Dunkirk D249 Series Commercial Boiler - Manual (Page 7)

Call For Heat
Control recognizes call for heat when power is applied
to 24V terminal on control module. Since control
receives signal from the thermostat, when call for heat is
terminated, heating cycle immediately terminates and all
control outputs will shut off.
Ignition Trial Period
Control energizes pilot gas valve and spark outputs for
ignition trial time of 12 seconds. If flame is sensed during
12 seconds of ignition trial, spark output is de-energized,
main gas valve is energized, and control enters steady heat
mode. If flame is not established within ignition trial period,
control de-energizes the spark and gas valve and operates
as described below in Ignition Failure/Re-try Sequence.
Ignition Failure/Re-Try Sequence
After an unsuccessful ignition trial, control checks to see
if maximum number of ignition trials (2 trials) has been
completed. If maximum number of ignition trials has been
completed, control will lockout. See Lockout (next page) for
details.
If maximum number of ignition trials has not been
completed, control delays for 5 minute inter-purge period.
After the inter-purge, control attempts another ignition
trial. Refer back to Ignition Trial Period for details.
Steady-State Heating
Control keeps pilot gas valve and main gas valve energized
while continuously monitoring call for heat and flame
status. Control will remain in this steady-state heating
mode until power is removed by (a) thermostat satisfied,
(b) pressure switch opening, or (c) flame being lost.
If call for heat is satisfied, power is removed from control,
de-energizing pilot gas valve and main gas valve.
If flame is lost, control will shut off main gas valve within
one second, leave pilot gas on, and immediately start
ignition trial. Control checks to see if maximum number
of flame losses (2 per call for heat) has been reached.
If maximum number of flame losses has been reached,
control locks out. See Lockout section for details.
PILOT
SEQUENCE OF OPERATION
7