Boiler Fault Finder

Thermostats – Over heat Cut Out

Over heat cut out thermostat (Set @ approx. 90 – 98 degrees C) Reasons to activate an over heat cut out thermostat Scale / magnetite deposits causing excessive heat build up Loss of system water Loss in manufacturers calibration Faulty pump or pump speed too low Too much air within system water Faulty expansion vessel (loss in air charge or split diaphragm) Faulty gas pressures / solenoid stuck in on position These thermally activated control devices may interrupt the electricity supply to a gas solenoid valve extinguishing the pilot and/or main burner, signalled by the printed circuit board. The overheat cut out thermostat must be manually reset to prevent further boiler cycling whilst a fault may be present (overheat lockout). To test an over heat cut out thermostat (when not at trip point): Electrically isolate boiler / system controls Remove wires from thermostat terminals Set multimeter on ohms scale for resistance readings, across thermostat terminals: Continuity across terminals = ok, No continuity across terminals = faulty To test an over heat cut out thermostat (when at trip point reaching 90-98 degrees C) Electrically isolate boiler /system controls Remove wires from thermostat terminals Set multimeter on ohms scale for resistance readings, across thermostat terminals: No continuity across terminals = ok, Continuity across terminals = faulty…

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