Flame Detector
The device that continuously proves a burner's flame is present and causes the gas supply to be shut off as soon as that proof is lost.
What does a flame detector do?
A burner never assumes its flame is lit — it establishes the fact by measurement. The flame detector is the device that supplies that proof to the flame safeguard control, the unit that sequences the firing cycle. While the signal stays valid, the valves in the gas train are allowed to remain open; as soon as it disappears, they close.
The stake is easy to state: gas entering a firebox without burning accumulates there. Flame supervision is the function that makes that scenario impossible in normal operation, and it acts within a short interval set by the appliance manufacturer.
Flame rod, UV scanner, infrared
The flame rod is an electrode placed directly in the flame. The ionized gases it contains pass a very small current, on the order of microamps, and the presence of that current is the proof. The method is inexpensive and dependable, but it needs solid contact with the flame and a sound ground — which is where most of its faults originate.
A UV scanner detects the ultraviolet radiation of combustion at a distance. It suits fireboxes that are larger, hotter or harder to reach, typical of high-capacity burners. Infrared detectors answer the same need on other configurations. In every case the detector is selected and positioned with the appliance; it is not something to improvise at replacement time.
Lockout and root cause
When flame proof is missing at ignition, or lost during firing, the flame safeguard closes the valves and puts the appliance into lockout. Restarting then requires a deliberate reset, so that a fault gets noticed rather than erased. A recurring lockout is not background noise: it points to a fouled or displaced detector, a degraded ground, weak ignition, or an upstream condition picked up elsewhere by a gas or air pressure switch.
What maintenance verifies
The usual checkpoints are the cleanliness of the electrode or scanner window, the condition of the porcelain and wiring, the position, the quality of the ground, and the signal value read against the manufacturer’s published range. A signal still strong enough to light but clearly weakened is a failure in progress, and measurement reveals it before the shutdown does. To that is added the shutdown test itself: interrupt the flame deliberately and confirm the gas burner locks out as designed.