What this symptom may mean
Gas furnaces follow a startup sequence: the inducer fan runs, safety switches confirm venting, the igniter heats, the gas valve opens, and a sensor confirms flame. If any step fails, the furnace stops and may lock out.
That design keeps the home safe, but many issues produce the same result: no heat. A few common causes are safe for a homeowner to check.
Possible causes
Thermostat in the wrong mode
It may still be on COOL from summer, or the setpoint may sit below room temperature.
Power to the furnace is off
Gas furnaces still need electricity, and a tripped breaker or off service switch stops everything.
Dirty filter and safety limit
A clogged filter can overheat the heat exchanger and trip a limit switch that shuts the burners down.
Ignition or flame sensor failure
A cracked igniter or dirty flame sensor can make the furnace light, then quit within seconds.
Safe checks you can make
- Switch the thermostat to HEAT and nudge the setpoint above the current reading. Fresh batteries help if the display is faint.
- Check that the furnace breaker and the service switch near the furnace are on. Reset a tripped breaker only once.
- Replace a dirty filter, then wait several minutes for the furnace to try again.
- Make sure the furnace door panel is fully closed, since many furnaces will not run with it loose.
What not to do
- Don't stay inside if you smell gas. Leave the home, then call the gas utility or 911 from outside.
- Don't remove the burner cover, touch the igniter or try to clean the flame sensor yourself.
- Don't keep resetting the furnace after repeated lockouts.
- Don't heat the house with the oven or an unvented space heater.
When professional evaluation may be appropriate
If thermostat, power and filter all check out and the furnace still will not light, or keeps lighting and quitting, the ignition, safety or gas parts need a trained technician.
Call to be connected with an independent provider for furnace repair in Montgomery, AL. Homes heated by a heat pump should see the page on a heat pump not heating instead.
- Any gas or rotten-egg smell (leave first, then call from outside)
- The furnace lights, then shuts off within seconds, again and again
- Soot, scorching or a yellow, flickering flame
- The carbon monoxide alarm sounds
What an HVAC provider may inspect
- Igniter, flame sensor and gas valve
- Pressure switch and vent piping
- Limit switches and heat exchanger condition
- Control board error codes and wiring
Keeping it from happening again
Furnaces here may sit idle for months, letting dust settle on burners and sensors. Test the heat on a cool fall day, replace the filter, and put fresh batteries in carbon monoxide alarms.
