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WD388/0/2004
6.1
ELECTRICAL CONTROL SEQUENCE
6.1.1
HEATING DEMAND
When the external control(s) are calling for heat, power will be fed to the boiler connection
strip at terminal SL (Switched Live) and N (Neutral) and the CH green light will illuminate.
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 will start.
(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.
(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. A dot is
illuminate in the lower right corner of the temperature display. 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 "E01"). 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.
6.1.2
HOT WATER DEMAND
When a hot tap is opened to allow flow of more than 2.5 litres per minute the DHW green
light will illuminate. If the user control setting of the DHW temperature is to a temperature
above the actual DHW temperature, and provided all safety sensors are confirmed as OK
the control PCB will make the following sequence.
(1)
The fan will start (if not already running for a CH demand)
(2)
The diverter valve, integral to the boiler pump, will move to the DHW plate heat
exchanger position.
(3)
As steps (2) to (7) above.
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.
Installation & Servicing Instructions
Chapter 6 : Fault Finding
6. FAULT FINDING
The Keston C36 & C36P Combi Boilers
Page : 28
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