Air Conditioning It works by removing heat and humidity from inside the building and releasing it outside, using a system of coils filled with a refrigerant. Modern HVAC systems are designed for efficiency and can include air filters, humidifiers and dehumidifiers to further refine the indoor environment.

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.

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.