How a Refrigerator Cools: The Four-Part Cycle

A refrigerator does not create cold. It moves heat out of the cabinet and releases it into the room. A liquid called a refrigerant circulates in a sealed loop, absorbing heat inside the fridge and giving it up outside. Four parts make this happen: a compressor, a condenser, an expansion device and an evaporator.

Why moving heat takes work

Heat flows by itself from warm things to cooler ones. A fridge has to push it the other way, from the cool inside to the warmer kitchen. A Penn State energy course describes the fridge as a heat mover, and says this reversal needs work, which the compressor supplies using electricity. This is also why the back or underside of a fridge feels warm: the heat from your food, plus the energy the compressor uses, ends up in the room.

The four parts

The refrigerant goes round the loop in this order, according to both Penn State and the repair site iFixit.

• Compressor: pumps the refrigerant round and squeezes the vapour, raising its pressure and temperature above that of the kitchen air. It is the main user of electricity, and its motor makes the humming you hear.
• Condenser: coils on the back or bottom. The hot refrigerant gives up heat to the room, often helped by a fan, and condenses from a gas into a liquid.
• Expansion device: the liquid passes through a narrow opening, either a thin capillary tube or a thermal expansion valve. Pressure and temperature drop sharply, and some of the liquid flash-evaporates into a cold mixture.
• Evaporator: coils inside the fridge or freezer. The cold refrigerant absorbs heat from the air and food and boils into vapour, which returns to the compressor.

iFixit names tetrafluoroethane as a common refrigerant and notes that the loop is sealed copper tubing. Other refrigerants are used in different models, and the choice has changed over the years. The sealed parts need special tools to open and refill, so iFixit recommends a professional for that kind of repair.

Control: when the fridge starts and stops

The compressor does not run all the time. A temperature sensor tells a control board when to switch parts on or off. iFixit describes a sensor called a thermistor that reports the temperature to the board. In many models a fan in the freezer blows air over the evaporator and into the fresh-food section, with a damper that controls how much cold air gets through.

Penn State gives recommended temperatures: 37 to 40 degrees Fahrenheit (about 3 to 4 degrees Celsius) for the fresh-food compartment and 0 to 5 degrees Fahrenheit (about minus 18 to minus 15 degrees Celsius) for the freezer.

Because the compressor only has to run when the inside warms up, a fridge in a cool room with a well-sealed door spends more of its time switched off. The sources do not give run-time figures, and real behaviour depends on the model, the room and how often the door is opened.

Frost, seals and cleaning

Frost builds up on the evaporator because moisture in the air freezes there. iFixit explains that many fridges have a defrost heater around the evaporator coils to melt it from time to time. If the heater fails, frost thickens, blocks heat absorption and the fridge struggles to cool.

The door seal matters for the same reason. If it leaks, warm, damp air enters and the fridge works harder. iFixit also lists dirty condenser coils among the common causes of poor cooling, since the heat cannot escape properly.

How efficiency is rated

Penn State notes that efficiency is stated as the volume cooled per unit of electrical energy per day. The page gives, as an example of the most efficient 2025 model it listed, a refrigerator of 27.4 cubic feet that uses 546 kilowatt-hours a year. Figures like these change as new models appear, and they depend on the test used.

In short

The compressor raises the refrigerant's pressure and temperature, the condenser releases the heat to the room, the expansion device makes the refrigerant cold, and the evaporator absorbs heat from the food. Then the cycle repeats. Air conditioners and heat pumps use the same basic principle. The details here come from the two sources listed below, as of 1 November 2026.

Sources

• Refrigerators, Penn State EGEE 102 course
• How Refrigerators Work, iFixit