The two main parts of a HVAC system are the heating and cooling components. The heating component is responsible for providing warm air to the interior of the building, while the cooling component is responsible for providing cool air. Both components are typically powered by a furnace, which is a device that uses fuel to generate heat. The furnace is connected to a series of ducts that distribute the heated or cooled air throughout the building. Additionally, the HVAC system may also include a thermostat, which is used to control the temperature of the air in the building.