A furnace that will not start is an obvious reason to seek professional help, but smaller changes can also point to a developing fault.
- Watch for cool air from supply registers, rooms that remain cold, weak airflow, frequent on-and-off cycling, delayed ignition, or a thermostat that does not seem to control the temperature.
- A sudden increase in operating noise, such as banging, rattling, squealing, grinding, or humming, deserves attention rather than guesswork.



