The average lifespan of a furnace typically ranges from 15 to 20 years. However, this can vary significantly based on several factors, including the type of furnace, its brand, how well it has been installed, and the consistency and quality of maintenance it receives over its lifetime. Regular maintenance, such as annual inspections and prompt repairs, can extend a furnace's lifespan, while neglect and excessive workload can shorten it. High-quality furnaces that are well-maintained and operated under normal conditions tend to last towards the upper end of this range or even longer.