Purchasing an older home with an existing furnace can be a worry; a furnace often needs replacing after 15 to 20 years, so knowing its age is helpful in determining if a new furnace should be in the budget. Newer units may have a manufacturer's date listed somewhere on the furnace; older furnaces require a bit more digging by looking at the serial number, which tells the manufacturer various pieces of information, including the age. Finding the serial number, however, may also require a bit of sleuth work.