5. FAULT FINDING
5.1
ELECTRICAL CONTROL SEQUENCE
When the external controls are calling for heat, power will be fed to the boiler connection
strip at terminals SL1 (Switched Live 1) &/or SL2 (Switched Live 2) and N (Neutral). If the
user control setting on the relevant knob is to a temperature less than the boiler flow
temperature the display will show “6 nn” where nn is the actual boiler flow temperature. If
the user control setting of the relevant knob is to a temperature above the actual boiler
flow temperature and provided all temperature thermostats and pressure switches are
closed the control PCB will initiate the following sequence.
(1)
The fan will start.
(2)
When the fan reaches running speed, the control PCB will start the ignition
sequence and the first digit of the display will show “1”.
(3)
After a pre-purge period of about 10 seconds, the first digit of the display will show
“2”, 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.
(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 first digit of
the display will show “3” or “4” depending on the external demand being served.
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 thermostat. 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 “E 02”). 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 or flue
overheat thermostat.
Any failure of the boiler to sequence in the above manner should be investigated using the
following trouble shooting flow diagram.
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.
WD536
Chapter 5 : Fault Finding
The Keston C45, C40P, C55 & C55P
Installation & Servicing Instructions
Page : 28