Heating · 5 min read · July 21, 2026

When the furnace will not light

Short heating seasons mean furnace faults surface on the first cold night, on equipment that has sat idle since spring.

Coastal Georgia does not ask much of a furnace, and that is exactly the problem. Equipment that runs a handful of weeks a year sits idle for months, and the first genuinely cold night is when everyone finds out what did not survive the summer.

On modern furnaces the usual suspects are the igniter and the flame sensor. A hot surface igniter is a consumable — it degrades with every cycle and eventually cracks. A flame sensor does not fail so much as foul: a thin oxide film builds up, the furnace lights, fails to confirm flame, and shuts down within seconds. That specific pattern, lighting then dropping out almost immediately, is close to diagnostic on its own.

A furnace that runs but cuts out after several minutes points somewhere different, usually a limit switch responding to a genuine overheat. The cause is often airflow: a filter left in far too long, a closed-off return, or a failing blower. The switch is doing its job, and replacing it without fixing the airflow just removes the protection.

Repeated short cycling, soot, or a burner flame that lifts and rolls are reasons to stop and have someone look properly rather than keep resetting the unit.

Gas equipment carries a safety dimension that cooling equipment does not. A cracked heat exchanger can put combustion products into the air you breathe, which is why an inspection of it belongs in any heating service call as a matter of course. Working carbon monoxide alarms in the house are not optional, and they are cheap.

What you can check: the filter, the thermostat batteries and mode, the breaker, the furnace door switch, and whether the gas supply is on. Everything past that is a service call.

Placeholder editorial written for this demonstration build.