gas furnaces typically last 20 to 25 years. Furnaces are usually replaced when the heat exchanger fails. Other minor components are typically replaced when they fail.
Some furnaces are known by reputation to fail early on a regular basis. Talk to local installers and service people to get a sense of which furnaces are troublesome in your service area.
Some furnaces last considerably longer than the statistical averages. We have seen conventional forced-air furnaces approaching 40 years of service.
The life expectancy of
furnaces is more difficult to predict, since these systems have not been around as long as conventional furnaces. Early experience suggests that they are higher maintenance, as might be expected since there are many more components. There is also some question as to the durability of the modern, restrictive heat exchanger.
Several different materials have been used for heat exchangers and inevitably some will perform better than others.
Gravity furnaces have not been made for more than 20 years in many areas. Whenever you see one, it is probably safe to say it is close to the end of its life.
Combination systems are relatively new, and since the furnace side is relatively simple, there is no reason to expect a short life expectancy. If the fan or coil in the furnace fails, it can probably be replaced without replacing the entire furnace.
There are several ways of determining the age of a furnace, although the most reliable is the data plate information on the furnace itself. Carrier Corporation has put together a Blue Book that gives an indication of the age of many furnaces and air conditioners based on their model number. This book is available from ASHI