Replace a furnace when the current unit is more than 15 years old and requires frequent fixing, or when repairs cost more than half the replacement cost.
Signs a furnace is going out:
Energy bills are increasing.
Rooms heat unevenly.
Fuel lines rust or system chamber cracks.
The thermostat malfunctions.
Furnace cycles on and off frequently or makes loud noises.
Dust or soot buildup appears.
Interior air smells like a gas leak.
The gas-furnace flame is yellow instead of blue.