The average life expectancy of a furnace can vary depending on the type, quality, and maintenance of the unit. However, most furnaces have a lifespan of 15 to 20 years, with some high-quality models lasting up to 25 years or more. Regular maintenance, such as changing filters, cleaning ducts, and inspecting the furnace components, can help extend the life of a furnace. It's essential to consult the manufacturer's guidelines and consult with a professional HVAC technician to ensure proper maintenance and care.