A fan blows indoor air across the cold evaporator coil where the heat inside the home is absorbed into the refrigerant. The cooled air is then circulated throughout the home while the heated evaporated gas is sent back outside to the compressor.

Heating or cooling through conduction typically takes place at the building envelope (the outside walls, windows and doors) where warm or cold air outside causes the molecules of the envelope to increase vibration or decrease vibration which in turn causes a heat loss or gain inside of the building.

Air conditioning, often the most recognized part of HVAC systems, cools indoor air during warmer periods. It works by removing heat and humidity from inside the building and releasing it outside, using a system of coils filled with a refrigerant.

A central heating system has a furnace that converts fuel or electricity to heat. The heat is circulated through the building either by fans forcing heated air through ducts, circulation of low-pressure steam to radiators in each heated room, or pumps that circulate hot water through room radiators.