The Flame Sensor & Flame Rectification Explained
A boiler proves its flame is really lit using flame rectification through a sensor rod. Here is how it works and why a dirty sensor causes flame-loss lockouts.
How does a boiler know its burner is actually lit, and not just dumping unburned gas? It uses a clever trick called flame rectification, sensed through a thin metal rod in the flame. When that sensor gets dirty, you get one of the most common nuisance faults: a burner that lights and then drops out.
How flame rectification works
A flame conducts a tiny electrical current and, because of its shape between the sensor and burner, lets that current flow more easily in one direction — it "rectifies" it. The control board sends a small AC voltage to the sensor and looks for that one-way (DC) signal. A real flame produces it; a faulty igniter sparking into thin air does not.
Why it matters for safety
This is how the boiler confirms a flame within a second or two of opening the gas valve. If it does not sense flame, it shuts the gas immediately and locks out. It is the difference between a safe appliance and one that could fill a space with gas.
The classic dirty-sensor fault
- Burner lights, runs a few seconds, then shuts down — repeating.
- A flame-loss or flame-failure code that clears briefly after a reset.
- More common on older units and after a long idle period collecting oxide film.
Key takeaways
- The flame sensor proves the burner is truly lit using flame rectification.
- A dirty or oxidised sensor is a leading cause of light-then-drop-out lockouts.
- Cleaning is part of a proper annual service — but the surrounding gas circuit makes diagnosis pro work.
Frequently asked questions
Why does my boiler light then shut off after a few seconds?
That timing is the signature of a flame-sensing problem: the boiler lights, fails to confirm a steady flame, and shuts the gas as a safety response. A dirty flame sensor or a weak ignition/ground path is the usual cause, confirmed during diagnosis.
Can I clean the flame sensor myself?
Because the sensor sits inside the sealed combustion chamber of a gas appliance, accessing it means opening the gas circuit. In BC that is licensed work, and cleaning is best done as part of a full combustion check during an annual service.
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