A home air conditioner, whether a window box or a split-system AC unit, is basically an efficient cooling device that consists of indoor and outdoor portions. An air conditioner appears to cool the air, but in reality, it is only making the area less warm by removing heat from the indoor air and transferring that heat to the outdoor air.

Air conditioners use refrigerant fluids (typically Freon) that readily change from a gas to a liquid and back again. This refrigerant is used to transfer heat throughout the unit.

An air conditioner's interior portion consists of an air filter, a fan, and a cooling coil. Warm, humid air is drawn through the filter and is blown over the cooling coil. The cooling coil then cools and humidifies the air before it is returned to the room.

The cooling coil contains the refrigerant fluid. The refrigerant fluid runs through a set of coils that allow the gas to absorb heat and cool down the air. Heat from the warm air passing over the coil causes the gas to evaporate.

The outdoor portion of the unit consists of a compressor, a fan, and a condenser. The refrigerant arrives at the compressor as a low-temperature, low-pressure gas. The compressor squeezes the fluid, which packs the fluid molecules together. This raises the temperature of the gas. The compressor then returns high-temperature and high-pressure refrigerant gas back to the condenser. While this is happening, the fan blows outdoor air over the condenser, where the high-temperature, high-pressure refrigerant gas is converted back a liquid.

The cooler outdoor air passing over the condenser dissipates the heat from the hotter refrigerant gas. This part, called the evaporator, typically has metal fins to help exchange the thermal energy with the surrounding air.

This closed process causes the refrigerant gas to change to a liquid state and back again. By the time the working fluid leaves the evaporator, it is once again a cool, low pressure gas. It then returns to the compressor to begin its trip all over again.

This transferring of heat continues over and over until the room reaches the temperature desired, and then the thermostat will shuts the air conditioner cycle.