Ignition: The Burners Never Lit
Modern furnaces use a hot surface ignitor or spark ignition. A cracked or worn ignitor glows weakly or not at all, so the gas valve never gets the signal to stay open. You may hear repeated clicking or see the furnace try and fail in cycles. Ignitors are wear parts, and repeated failed starts are the furnace protecting your home, not a quirk to live with. Leave gas and ignition work to a technician: the parts are inexpensive to test and risky to guess at.
Flame Sensor: It Lit, Then Quit
If the burners light with a whoosh and then cut off after a few seconds, the flame sensor is the usual suspect. This small rod confirms flame is present. When its surface coats with oxidation, it cannot prove flame and the control shuts the gas off as designed. Cleaning or replacing the sensor is a common, honest fix. A furnace that short-cycles like this will leave the house cold while the blower keeps pushing unheated air.
The Filter and Blower Side
Not every cold-air complaint is a burner problem. Check these before you assume the worst:
- Fan set to On: With the fan on On, the blower runs constantly and pushes room temperature air between heating cycles. Set it to Auto for heat.
- Clogged filter: A choked filter can overheat the furnace, trip a limit switch, and shut the burners down while the blower keeps running to cool things off.
- Thermostat placement and batteries: A weak battery or a thermostat reading a warm spot can end the call for heat early.
- Duct leaks in the crawlspace: Many La Vergne ranch homes have crawlspace ductwork. A disconnected or torn duct mixes cold crawlspace air into the supply, so vents feel lukewarm even with a healthy furnace.
What the Pattern Tells Us
| Pattern | Likely cause | Next step |
|---|
| Clicks, no ignition, repeats | Ignitor or gas supply issue | Technician tests ignitor and valve |
| Burners light, quit in seconds | Dirty flame sensor | Sensor cleaning or replacement |
| Blower runs, burners stay off after heating a while | Limit switch trip from poor airflow | Filter and airflow check first |
| Air is lukewarm everywhere, furnace sounds normal | Duct leak or heat loss | Duct inspection in crawlspace or attic |
Homes in Brookside and Woodland Hills with older furnaces often need the ignitor and sensor checked together, since both age at the same pace. If you smell gas at any point, leave the house and call your gas utility or emergency services first, then call us. For no-heat calls on cold nights, our emergency HVAC repair line stays open around the clock.
