There are many factors that contribute to the lifespan of gas furnaces, including the quality of the furnace, installation quality, how hard it works, and regular maintenance as per the manufacturer’s recommendations. On average, a furnace will last somewhere between 15 and 20 years. Unfortunately, many furnaces simply give out at the end of their lifespan without a great deal of warning. Therefore, if your gas furnace has passed the 15-year mark, it is a good idea to begin researching replacement options.