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21
220459C
Re-connect boiler overheat cut-off to points
A and B. Disconnect thermocouple at point
D of the gas valve. Test continuity between
point C and the body of the gas valve.
Is there continuity?
Test continuity of thermocouple between inner
connection point D and point E.
Is there continuity?
Faulty boiler overheat cut-off,
renew.
Thermocouple and boiler overheat circuit
satisfactory.
NO
YES
YES
Faulty cut-off connectors into gas
valve, either at A or C.
Renew where faulty.
Faulty thermocouple, renew.
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 3.4.
Test continuity of the overheat cut-off. Is there continuity?
YES
NO
NO
11.7 Thermocouple and Over Heat Cutoff
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 diagram 11.2
and diagram 11.1.
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.
11 Fault Finding
THERMOCOUPLE
Diagram 11.1
THERMOCOUPLE and BOILER OVERHEAT CUT-OFF
Diagram 11.2
2487
CONNECTION 'E'
CONNECTION 'D'
CONNECTION 'B'
CONNECTION 'C'
GAS VALVE
BOILER
OVERHEAT
CUT-OFF
CONNECTION 'A'
2327
