There’s no such thing as a furnace that never dies. According to David Kenyon of Sears Home Services, “the typical unit lasts about 15 or 20 years.” Unfortunately, in evaluating any given unit, "there's no way to know its expiration date in advance." All you can do is exercise caution as the furnace enters the final quarter of its life. If a previous owner installed the furnace, be advised that it may not be easy to determine exactly how long it's been there. Check the manual if you have it. Otherwise, "look for a serial number," Kenyon suggests, or present a photo of the unit to the company that manufactured it. Make no mistake: “It would be unwise to replace a fully functional furnace only because it’s old,” Kenyon says. Still, knowing its age may help put any performance problems in context.