What is a hot surface igniter?
A hot surface igniter is a ceramic element that glows to about 2,500°F when energised, lighting the gas at the burners. It replaced the standing pilot in residential furnaces from the 1980s onward: there is no small flame burning all summer, so the furnace wastes no gas between cycles. It is also the most commonly replaced part on a modern gas furnace, because it is a brittle element that is heated to incandescence several times a day.
Common questions
-
Silicon carbide or silicon nitride — which do I want?
Nitride, if your furnace will take one. Carbide is the older material and it is effectively glass: it cracks if you look at it wrong, and it degrades a little on every heat cycle. Nitride is denser, survives handling, warms up faster and typically lasts two to three times as long. Universal nitride kits replace most carbide igniters — the element differs, the mount and the plug are what have to match.
-
Why do they fail so often?
Three reasons, in order. Thermal cycling: heating a ceramic to 2,500°F and back to ambient several times a day is a fatigue test. Handling: skin oil bakes into a hot spot that cracks on the next cycle, which is why every guide says hold it by the base. And short-cycling: a furnace with a dirty filter that trips its limit and restarts is running the igniter twice as often as it was designed for.
-
Mine glows but the furnace still won't light. Is it bad?
Probably not — a glowing igniter is doing its job, and the problem is downstream. Look at the gas valve, the pressure switch, or the flame sensor next: if the burners light and then die a few seconds later, that is the sensor, not the igniter. An igniter that reads in range on a meter (roughly 40–70 Ω cold for nitride) and glows steadily is fine.