrost system is controlled by an electronic control board, a thermistor issues to read the evaporator temperature. The temperature of the thermistor is monitored by the electronic control board and if the evaporator is warm, the electronic control will not allow the defrost heater to come on.

The Defrost Timer

A defrost timer is a small mechanical mechanism that includes a motor, a series of cams, gears and switch contacts. The defrost timer was the standard since the beginning of automatic defrost systems. Of course today lots of refrigerators are using electronic control boards. The refrigerator defrost timer can be located almost anywhere. Refrigerator manufacturers have installed the defrost timers behind the kick plate on the base of the refrigerator, inside the fresh food section somewhere, and in the past on older refrigerators, somewhere even located on the back. Regardless of the location of the defrost timer, they all do the same thing. The timer closes a set of contacts that allow the compressor to run when the contacts on the refrigerator control thermostat are closed. The amount of time a refrigerator is in defrost is controlled by the timer. In the past refrigerator defrost timers were set to run anywhere between eight to twelve hours of compressor time, with 20 minutes of defrost time. After eight or twelve hours of compressor runtime, the timer switches over and closes a set of contacts in the defrost circuit. If the contacts on the defrost terminator are closed, the circuit to the heater will be complete and the heater will come on. That heater will run until the defrost terminator opens the contacts to the heater, or the defrost timer switches back to compressor runtime.

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