How a Refrigerator Keeps Food Cold

A fridge moves heat. It doesn’t make cold

Open the door and the cold air feels like something the fridge produced. It isn’t. No part inside a refrigerator manufactures cold. As ENERGY STAR puts it, a refrigerator removes heat from an enclosed box and expels that heat to the surrounding air. What stays inside feels cold because it holds less heat than before.

That sounds like a play on words, but it explains almost everything about the machine: why the back of it is warm, why it hums on and off, why its coils collect dust, and why leaving the door open never cools a kitchen.

The hard part is direction. Heat flows on its own from warm things to cold things, never the other way. Your kitchen is warmer than the inside of the fridge, so heat is always leaking in through the walls and every time the door opens. Pushing it back out, uphill, takes energy. That is the compressor’s job.

The trick: a liquid that boils below freezing

The heat is carried by a refrigerant, a fluid that circulates through a sealed loop of tubing. What makes a refrigerant useful is how easily it switches between liquid and vapor. When a liquid boils, it soaks up a large amount of heat without getting any warmer. When the vapor condenses back into a liquid, it gives that heat away again.

The temperature at which this happens depends on pressure. At low pressure, the refrigerant boils at a temperature colder than the inside of your freezer. At high pressure, the same fluid condenses at a temperature warmer than your kitchen. So by squeezing the refrigerant in one part of the loop and letting it expand in another, the fridge makes it boil, and absorb heat, inside the cabinet, then condense, and release heat, outside it.

Engineering notes from the University of Florida describe this vapor-compression cycle with four basic parts: a compressor, a condenser, an expansion valve and an evaporator. Heat is absorbed at a low temperature and rejected at a higher one, again and again.

One loop, four stops

Follow a single bit of refrigerant around the loop:

1. Compressor. The vapor is squeezed. Its pressure and temperature both rise until it is hotter than the room.
2. Condenser. In the coils outside the insulated box, the hot vapor gives its heat to the kitchen air and condenses into a liquid.
3. Expansion device. The liquid is forced through a narrow restriction. Its pressure drops sharply and it becomes very cold.
4. Evaporator. Inside the cabinet, the cold refrigerant absorbs heat from the air around your food, boils back into a vapor and returns to the compressor.

Nothing gets used up. The same refrigerant goes around for the life of the fridge, picking heat up on the inside and dropping it off on the outside.

Why the back of your fridge feels warm

All the heat pulled out of your food has to go somewhere, and the condenser is where it leaves. On older fridges you can see it as a black grid of tubing on the back. Many newer models hide it underneath or behind panels, where condenser and evaporator fans move air across the coils.

The condenser doesn’t only release the heat taken from the food. It also releases the energy the compressor put in to do the job. So the heat that leaves a fridge is always more than the heat it removed from inside. That is why a fridge with its door left open will never cool a kitchen. The room ends up slightly warmer instead.

It also explains two pieces of advice from ENERGY STAR: keep the fridge away from heat sources such as an oven, and keep the condenser coils clean if you have an older model. A coil coated in dust, or sitting next to a hot appliance, struggles to get rid of heat, so the compressor has to run longer.

What the thermostat really controls

The thermostat, or the temperature sensor and control board in a newer fridge, does not set how cold the refrigerant gets. It decides when the compressor runs.

In a conventional fridge, the compressor has one speed. When the inside warms past the set point, the control switches it on at full power. Once the cabinet is cold enough, it switches off. Whirlpool notes that a single click can occur when the compressor starts or stops. Turning the dial colder doesn’t make the compressor work any harder. It only makes it run for longer before it rests.

Many newer fridges use a variable-speed, or “inverter”, compressor instead. Rather than cycling between full power and off, it changes its motor speed to match how much cooling is needed. Compressor maker Secop says its variable-speed compressors typically run at minimum speed during normal operation, which is where they are most efficient. It reports energy savings of up to 40 percent, depending on the system design, and average noise up to 5 dB(A) lower. Those are a manufacturer’s best-case figures, but the principle is simple: a motor that runs gently most of the time wastes less than one that keeps starting and stopping at full speed.

Where the frost goes in a frost-free fridge

The evaporator is the coldest surface in the fridge, and moist air keeps reaching it, from food and from the warm room air that rushes in whenever you open the door. That moisture freezes onto the evaporator as frost. Left alone, frost acts like a blanket on the coil and stops it absorbing heat, which is why older fridges had to be defrosted by hand.

A frost-free fridge does the defrosting for you. Every so often, a timer or control board pauses cooling and switches on a small heater next to the evaporator. The frost melts, the water runs down a drain into a pan beneath the refrigerator, and normal cooling starts again.

This routine is behind several sounds that worry new owners. GE Appliances lists a hissing or sizzling noise when water drips onto the defrost heater, a drip as defrost water falls into the drain pan, and a clicking or tick-tock noise from the defrost timer switching on and off. A boiling or gurgling sound is caused by refrigerant circulating through the tubes.

What’s inside the tubes

The refrigerant in your fridge depends on when and where it was made. For years, many household fridges used HFC-134a. In the United States, the EPA has listed HFC-134a as unacceptable in new household refrigerators and freezers since January 1, 2021. Two hydrocarbons are among the accepted substitutes: isobutane, known as R-600a, and propane, known as R-290. Both are listed as acceptable subject to use conditions.

Isobutane and propane are flammable. Those refrigerants come with EPA use conditions, and that is one more reason the sealed system is a job for a certified technician, not a weekend repair. The rating label on a fridge usually names the refrigerant it contains.

The right temperature, and how to check it

The FDA advises keeping the refrigerator at or below 40°F (4°C) and the freezer at 0°F (−18°C). ENERGY STAR suggests a range of 35 to 38°F (about 2 to 3°C) for the fridge.

The numbers on many fridge dials are settings, not degrees, so they can’t tell you the real temperature. The FDA says appliance thermometers are the best way of knowing the real temperature, and they are generally inexpensive. Put one in the middle of a shelf, leave it overnight, then adjust the control one step at a time.

Small habits that help the machine

• Check the door seals. ENERGY STAR advises making sure they are airtight and replacing them if they are not. A leaky seal lets warm, moist air in all day.
• Give the condenser space. Keep the fridge away from the oven and other heat sources.
• Clean the coils on older models. Follow the user manual for cleaning them safely; unplugging the fridge first is standard practice.
• Close the door quickly. Every opening lets in warm air that has to be cooled, and moisture that ends up as frost.
• When you replace a fridge, compare efficiency. ENERGY STAR certified refrigerators are about 9 percent more energy efficient than models that meet the US federal minimum standard.