Honeywell Photo resistor flame sensor MZ770S 50003

This Honeywell photo-resistor flame sensor, model MZ770S (SKU 50003), is designed for flame monitoring in oil-fired burners. It detects visible or infrared light using a photo-resistor sensing element and outputs a resistance change. The sensor is 243 mm long and comes with two extensions. It is compatible with Satronic burner controls, including Honeywell Satronic versions.

Parts of this type are selected by matching the spectral sensitivity (visible, infrared, or combined) to the fuel and burner type, and by ensuring the response time is compatible with the burner control’s safety time. Cable length and connector type determine ease of replacement; some sensors have a fixed flying lead, others a plug. The mounting sleeve or holder must fit the existing bracket, and the sensing window should be positioned so soot or debris cannot easily obscure it. Before ordering, measure the sensor length and check the focal distance to the flame.

Typical failure modes include a dirty or cracked window that reduces light transmission, an aged photo cell producing a weakened signal that causes nuisance lockouts, and a broken sheath allowing moisture ingress. When such symptoms occur, check the lens for contamination and verify the replacement sensor is positioned at the same focal distance from the flame as the original.