Should I Repair Or Replace My Furnace?
How Do I Know When To Repair Or Replace?
If your furnace is old, or has a serious malfunction that will cost several hundred dollars to fix, it may be wise to replace it. As a general rule of thumb, if your furnace is more than ten years old and costs more than $500 to fix, it should probably be replaced in place of repair.
If your furnace is old but not broken, deciding when to replace it can be difficult. Average life expectancy of furnaces in homes today is between 16 and 20 years. If your furnace is close to this age or older, begin shopping.
--Minnesota Department of Commerce Energy Information Center