An HVAC furnace is a heating system that uses a fuel source, such as natural gas, propane, or electricity, to heat air and distribute it throughout a building. The furnace consists of a blower, heat exchanger, and burner, which work together to heat the air. The blower circulates the air through the ductwork, while the heat exchanger absorbs the heat from the burner and transfers it to the air. The burner ignites the fuel source, which heats the air and is then distributed throughout the building.