Call-Now Signs
- Gas odor: leave the building, avoid switches, and call your gas utility from outside. This is not an HVAC-first situation.
- Carbon monoxide detector alarm: get everyone to fresh air, then treat the furnace as off-limits until inspected.
- No heat in freezing weather: a comfort problem that becomes a frozen-pipe problem on a delay. Call promptly and use safe stopgap heating.
- Banging at startup: often delayed ignition, where gas builds before lighting. Turn the furnace off and call.
- Scraping or metal-on-metal sounds: shut the system down; continued running can cascade the damage.
Schedule-Soon Signs
- Starts, runs briefly, shuts off, repeats: the classic fouled flame sensor pattern, or a limit switch tripping from restricted airflow. Often a modest repair; rarely an emergency.
- Cold or lukewarm air from vents while the system runs.
- Frequent cycling with the house never quite reaching temperature.
- New rattles, squeals, or hums. Bearings, belts, and loose panels all announce themselves before they fail completely.
- A yellow or flickering burner flame where you can see it (many sealed furnaces do not show the flame at all). Proper combustion burns blue; anything else deserves an inspection.
- Rising gas bills without colder weather or rate changes.
Probably-Normal Signs
- A dusty smell for the first few minutes of the season's first run, fading quickly.
- Cool air from vents for a moment before the burners warm the exchanger.
- The blower running briefly after the burners shut off, pushing the last heat through.
- Metallic ticking as ducts expand and contract with temperature swings.
What Never to Do
Do not open the furnace cabinet to poke at ignition components, do not bypass safety switches, do not ignore a CO alarm because "it is probably the batteries," and do not keep resetting a furnace that keeps locking out. Lockouts are the system protecting you; overriding them removes the protection. Anything involving gas, combustion, ignition, or electrical components is professional territory.
What Happens Next
When you call, describe the symptom in the terms above: "starts and stops every few minutes" is far more useful than "acting weird." The furnace repair page explains the diagnosis process, and emergency HVAC covers urgent situations. Verify licensing and insurance before authorizing any work.