Furnaces have an average life expectancy of 16 to 20 years, and EnergyStar.gov recommends replacing furnaces that are more than 15 years old. Other indications for replacement include increased repair frequency, loss of heating efficiency, uneven heating in the house, and unusual noises from the heating system.
Older furnaces can be dangerous as well as inefficient. As a furnace ages, it can develop cracks in its heat exchanger, increasing the risk of releasing carbon monoxide gas into the home. Indications of the presence of carbon monoxide include frequent headaches, flu-like symptoms, burning eyes or nose, nausea and disorientation. Other signs to watch for include soot stains near the furnace, excessive moisture on cold surfaces, and rusting of chimneys, vents and flue pipes.
According to Bob Vila, a furnace can be professionally rated by using the annual fuel utilization efficiency method of evaluation, which ranks a furnace based on its efficiency. If a furnace scores under 80 percent, it is probably more cost efficient to have it replaced.