Flame Sensing Electrodes and Why the Boiler Won't Stay Lit
The flame sensing electrode confirms the burner has actually lit and keeps confirming it while the boiler runs.
The flame sensing electrode confirms the burner has actually lit and keeps confirming it while the boiler runs. A boiler that ignites briefly then cuts out, over and over, is a classic sign this electrode is dirty or failing rather than anything wrong with the gas supply.
What it does
It measures a tiny electrical current that only flows through an actual flame, proving to the control board that ignition succeeded and combustion is continuing safely.
How it fails
A film of carbon or oxide builds up on the electrode tip and weakens the signal until the control board decides the flame isn't there and shuts the gas off.
Repair or replace
Cleaning the electrode with fine abrasive paper resolves many cases. If the ceramic insulator is cracked or the electrode is badly corroded, it's replaced instead.
Fix a boiler that keeps cutting out
Tell us how many seconds it runs before cutting out and whether it re-lights on its own, and we'll bring the likely parts to the visit.
Questions we get asked
Is flame sensing something only a registered engineer should touch?
Yes, it's part of the gas appliance's safety sequence and cleaning or replacing it is Gas Safe registered work.
Can I clean the electrode myself?
This involves accessing the burner assembly on a live gas appliance, so it's not something we'd advise doing yourself.
Is this the same as the ignition electrode?
No, though they often sit close together — the ignition electrode creates the spark to light the gas, the flame sensing electrode then proves it's still burning.