b5a5d860_for_any_fuel_gas_fired__boiler__Item
[Safety] The flame detector has a mechanical shutter which blocks the detector tubes view of the flame periodically. If the detector tube still sees flame when blocked, the detector has failed and the burner shuts off. [Reason] If a flame detector tube fails it must be replaced. Sometimes these tubes can fail in the "flame OK" position Â– if this happens the burner could go out and the detector would not know. NFPA guidelines require self-checking detectors when the burner operates more than 24 hours without shutdown. [Item] 
"Self-Checking" Flame Detector
Safety controls for any fuel gas fired  boiler
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