Chapter 4 Sequence Of Operation
4-2 750-177
Blower Motor Starter Circuit
• Blower motor starter (BMS)
Running lnterlock Circuit
• Blower motor starter interlock (BMSI)
• Combustion air proving switch (CAPS)
Low Fire Proving Circuit
• Low fire switch (LFS)
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
• Damper motor transformer (DMT)
• Modulating damper motor (MDM)
• Manual-automatic switch (MAS)
• Manual flame control (MFC)
• Modulating control (MC)
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.
High Fire Proving Circuit
• High fire switch (HFS)
Running Interlock and Limit Circuit
• Low oil pressure switch (LOPS)
• High oil pressure switch (HOPS)
• Auxiliary low-water cutoff (ALWCO)
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 controls wired in 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 modulating
damper motor (MDM) to open the air damper. The damper
begins to open and drives to its full open or high fire position.
Opening the damper motor allows a flow of purging air
through the boiler prior to the ignition cycle.
On certain boilers the circuitry will include a high fire switch
(HFS). The purpose of the switch is to prove that the
modulating damper motor (MDM) has driven the damper to
the open position during the pre-purge cycle. In this instance,
the “high fire proving circuit” is utilized.
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 modulating damper motor (MDM) to drive
the air damper to its low fire position.
To assure that the system is in low fire position prior to
ignition, the low fire switch (LFS) must be closed to complete
the “low fire proving circuit.” The sequence will stop and
hold until the modulating damper motor (MDM) has returned
to the low fire position and the contacts of the low fire switch
(LFS) are closed. Once the low fire switch is closed, the
sequence is allowed to continue.
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.
Note: An oil-fired burner may be equipped
with a direct spark rather than a gas pilot.
The ignition sequence of both is identical.
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