Siemens Flame sensor QRB4A-B070B70A

This flame sensor is designed for flame monitoring in oil-fired burners using diesel fuel or heavy oil. It operates on the photoelectric principle, detecting light emission from the flame and sending a signal to confirm combustion has started. The device is built into the burner and has a heat-resistant plastic housing. It is intended for boilers, heat generators, and burner devices, and is compatible with Siemens LOA and LMO burner controls.

Flame sensors of this type are selected by detection principle—UV or infrared—and must match the spectral range of the flame being monitored. Before ordering, check the cable length, connector type, and mounting sleeve dimensions, because the sensor must fit the existing sight port and reach the control box without strain. Always verify the replacement uses the same detection principle; substituting a UV sensor for an infrared type, or the reverse, will not work with the burner control.

Common failure causes include a contaminated or scratched quartz window, an aged photocell that no longer produces a strong enough signal, or a cracked sheath that allows moisture ingress. When the flame sensor fails, the burner control shuts down the fuel supply and goes into lockout within the safety time because it does not receive confirmation that combustion started. Cleaning the window with a soft dry cloth may restore function, but a sensor with a weakened cell should be replaced.