Replacing Your Defrost Thermostat

The Defrost Thermostat

Known commonly as the defrost thermostat, this handy device is often the culprit if you're noticing some frost build up in your no-frost refrigerator.  Other names include defrost bi-metal or simply bi-metal.  Unfortunately, the technology used in this part isn't the kind of thing you can test at home, so if you suspect it is the problem, the solution is replacement.

A Little Science

A bi-metal thermostat is a clever mechanism that uses two metals fused together in such a calibration that at a certain temperature (depending on its function), it will pop outward like a tiny diaphragm, lifting an electrical switch and breaking a circuit.  In the case of your refrigerator, frost accumulates when moisture in your freezer touches the evaporator (the grid at the back or on the bottom of your freezer compartment).  The refrigerant passing through the evaporator is in the process of changing from a liquid to a gas, which sucks heat out of the freezer and passes it to the outside.

At this point we should remind you that refrigeration involves removing heat.  "Adding" cold is a physically impossible. There is only one measure of temperature in the universe, and that is heat.  Since the heat is leaving fastest at the coils of the evaporator, it is the coldest spot in the freezer.  When enough frost accumulates on the evaporator, the internal temperature of the defrost thermostat will reach 30 degrees, flattening out the bi-metal, which in turn lowers a pin that completes a circuit.  The circuit powers a heating element that melts the frost and warms up the thermostat to about 55 degrees.  At this temperature, the bi-metal defrost thermostat bends upward, lifting the pin and disconnecting the circuit.

Done Easily, By Yourself

Usually mounted directly on the evaporator, the defrost thermostat should not be terribly difficult to access, and anyone with a rudimentary understanding of electricity should be able to replace one.  Aftermarket options are available if you do not want to go through the manufacturer, but prices for defrost thermostats are reasonable, usually around $16.