The energy efficiency of your furnace is dependent on its Annual Fuel Utilization Efficiency (AFUE), which basically tells you how much of your fuel is used for heating and how much is lost due to combustion (usually out of your vents).
Thermostat and Gas Valve
As we head into winter, it's a good idea to learn a thing or two about the machine that will provide us with heat and comfort throughout the cold, winter days—most likely, a gas furnace. The more you know how a furnace works, the better you can troubleshoot it if something goes wrong. And since space heating accounts for roughly 45% of energy bills paid by U.S. homeowners, it's no wonder why people want to know more about how they work (U.S. DOE).
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.