How Do Defrost Bimetal Thermostats Work?

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A defrost bimetal thermostat is a common feature of refrigerators and freezers. This device helps to keep temperatures down in refrigeration systems. When a defrost bimetal thermostat isn't working properly, a refrigerator/freezer will have to work harder to keep things cool. If the bimetal thermostat fails to function, the refrigerator or freezer will eventually stop working.

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Defrost bimetal thermostats prevent the harmful buildup of frost on refrigerator and freezer evaporators. These devices trigger a defrost heater that stops frost from forming on refrigeration evaporator coils, which helps keep the refrigerator and freezer running efficiently.

Role of the Bimetal Thermostat

A buildup of frost on a refrigerator or freezer's evaporator leads to a layer of insulation that reduces the system's efficiency. This results in increased running costs for your refrigerator, as the system must work extra hard to keep temperatures down. This may also result in the refrigerator or freezer becoming too warm at times, which will negatively affect the contents.

How Defrost Bimetal Thermostats Work

A defrost bimetal thermostat works separately from the refrigerator or freezer. This device, which turns on several times a day, senses the temperature of cooling coils. When these evaporator coils become so cold that frost starts to build up, the defrost bimetal thermostat facilitates the melting away of any frost that has formed on the cooling coil. The defrost bimetal thermostat does this by activating a hot gas valve or an electrical heating element that raises the temperature near the evaporator, which then melts the frost that has formed.

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The melting away of frost buildup protects your refrigerator and freezer's evaporators from overheating during the defrost cycle. The bimetal thermostat and defrost heater work in tandem. When the frost is all melted, the bimetal thermostat will sense the temperature increase and trigger the defrost heater to turn off.

The Bimetal Thermostat’s Crucial Role

For the proper and efficient functioning of your refrigerator/freezer, it's critical that your bimetal thermostat is in working order. Without this device working properly, your refrigerator and freezer will have to strain to function. This can mean increased electricity bills, and it will decrease the life of your refrigerator. If the bimetal thermostat doesn't work at all, your refrigeration unit will soon fail, requiring you to buy a new one.

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Signs of a Failing Bimetal Thermostat

If your refrigerator or freezer bimetal thermostat is failing, the thermostat may not be able to correctly sense the temperature of the coils. This can lead to the defrost heater turning on sporadically or not at all.

Common signs that your refrigerator or freezer's bimetal thermostat is failing include:

  • The refrigerator or freezer is leaking. If you see water on the floor, this could be a sign of a bimetal thermostat failure.

  • The refrigerator or freezer is demonstrating inconsistent temperatures. The interior may become too hot or too cold.

  • There is ice and frost buildup on the interior. This is especially a problem in freezers that won't defrost, but you may also see ice or frost buildup in refrigerators.

  • The appliance seems to be running too long or never shuts off.

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Julie Bawden-Davis is a widely published home and garden writer and a University of California Certified Master Gardener. She has written several gardening books, and her work has appeared in a wide variety of publications, including Parade.com, Organic Gardening, Wildflower, Better Homes and Gardens and The Los Angeles Times. In her free time, Julie gardens in her Southern California backyard, certified by the National Wildlife Federation as a backyard habitat.