By the time a furnace is 20 years old, homeowners should start planning for replacement. That's especially true if there have been some malfunctions in recent years and a few calls for repair service. A new furnace will be quieter, more energy-efficient, and will be more unlikely to break down. Old furnaces also can eventually become health hazards. Energy Efficiency Considerations Higher energy efficiency means decreased use of heating fuel and lower utility bills than would otherwise be the case.