If your furnace is more than 15 years old it is nearing the end of its life. There are some signs that may indicate it's time for a replacement unit. While many problems can be repaired, on an older unit it will not be cost effective to make repairs at a certain point. If you see a sudden unexpected spike in heating bills, if the unit cycles frequently, if your home is being heated unevenly, or if you hear unusual noises coming from the unit, it's a good idea to have it inspected, and to begin considering a replacement.