Honeywell Flame sensor MZ770 37-90-10118

This Honeywell MZ770 flame sensor (SKU 37-90-10118, catalogue article 852-211-016) is a direct-acting flame detector. It is mounted with a direct line of sight to the burner flame and confirms to the burner control that combustion has started and is stable. During the ignition sequence, the control opens the fuel valve and energises the ignition spark; the sensor must detect the flame within the set safety time, otherwise the control locks out and stops fuel flow.

Parts of this type are selected by matching the detection principle (ionisation, UV, or IR) to the burner control and the fuel's flame characteristics. Cable length, connector type, and mounting arrangement (thread size, insertion depth) must correspond to the original part to ensure the sensor is positioned correctly. The sensor must be mounted with a direct line of sight to the burner flame.

Common failure modes include a contaminated optical window, a weakened signal from an aged cell, or a broken sheath or cable causing an open circuit. If the sensor fails, the burner control receives no flame signal within the safety time, locks out, and stops fuel flow. Check the optical window for soot or damage and verify the cable and connector for continuity.