Boiler Operational Sequence - Thermal Solutions EVO-500 Installation, Operating And Service Instructions

VI. Boiler Operational Sequence
Boiler in Standby
Call for Heat
Combustion Air Flow Switch
Contacts Make
Prepurge Begins
30 sec.
Pilot Flame Establishing
Period Begins
5 sec.
Early Spark Termination
5 sec.
Main Flame Establishing
Period Begins
3 sec.
Run Period Begins
Run Period
Post Purge
Standby
NORMAL SEQUENCE OF OPERATION
Limits and operating controls such as safety and operating temperature limits,
water flow switch, thermal fuse switches and high/low gas pressure switch
contacts must be closed to initiate and continue burner sequence. Flame
signal must not be present. Airflow switch must be open.
Control terminal 5 is energized and all safety and operating control contacts
are closed. Control terminal 4 is energized to start the combustion air fan.
Contacts close when fan generates enough differential air pressure to actuate
combustion air flow switch diaphragm.
Fan purges combustion chamber and venting for 30 seconds.
Pilot gas valve(s) and ignition transformer are energized. Pilot gas flows to
pilot burner and is ignited.
Ignition transformer is de-energized after 5 seconds. Presence of pilot flame
is detected by ultraviolet scanner. Flame signal exceeds 1.5 volts DC.
Main gas valves are energized. Gas flows to main burner and is ignited by
pilot flame. Pilot and main flames are detected by ultraviolet scanner. Flame
signal continues to maintain 1.5 to 5.0 volts DC.
After 5 seconds, pilot gas valve(s) is de-energized. Gas flow to pilot stops.
Pilot flame is extinguished. Main flame is detected by ultraviolet scanner.
Flame signal continues to maintain 1.5 to 5.0 volts DC.
Main burner continues to operate until call for heat ends, contacts of a safety
or operating control open, electrical power is interrupted, or the flame signal
drops below 1.5 volts DC. Modulating boilers operate based on the load,
determined by the difference between the bulk boiler water temperature and
the boiler water set point.
Main gas valves are de-energized. Gas flow to main burner stops. Main
flame is extinguished. Fan continues to operate for 60 seconds to EVOcuate
combustion chamber and venting.
After 60 seconds fan stops.
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