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24
End of a domestic
hot water overrun
Escape pause by
resetting the mains
electricity supply
OFF-ON
Anti-cycle delay
Main gas valve
OFF
6
EITHER
AUTOMATIC START-UP SEQUENCE
1. Central Heating Mode
NO
PASS
PRESENT
PRESENT
ABSENTABSENTFAIL
YES
Central heating
temperature 5°C
above set point
Operating Switch
(or programmer)
OFF
Fan overrun
4 minutes
Continue
pump overrun
for 6 secs pump
pulse after
2 miniutes.
Gas valve OFF
Room thermostat
OFF
Burner pressure
1.0 mbar for 60
seconds.
Burner pressure
automatically
adjusted for range
rating and then
modulated to
maintain set flow
temperature
AUTOMATIC
START-UP
SEQUENCE
CENTRAL
HEATING
DEMAND
SATISFIED
Water flow
temperature
monitored by central
heating sensor
Pump on
Operating Switch
(or programmer)
and room
thermostat on
CENTRAL HEATING
DEMAND
Internal
"Autofrostat"
demand
Is fan in
overrun mode?
Air pressure
switch test
Fan ON
Sense air
pressure
Main valve
opens
Sense
flame
Initiate
sparking
Stop
sparking
Wait
(fan or fan and
pump on)
Ignition Time -
out (after 9secs)
Ignition
Lock-out
Wait
(fan or fan and
pump on)
Wait (pump on
in central
heating mode)
17. Operational Flow Diagrams