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How Refrigeration Thermostats/Cold Controls Work

Macro detail of typical refrigerator thermometer.

The first step to understanding how refrigeration thermostats (aka cold controls) work is to understand a counter-intuitive fact about refrigeration physics:

Cooling is achieved by sensing and removing warm air, not by adding cool air.

A thermometer, then, measures how much heat is present. If the heat level reaches a certain, um, level, the refrigeration mechanism kicks, and the refrigerant starts its trip through a maze of intestine-like coils, and the warm air is removed.

That’s the big picture, anyway. Let’s dive in and explore the process in greater detail.

A refrigerator thermostat/cold control is basically the brains of the refrigerator cooling system—it runs the show.

Thermostats are typically found inside the refrigerator and have a knob that allows users to adjust the temperature setting. Once a user sets the desired temperature, the thermostat maintains that temperature by controlling the flow of electricity to the compressor. If the thermostat is the brain, the compressor is the heart of the operation, responsible for pumping the refrigerant through the coils.

When the air inside the refrigerator is at the desired temperature, the thermostat stops the flow of electricity to the compressor. When the thermostat senses too much heat, it allows electricity to flow, activating the compressor.

How does the thermostat control the electricity, you ask?

In most commercial refrigerators, the thermostat has a capillary tube filled with gas. As the temperature in the fridge increases, the gas expands and pushes on a diaphragm, which operates a set of contacts which in turn operates the compressor.

Pretty cool, huh?

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About Jeb Foster

With a passion for content, Jeb is Tundra’s email guru and works to deliver a perfect mixture of content and deals in our emails to keep even the finickiest subscriber overwhelmed with email happiness. When he’s not at Tundra, he likes to spend his time playing air guitar and remembering the good old days of snail mail.

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7 comments

  1. Thanks man !!!!!

  2. You mean when we open our fridge , tempertaure inside the fridge goes up.. And again themostat senses heat and electricity start flowing?
    Is it the reason we always hear more sound when we openup the fridge?
    And can you plz tell what all gases are in that capillay tube have?

    • @Khushu the compressor won’t switch on when you open your door it takes a bit longer like having your freazer-fridge door open for at least 10min the thermostat works like an normal light switch per say it’s only bridged now how it works is like the author says the gas expands and the contactors switch off think an spring because thats basically what happens gas expands and the sealed spring expands too thus disconnecting and connecting contactors and sometimes another contactor switches on and thats the defrost element that’s basically just an very thin wire that gets hot and melts the ice

  3. The thermostat has a capillary tube filled with gas. So what type of gas is inside? Will a severe bend in the tube affect operation? My refrigerator didn’t regulate properly for years, and later it was discovered that the capillary tube had a sharp bend. Could that explain it?

  4. Hi Pete,

    Yes, that would definitely affect operation. They put a type of refrigerant in there that reacts to the temperature and puts pressure on a switch in the body. If it’s pinched, then there would not be enough pressure to operate the switch correctly. Also, those thermostats are not designed to be adjusted a lot; they should be set to the middle setting (usually #5) and adjustments should be made only after the unit has had 4 hours to settle into the new setting. If additional adjustments are needed, only move it by one number at a time and give it time to settle again. Be happy with 36-38 degrees—trying to pull too low will lead to frozen coils and additional repair cost.

    – Paul
    Tundra Team

    • I was wondering if that is the case with my refrigerator. It had stopped cooling in both the freezer and the fridge. We cleaned the fridge out and turned it off for one day and night. The next day when we turned it on, it worked fine. However, the setting at that time was number 6 for the fridge. It was working great for one week and now it’s not cooling again. What could be the problem? There is a lot of frost in the back of the freezer. I’m assuming the coils are frozen again. If it’s the thermostat, can I replace it?

      • Hi Anna,

        It does sound like a thermostat problem. If the unit doesn’t know to turn off, it will freeze up. It also sounds like you don’t have a commercial unit. I’d reach out to the manufacture direct and find out what they can provide you.

        – Paul
        Tundra Team

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