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What is a Defrost Thermostat?

T. Doucette
T. Doucette

A defrost thermostat is the temperature-controlling device within the automatic defrost system of a refrigerator. There are three components to the defrost system: a timer, a thermostat, and a heater. When the coils within a refrigerator become too cold, the defrost timer cues the heater to click on and work to melt any excess ice buildup. The function of the thermostat is to prompt the heater to turn off when the coils return to the correct temperature.

When a defrost cycle begins, the metal alloy switches of the defrost heater are triggered by the timer to switch to warm. When the circuit closes again, it will then cool off the metal. As the thermostat detects this new temperature, it prompts the heater to turn off. A defective thermostat can cause the heater to either overheat or fail to operate at all.

The defrost thermostat in a refrigerator helps the coils maintain the correct temperature.
The defrost thermostat in a refrigerator helps the coils maintain the correct temperature.

The defrost thermostat differs from the refrigerator thermostat, which is the temperature gauge used to regulate the cooling system of the refrigerator. This device works to measure internal temperatures and prevents the refrigerator from becoming too hot or too cold. If temperatures do become extreme, the thermostat prompts the refrigerator's compressor to turn on or off.

A defrost thermostat is part of a refrigerator.
A defrost thermostat is part of a refrigerator.

Typically located underneath the refrigerator, the defrost thermostat may alternately be found in the back of a side-by-side refrigerator/freezer. It is attached to wire plugs near the coils of the defrost system, and connected to a wiring harness that will allow for its removal. In some cases, it is wired directly to the harness and will need to be removed by cutting the wires.

Inside the thermostat is a small vile of mercury used to measure the temperature of the refrigerator's coils. The thermostat should read 0°F (-17°C) at its coldest. Readings for this can be done with an Ohm meter, or multimeter, in cases where the thermostat needs to be tested for operation. Often, the vial can come out of alignment and will need to be reset. If it still does not read within limits, it is most likely defective.

Many newer-model refrigerators feature adaptive defrost control boards. These digital boards replace the traditional mechanical defrost timers and thermostats, and plug directly into the refrigerator. The defrost thermostat in these boards works in the same manner, but the entire board can detect failures in the thermostat, compressor, or heater features. The adaptive defrost control board will automatically reset itself after repairs are made.

Discussion Comments


I was always clueless about how a freezer defrosted itself as well. I remember the first apartment I lived in had one of the older freezers that didn't defrost. It was a nightmare to get rid of the ice. You could run the defrost setting, but then you ran the risk of ruining your food. Scraping it never seemed to help either, and there always warnings that using something like a screwdriver or ice pick could puncture the wall and ruin the freezer. I'm thankful for whoever invented the self-defrost.

Anyway, I notice that sometimes a small amount of frost starts to build up in my freezer. I wonder if maybe my thermostat is out of line like the article mentioned. We were moving the fridge recently while were were retiling the floor and painting the walls. Does anyone know of an easy way to test the defrost thermostat and fix it if it is not at the right settings? Maybe it is something that I will have to call a repairman for.


@SarahSon - That is odd that the freezer defrost thermostat wouldn't work right if the power went off. I wonder if that is what is always happening to my upright freezer, though.

Not more than a month after we got it, I noticed a giant puddle of melted ice cream on the bottom. Basically, everything in the freezer was ruined. I called the store to get someone to fix it. I think he said it was something like the condenser that wasn't working right. He replaced that, but we still had the same problem after that. The freezer worked fine for a while and then stopped. They finally just replaced our freezer with another.

Like you said in your case, though, we also have the power go out a lot more than it should, which is a whole other story. Anyway, I wonder if maybe those power outages were causing the problem. In any case, this new freezer works like it is supposed to, fortunately.


@jcraig - I think I actually heard the answer to your question a couple of weeks ago. It is basically a process called sublimation. Sublimation is kind of like the reverse of condensation or one of those processes. It is basically when water turns from a solid to a gas without going through the liquid stage.

Like you mentioned at the beginning, there isn't a lot of water that makes its way into the freezer, in general, otherwise this probably wouldn't work as well. I didn't really understand the science of how water can go straight to a gas form, but apparently it happens a lot in freezers. Anyway, if you ever feel the side of your freezer sometime, there will be a very thin layer of ice, but at some point, it sublimates and disappears. Enough sublimation happens to get rid of water that there is never any ice that stays left over.


I have always wondered how a self defrosting freezer worked. I knew it had to have some sort of a sensor in it that could tell what it was getting too cold, but I never knew that the coils actually heated up at any point. Like someone else mentioned, that's really counter-intuitive as to what you would initially think.

Does anyone have any idea how often a defrost cycle happens in a normal freezer? I know it takes a couple weeks for ice to start building up in a normal freezer, so I'm sure they defrosts don't happen too often.

If I remember correctly, when ice starts to build up in a freezer, it is mostly from molecules of water vapor getting into the freezer when you open and close the door. With a freezer that defrosts itself, what happens to the water particles? They don't turn to frost, but there aren't puddles of ice in the freezer either? Does anyone have any ideas?


I find it interesting there is a heater in a freezer defrost system, but it makes sense why it is needed.

I have learned more than I ever wanted to about how refrigerators and freezers work because mine have malfunctioned so often.

Thankfully I purchased a warranty on them so have not had to spend a lot of money on appliance repair, but I am worried what will happen once the warranty runs out.

This was a brand of refrigerator/freezer that I had never used before, and I won't be buying another one. Most people have only positive things to say about them, but this hasn't been the experience for me.

When my defrost thermostat in my freezer quit working I was gone for a long weekend. By the time I got home, most of the food in my freezer was ruined. Ever since then I have not kept a lot of food in the freezer because I don't feel like I can totally trust my freezer to work right.


@LisaLou - I have also defrosted my share of freezers and would always put it off way too long. The only reason I stayed with freezers like that was because I was told your food would stay frozen longer in a freezer that did not defrost automatically.

Like you said, I found myself throwing away a lot of food and wondered if it was worth it. There is more that can go wrong with an auto defrost freezer though.

We have frequent power outages and have had problems with our freezer defrost thermostat when that happens. More than once we have had to unplug the refrigerator and plug it back in again to restart everything.

Usually the only way I notice it is when the items in the freezer start to feel a little soft. So far I have not lost my food because of this, but I know I have to watch it closely.

I have not had to replace the defrost thermostat yet, but wonder how hard that is on it every time something like that happens.


We have a large family and have always had more than one freezer. The small freezer that is attached to our refrigerator is not nearly big enough for our needs.

In the past, my extra freezers have been upright freezers that did not automatically defrost. If you have ever defrosted a freezer you know what a pain this can be.

First you have to take everything out of it and wait until all ice that has built up melts. You do have a lot more room when this is done and you also find a lot of stuff that is too old to keep, but it is not a fun process.

I finally said no more freezers like this, and insisted on one that automatically defrosts. What a difference this makes. As long as that defrost thermometer keeps working I know that I won't have to defrost another freezer again.

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    • The defrost thermostat in a refrigerator helps the coils maintain the correct temperature.
      By: Manh Luong Bui
      The defrost thermostat in a refrigerator helps the coils maintain the correct temperature.
    • A defrost thermostat is part of a refrigerator.
      By: Piotr Pawinski
      A defrost thermostat is part of a refrigerator.