CleaverBrooks CB - Sequence of Operation: Oil or Gas; Pre-Purge Cycle

750-96 (revised 2010)
Model CB Packaged Boiler Manual
3-3
3.3 — Sequence of Operation: Oil or Gas
To comply with requirements of insurance underwriters such as Factory Mutual (F.M.), Industrial Risk Insurers
(I.R.I.), or others, additional interlock devices may be used in addition to those mentioned above.
3.3 — 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.
3.3.1 — 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, No. 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. This 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 this 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.
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)
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