When the refrigerator operates in its cooling cycle, the evaporator coils extract heat from the interior, causing moisture in the air to condense and freeze on the coils. Over time, this forms a layer of frost. To prevent excessive buildup, the defrost timer or control board activates the defrost cycle at regular intervals, typically every 6 to 12 hours, depending on the model.

A valve diverts the hot, high-pressure refrigerant gas from the compressor directly into the evaporator coils. As the hot gas flows through the coils, it transfers its heat to the frost, causing it to melt. The resulting water is drained away, and once the defrost cycle is complete, the valve redirects the refrigerant back to its usual cooling circuit.