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20
221481C
11 Fault Finding
THERMOCOUPLE AND BOILER OVERHEAT CUTOFF FAULT FINDING
Diagram 11.2
With the boiler cold, check connections of the thermocouple, boiler overheat cut-off and gas valve.
Disconnect overheat cut-off connectors at points A and B at the gas valve, see diagram.
Test continuity of the overheat cut-off. Is there continuity ?
Re-connect boiler overheat cut-off to points A and
B. Disconnect thermocouple at point D of gas
valve. Test continuity between point C and the
body of the body of the gas valve. Is there
continuity ?
YES
Faulty boiler overheat
cut-off, renew
Faulty cut-off
connectors into gas
valve, either at A or C.
Renew where faulty
Faulty thermocouple,
renew
NO
YES
Thermocouple and boiler overheat circuit Satisfactory.
YES
Test continuity of thermocouple between inner
connection point D and point E. is there
continuity ?
NO
NO
THERMOCOUPLE
Diagram 11.1
CONNECTION 'E'
CONNECTION 'D'
CONNECTION 'B'
GAS VALVE
BOILER
OVERHEAT
CUT-OFF
CONNECTION 'A'
2487
CONNECTION 'C'
11.7 Thermocouple and Overheat Device.
To test the thermocouple a meter with a range of 0 to 30mV is
required.
Symptoms: The pilot burner fails to stay alight.
Test the thermocouple, overheat cutoff and thermocouple
connectors as described in fault finding chart 11.2.
Check the millivoltage of the thermocouple closed circuit at
points “A” and “E”, see diagram 11.1.
This should be within the range 6 to 13mV.
Take the millivoltage drop reading and refer to thermocouple/
overheat cutoff fault finding diagram 11.2.