When the thermostat sends its signal to the furnace, the furnace gas valve opens and ignites the burner component beneath the combustion chamber. The gas valve works with the thermostat to regulate the amount of gas that flows into your furnace.
This system is also known as a ducted warm-air or forced warm-air distribution system.
The main parts of your furnace are the control system (thermostat and electrical controls), gas valve, burners, heat exchanger, blower, and duct and ventilation system. When your furnace creates heat, combustion gases are vented out of your home via a flue pipe.
An efficient and well-maintained heat exchanger is essential for a working furnace. During the process of heating the heat exchanger, combustion gases should be venting safely out of the house.