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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