Appliance Parts

Mechanical contact defrost thermostats were developed in the 1950’s and are still widely used on small domestic refrigeration. However if you have an electronically controlled refrigerator, it won’t have this part. In place of this device on an electronic refrigerator, you have a thermister which communicates with the main controller board. Characteristics of this device won’t apply to you. If you have a mechanical contact defrost thermostat it is a drum shaped device about the diameter of a quarter.

Sometimes also called a defrost limit switch or a defrost termination switch, this device is present on all standard frost free refrigerators that use electric elements for defrosting the evaporator. The temperature in which the internal contacts open is not adjustable. These universally sized thermostats come factory preset to open at temperatures ranging from 45 to 90° F ( 7 to 32° C) depending on the application and design of the refrigerator.

Since 1997, as an energy savings feature, the defrost thermostat can also be configured in the electrical circuit so that the evaporator fan motor won’t come on unless its contacts are closed. This can save energy, because in this way, the fan won’t circulate the heat and moisture present in the coil area right after the defrost cycle. The fan will then only come on once the evaporator is at the temperature at which the thermostat contacts close again.

Purpose

Once the evaporator is free of frost and moisture, towards the end of the defrost cycle, the defrost thermostat disconnects the main defrost element. See it’s placement in the electrical circuit diagram below. This circuit is typical for a frost free refrigerator. It’s main and only purpose is to prevent the evaporator and freezer section from getting too warm near the end of the timed defrost cycle, which would cause the food in the freezer section to slightly thaw, several times a day.

Note dotted line: since 1997 as an energy savings feature neutral to the freezer fan motor may be fed through the defrost thermostat.

How it works

With the body of the thermostat strategically placed at a specific sensing point determined by the engineers, the internal contact remains closed providing a circuit path through to the main defrost element. Once all the frost is melted off the evaporator, and its temperature rises to the factory set point of the thermostat, the contact opens and disconnects the element.

Location on the cabinet

When a resistive type defrost element is used, the switch body itself is usually embedded, or firmly attached to the surface of the evaporator. With designs that use radiant heaters, the thermostat is placed on or near the evaporator in the coil cover area.

How it fails

The internal contact burns out and the device will not reset. This causes the defrost element not to come on in the defrost cycle, and the refrigerator soon builds up frost throughout the coils.

Variations

Defrost thermostats are usually mounted with separate hold down clips, however at this time some now have built in clips that attach it directly to a refrigerant line on the evaporator.

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Frigidaire Refrigerator Defrost Thermostat 216872200

Kenmore Refrigerator Defrost Thermostat 216872200