A central air conditioning system consists of two main components: an indoor air handler and an outdoor compressor. The air handler contains an evaporator coil and a fan to force air over the pipes, while the outdoor unit contains the compressor, a condenser coil and an exhaust fan.
The two halves of a central air system function as a one-way heat pump. Coolant flows through pipes to the evaporator coil, where it absorbs heat from the air passing through the air handler and becomes a gas. The low-pressure coolant gas flows to the compressor, where it is pressurized and its temperature increases. The pressurized gas reaches the condenser coil, where it releases its heat into the outside atmosphere and cools rapidly. The chilled coolant then flows back inside to absorb more heat.
In addition to cooling the air, a central air system also serves as a whole-house dehumidifier. As air cools, the amount of water vapor it can hold decreases. This excess moisture condenses onto the chilled surface of the evaporator coil, where it flows into a collector pan and then out through a drain.
A central air conditioner can only transmit heat in one direction. A heat pump, on the other hand, is reversible. In the winter, the system absorbs heat from the outdoor air or ground and releases that heat indoors.