What is the average life expectancy of a furnace?

It depends on how me maintain the furnace, how it was installed and how often do we use it. If you have a properly installed and perfectly maintained furnace it would last for about 15-20 years. But if it is not so, it will decrease by a couple of years. To increase its life make sure that you get annual tune-ups and regularly change your air filter. I had my furnace installed by the High Efficiency Heating and Cooling Inc, and it is working okay.