If your furnace won't heat, confirm the thermostat is set to Heat, the furnace service switch and breaker are on, the filter is clean, and the front panel is seated. Common repairs include a dirty flame sensor, failed igniter, or clogged condensate drain. If you smell gas or a CO alarm sounds, get outside and call 911.
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Atlanta's winters are mild compared with much of the country, but cold snaps do arrive, and a furnace that sits idle for most of the year can surprise you on the first chilly night of fall. Many Metro Atlanta homes pair a natural gas furnace with a central air conditioner, while others use a heat pump or a dual-fuel combination of both.
Quick checks before you call
- Thermostat: set to Heat, setpoint above room temperature, fresh batteries.
- Power: the furnace has a light-switch style service switch nearby, often mistaken for a regular light switch. Make sure it's on, and check the breaker.
- Filter: a clogged filter can cause the furnace to overheat and shut itself off on a safety limit.
- Gas supply: confirm other gas appliances work and the gas valve to the furnace is open.
- Furnace door: many furnaces won't run if the front panel isn't fully seated on its safety switch.
Common furnace problems
- Dirty flame sensor. The burners light, then shut off after a few seconds. Cleaning or replacing the sensor is a common, quick repair.
- Failed igniter. Modern furnaces use a hot surface igniter that eventually cracks or burns out.
- Clogged condensate drain. High-efficiency furnaces produce water and will shut down if the drain or trap is blocked.
- Blower motor or control board issues. Symptoms include no airflow, odd cycling, or blinking error codes. The code chart on the panel door can help a technician diagnose faster.
- Short cycling. Frequent on-off cycles can come from an overheating furnace, an oversized unit, or a poorly placed thermostat.
Safety comes first with gas heat
A gas furnace burns fuel, and its heat exchanger keeps combustion gases separate from the air you breathe. A cracked heat exchanger or a blocked vent can allow carbon monoxide into the home. Take these steps seriously:
- Install carbon monoxide alarms on every level and near sleeping areas, and test them regularly.
- If a CO alarm sounds, or people feel headachy, dizzy, or nauseated when the heat is on, get everyone outside and call 911.
- If you smell rotten eggs (the odor added to natural gas), leave immediately and call 911 or Atlanta Gas Light's emergency line from outside. Don't flip switches or use your phone inside.
- Keep the area around the furnace and water heater clear of stored items, paint, and chemicals.
Repair or replace?
Gas furnaces often last longer than air conditioners, but age, repeated repairs, a cracked heat exchanger, or rising bills can tip the balance toward replacement. If your AC is also near the end of its life, replacing both together can make sense because the indoor components should be matched. If you're getting quotes, ask whether a heat pump or dual-fuel system fits your home. Request a heating contractor to get a licensed technician out to diagnose the problem.
Frequently asked questions
Why does my furnace turn on and then shut off?
A dirty flame sensor is a frequent cause: the burners light, the sensor fails to confirm the flame, and the furnace shuts down. Overheating from a clogged filter or a blocked condensate drain on high-efficiency furnaces can also cause short cycling.
What should I do if I smell gas?
Leave the house right away without flipping switches or using your phone inside. From outside, call 911 or Atlanta Gas Light's emergency line. Don't go back in until you're told it's safe.
How long does a gas furnace last?
Gas furnaces often last longer than air conditioners, frequently 15 to 20 years or more with maintenance. Age alone isn't a reason to replace, but a cracked heat exchanger or repeated major repairs usually is.