750-91 (revised 2009)
Model CB-LE Packaged Boiler Manual
4-3
4.3 — Sequence of Operation: Oil or Gas
To comply with requirements of insurance underwriters such as FM Global (Factory Mutual), XL GAP (GE GAP/
IRI), or others, additional interlock devices may be used.
4.3 — 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.
4.3.1 — 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. When firing oil, the air compressor motor starter (ACMS - if
provided) is also powered. The air purge valve (APV - nos. 5 and 6 oil only) remains de-energized.
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.
Firing Rate Circuit
• Damper motor transformer (DMT)
• Modulating damper motor (MDM)
• Manual-automatic switch (MAS)
• Manual flame control (MFC)
• Modulating control (MC)
High Fire Proving Circuit
• High fire switch (HFS)
Running Interlock and Limit Circuit
• Low oil pressure switch (LOPS)
• High oil pressure switch (HOPS)
• High oil temperature switch (HOTS)
• Auxiliary low-water cutoff (ALWCO)
Circuit Components
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