How a Gas Furnace Works
A gas furnace heats your home in a simple sequence: the thermostat calls for heat, the ignitor lights the gas burners, the burners heat the heat exchanger, and the blower pushes warm air through your ducts. Every step has a safety control watching it — a flame sensor that confirms the burners lit, a limit switch that stops the furnace if it overheats, and a venting system that carries combustion gases outside. When any one of those steps fails, the furnace shuts itself down rather than run unsafely. That is why a gas furnace that will not run is usually protecting you — and why the fix is a diagnosis, not a guess.
Common Gas Furnace Problems
- Ignition failure — a worn-out hot-surface ignitor or a failed pilot assembly means the burners never light
- Dirty flame sensor — the furnace lights, then shuts off within seconds because it cannot sense the flame
- Gas valve problems — a valve that will not open, or opens only partway, starves the burners of fuel
- Burner problems — dirty or corroded burners that light unevenly or produce a weak, yellow flame
- Heat exchanger cracks — the most serious gas furnace failure, and the one a safety inspection is designed to catch
- Venting problems — a blocked or damaged flue that cannot carry combustion gases outside
- Control board failure — the furnace's brain stops sending the right signals at the right time
Carbon Monoxide: What Every Montana Homeowner Should Know
Carbon monoxide is the serious risk that comes with any gas-burning appliance. It is colorless and odorless, and it is produced when gas does not burn completely — most often because of a cracked heat exchanger or a blocked vent. Every Montana home with a gas furnace should have a working carbon monoxide alarm on every level, and the alarm should never be ignored. Symptoms of carbon monoxide exposure — headache, dizziness, nausea, confusion — are a reason to leave the house and call 911 immediately. A furnace safety inspection, including a heat exchanger check and a carbon monoxide test, is the best way to make sure your furnace is burning gas safely.
Never Attempt Gas Repairs Yourself
Gas furnace repair is not a do-it-yourself job. Gas lines, gas valves, burners, and heat exchangers require training, the right tools, and an understanding of combustion safety. A mistake can cause a gas leak, a fire, or carbon monoxide exposure. If your gas furnace is not working, call a trained repair professional. Homeowners can safely check the thermostat, the filter, and the breaker — everything past that belongs to a professional.
Related Services
See also: Furnace Safety Inspection, No-Heat Furnace Repair, and Emergency Furnace Repair.
Frequently Asked Questions
My gas furnace lights, then shuts off a few seconds later. What is wrong?
That is almost always a dirty flame sensor. The sensor is a small rod that sits in the burner flame and confirms the furnace is lit. When it gets coated with carbon, it cannot sense the flame, and the furnace shuts off for safety. Cleaning or replacing the sensor is a quick repair.
What does a yellow burner flame mean?
A healthy gas burner flame is mostly blue with small yellow tips. A mostly yellow, lazy, or flickering flame can mean dirty burners or a combustion problem. It is worth having inspected — and it is one more reason to keep a carbon monoxide alarm working in your home.
How do I know if my heat exchanger is cracked?
You usually cannot tell by looking — a cracked heat exchanger is found during a safety inspection, with a visual check and a combustion test. Warning signs include a carbon monoxide alarm going off, soot around the furnace, or a strange smell when the furnace runs. If any of those appear, turn the furnace off and call immediately.
Is it safe to run my gas furnace if it smells odd?
No. If you smell gas, leave the house and call your gas utility or 911 from outside. If you smell a burning or electrical odor from the furnace, turn it off at the thermostat and call a repair professional. Do not keep running a furnace that smells wrong.
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