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Chapter 4
Blower Motor Starter Circuit
• Blower motor starter (BMS)
Running lnterlock Circuit
• Blower motor starter interlock (BMSI)
• Combustion air proving switch (CAPS)
Pilot Ignition Circuit
• Gas pilot valve (GPV)
• Ignition transformer (IT)
• Gas pilot vent valve (GPVV) (if provided)
Flame Detector Circuit
• Flame detector (FD)
Main fuel valve circuit
• Main gas valve (MGV)
• Main gas vent valve (MGVV) (if provided)
• Oil valve (OV)
• Main fuel valve light (FVL)
Firing Rate Circuit
Boiler firing rate is controlled by the PLC, which sends
individual modulation signals via Modbus to actuators for air,
fuel, and FGR (if provided).
An auto/manual switch on the touchscreen HMI allows
operator control of firing rate.
To comply with requirements of insurance underwriters such
as Factory Mutual (FM), Industrial Risk Insurers (IRI) or
others, additional interlock devices may be used in addition to
the circuits mentioned in Section B.
Running Interlock and Limit Circuit
• Low oil pressure switch (LOPS)
• High oil pressure switch (HOPS)
4-2
C. SEQUENCE OF OPERATION - OIL
OR GAS
On a combination fuel unit, the gas/oil switch must be set for
the proper fuel.
The following sequence occurs with power present at the
program relay (PR) input terminals and with all other
operating conditions satisfied.
Pre-Purge Cycle - When the burner switch (BS) is turned
"on," and the "limit" and "fuel valve interlock" circuits are
closed and no flame signal is present, the "blower motor start
circuit" is powered, energizing the blower motor starter
(BMS). The load demand light (LDL) turns on.
At the same time, the program relay signals the PLC to drive
the air damper actuator to its full open or high fire position.
Opening the damper allows a flow of purging air through the
boiler prior to the ignition cycle.
The controls wired into the "running interlock circuit" must
be closed within 10 seconds after the start sequence. In the
event any of the controls are not closed at this time, or if they
subsequently open, the program relay will go into a safety
shutdown.
At the completion of the high fire purge period, the program
relay signals the PLC to drive the air damper to its low fire
position.
To assure that the system is in low fire position prior to
ignition, the sequence will stop and hold until the damper
actuator has returned to the low fire position and the low fire
relay (LFR) is closed.
Note: The ignition trial cannot be started if
flame or a flame simulating condition is
sensed during the pre-purge period. A
safety shutdown will occur if flame is
sensed at this time.
Ignition Cycle - The ignition transformer (IT) and gas pilot
valve (GPV) are energized from the appropriate pilot ignition
terminal.
The pilot flame must be established and proven by the flame
detector (FD) within a 10 second period in order for the
ignition cycle to continue. If for any reason this does not
happen, the system will shut down and safety lockout will
occur.
Note: Depending upon the requirements of
the regulatory body, insurer or fuel being
burned, either the 10 or 15 second pilot
ignition terminal may be used. Both provide
the same function but differ in time interval
allowed for proving main flame ignition.
Refer to the boiler wiring diagram.
With a proven pilot, the main fuel valve(s) (OV or MGV) is
energized and the main fuel valve light (FVL) in the panel is
lighted. The main flame is ignited and the trial period for
Sequence Of Operation
750-364
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