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WD420/3/2007
6.1
ELECTRICAL CONTROL SEQUENCE
When the external control(s) are calling for heat, power will be fed to the boiler connection
strip at terminal SL1 (Switched Live) and N (Neutral) and the radiator symbol with a "1" will
appear in the top left of the display. If the user control setting of the CH temperature is to a
temperature above the actual boiler flow temperature and provided all safety sensors are
confirmed as OK the control PCB will initiate the following sequence.
(1)
The fan and pump will start (indicated by a pump and a fan symbol on the top row
of the display).
(2)
When the fan reaches running speed, the control PCB will start the ignition
sequence.
(3)
After a pre-purge period of about 5 seconds, the gas valve will open to allow gas
to mix with the air at the suction side of the fan and the ignition spark will occur at
the main burner (indicated by a flashing flame symbol on the top row of the
display).
(4)
When the burner ignites, the flame is detected by the control PCB through the
combined flame sensor/ignitor and the ignition spark is stopped. The flame
symbol on the top row the display stops flashing and becomes constantly
illuminated. The boiler is now in its normal run condition.
(5)
The boiler will automatically and continuously adjust its power output and pump
speed to best match the demands of the system at any time.
(6)
The burner will continue to operate until the gas valve interrupts the gas supply.
The gas valve will be closed by the control PCB if power is interrupted to the
boiler by any external control or the boiler exceeds the requested temperature. If
an interruption to the gas supply causes loss of the flame, the control box will
pause for approximately 10 seconds and then attempt to re-ignite the unit. If this
attempt fails, i.e. due to continued lack of gas supply, the boiler will make four
further attempts to ignite at intervals of approximately 10 seconds and will then go
to a lockout state (the display shows "E02"). Once the gas supply has been
resumed the boiler can be reset by pressing the "Reset" button.
(7)
The boiler can also be shut down by the cabinet temperature sensor, water
pressure sensor or flue overheat sensor.
Any failure of the boiler to sequence in the above manner should be investigated using the
following trouble shooting table.
Before attempting any electrical fault finding, always carry out preliminary electrical system
checks. On completion of any service/fault finding task which has required the breaking
and remaking of electrical connections, the checks, earth continuity, polarity, short circuit,
resistance to earth must be repeated.
Installation & Servicing Instructions
Chapter 6 : Fault Finding
6. FAULT FINDING
The Keston Qudos 28s & Qudos 28sP Boilers
Page : 31
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