20
221460C
8 Fault Finding
Diagram 8.1
THERMOCOUPLE
Diagram 8.2
GAS VALVE
CONNECTION 'E'
CONNECTION 'D'
BOILER
OVERHEAT
CUT-OFF
CONNECTION 'B'
CONNECTION 'C'
CONNECTION 'A'
2487
2327\A
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
8.7 Thermocouple and Overheat Cutoff Device
To test the thermocouple, a meter with a range of 0 to 30 mV is
required.
Symptom: The pilot burner fails to stay alight. Test the
thermocouple, overheat cutoff and thermocouple connectors,
as described in fault finding chart 8.2, see also diagram 8.1.
Check the millivoltage of the thermocouple closed circuit at
points “A” and “E”, see diagram 8.1. This should be within the
range of 6 to 13mV.
8.8 Electrical
Important: The preliminary electrical systems checks in a
multimeter instruction book are the first checks to be carried out
during a fault finding procedure. On completion of the servicing/
fault finding task which has required the breaking and remaking
of electrical connections then checks, earth continuity, polarity
and resistance to earth must be repeated.
To check the control thermostat and gas valve, see diagram 8.3
and functional flow wiring diagram 8.4.
To check control thermostat pump over run cutoff device circuit
see diagram 8.3 and functional flow wiring diagram 8.5.
8.9 Pilot
Refer to fault finding for pilot, see diagram 8.5.