Definition
A flame sensor is a thin metal rod positioned in a gas furnace's burner flame that confirms ignition by passing a tiny electrical current through the flame, a process called flame rectification. If the control board does not detect that signal within seconds, it closes the gas valve as a safety measure. A dirty sensor causes nuisance shutdowns.
Also called: Flame rod, Flame sensing rod
Why it matters to you.
A sooty or oxidized flame sensor is one of the most common reasons a furnace lights, runs for a few seconds and then shuts off, and in Central Kentucky it often shows up on the first cold nights of fall. Repeated lockouts during a January cold snap can leave a house without heat while pipes are at risk of freezing.
Practical guidance.
If the furnace repeatedly lights and drops out, note any blinking code on the control board's indicator light and call for service rather than resetting it over and over. Cleaning or replacing the flame sensor means working in the burner compartment and is technician work, typically part of a fall furnace tune-up.
About this definition
Written by the Feistritzer Heating & Air Conditioning team — family owned and operated in Danville since 1976, Kentucky HVAC license M02584. Definitions inform; a whole-system diagnosis in your home decides.