A good rule of thumb is to replace a Lennox furnace when the cost of repair approaches 50 percent of the value of the heating system. Lennox suggests planning to replace the furnace if it is 12 years or older.
The average furnace lasts 10 years or longer, and some may last up to 20 years. This average varies based on the quality of the furnace, how much it is used and how well it has been maintained. One problem in trying to determine when to replace your furnace is that most furnaces do not show preliminary signs that something is wrong or going bad and typically just stop working.