The parts that make up an HVAC, or heating, ventilation and air conditioning, unit are a compressor and condenser coil, a fan, and refrigerant lines. The system also includes a plenum, an evaporator coil, a condensate drain and a blower.
An HVAC unit also includes a furnace, supply and return ducts, and supply and return registers. The system might also have power humidifers to keep the humidity of the air constant and comfortable. A thermostat controls the temperature.
The compressor and condenser are found outside the building and help to remove heat from the air. They pressurize refrigerant gas, which loses heat and liquefies. The coil transfers the lost heat to the outside air by means of a fan. The cooled refrigerant then flows through refrigerant lines to the evaporator coil. There, the refrigerant absorbs heat from the air from the furnace blower. The cool dry air then moves into the plenum, a large duct that emerges from the furnace and divides into other ducts throughout the building. In the meantime, the refrigerant, which is once again a gas, returns to the condenser.
Forced air heating units can use the same ductwork as air conditioning units. These furnaces heat the air and force it through the ducts via a blower. The air is then pulled back through return ducts.