Symptoms
- Air comes out of vents but feels cool or room temperature
- Furnace fan runs but you never hear ignition
- Problem may be constant or intermittent
Likely causes
- CommonThermostat fan set to 'On' instead of 'Auto''On' runs the blower continuously, including between heating cycles, which feels cold.
- CommonIgnition failure (pilot or igniter not lighting burners)The blower can still run even if the burners never light.
- CommonDirty flame sensor shutting off burners quicklyA dirty sensor makes the furnace think there's no flame and it shuts the gas off.
- PossibleGas supply issue (closed valve, empty propane tank)No fuel means no flame even if everything else works.
- Less commonCracked heat exchanger tripping a safety limitA safety feature can stop burners while still allowing the fan to run.
What to do
- Set the thermostat fan setting to 'Auto', not 'On', and see if warm air returns.
- Watch or listen through a full cycle: you should hear clicking, then a whoosh of ignition within about a minute of the blower starting.
- If you never hear ignition, check that the gas valve at the furnace and any propane tank valve are fully open.
- Check the pilot light (older units) is lit; relight following the label instructions if it's out.
- Replace the air filter if it's dirty, since restricted airflow can trigger safety shutdowns.
- If the furnace tries to ignite but shuts off after a few seconds, the flame sensor is likely dirty and needs cleaning.
- If ignition never happens after these checks, stop troubleshooting gas components yourself and call a technician.
Tools and parts
- Flashlight
When to call a pro
Call an HVAC technician if the burners won't ignite after checking fan settings, gas supply, and pilot light, or if you smell gas at any point.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.