It’s not always obvious when a furnace is struggling — until it quits completely. Some early warning signs include weak or inconsistent heat, loud clanging or buzzing noises, and unusual smells when the system first turns on. You might also notice the furnace cycling on and off frequently, energy bills rising without explanation, or that your system is 15 years old or older. Even if it’s still running, an inspection now could save you from a bigger repair — and a much colder night — later.