Furnace Not Heating? A Homeowner's Troubleshooting Guide
Before you call, check the easy things: thermostat mode and batteries, the furnace switch and breaker, a clogged filter, and that the gas is on. If you smell gas, hear repeated clicking without ignition, or see a flashing error light, stop and call a professional.
Safe checks you can do in five minutes
- Thermostat: set to HEAT, temperature above room temp, fresh batteries
- Power: the furnace switch (looks like a light switch) is on and the breaker isn't tripped
- Filter: a clogged filter chokes airflow and can trip safety limits — replace if dirty
- Vents/registers: open and unblocked in the main rooms
- Gas: other gas appliances are working (confirms gas service)
Stop and call a professional if…
Some symptoms point to safety issues or component failures that should not be DIY-diagnosed.
- You smell gas or a persistent burning odor — leave and call from outside
- Repeated clicking but the burners never light
- Short-cycling (turns on and off every minute or two)
- A blinking status light on the furnace (it's a diagnostic code)
- Any sign of soot, water around the unit, or a tripped CO alarm
A carbon-monoxide alarm sounding is an emergency — get everyone outside and call 911, then a heating professional.
Why filters cause so many 'no-heat' calls
A dirty filter is the single most common cause of avoidable furnace problems. Restricted airflow overheats the heat exchanger, which trips a safety limit switch and shuts the burners down. Replacing a filter on a sensible schedule prevents a surprising number of no-heat nights.
Frequently Asked Questions
Why does my furnace blow cold air?
Common causes include the thermostat fan set to ON instead of AUTO (so the blower runs between heating cycles), a dirty filter tripping a safety limit, or an ignition/flame-sensor problem. If basic checks don't fix it, have it inspected.
How often should I change my furnace filter?
For a standard 1-inch filter, every 1–3 months is typical — more often with pets or during heavy-use months. Thicker media filters can last longer. A clean filter protects the system and your air quality.
Is a flashing light on my furnace bad?
It's a diagnostic code, not necessarily a disaster, but it means the furnace detected a fault. Note the blink pattern and call a professional — it speeds up the diagnosis.
Related Resources & Next Steps
This is general educational guidance, not a diagnosis. For a specific answer about your system, call Mountain West at (775) 737-1917.
Have a question about your system?
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