A gas furnace consists of two parts, an outdoor air conditioning unit and indoor furnace that work together to draw in, heat and circulate winter air and cool, then circulate, warm summer air. These two units contain components including a burn, flue and heat exchanger that contribute to making the process complete.
The basic furnace unit contains a burner, heat exchanger, ductwork and flue or vent pipe. In the winter, the burner starts the air heating process by delivering gas to the heat exchanger, which transfers heat produced from the gas to the air distribution system. This system adjusts the air according to the thermometer setting, then sends it out through ducts for circulation. The last step in emitting air involves a bit of cleansing; flues and vent pipes complete the process by exhausting harmful byproducts, such as carbon monoxide and other noxious fumes outdoors. In the winter, the outdoor unit pulls in warm air through vents, which is circulated by the furnace fan to cool the household interior.