Chapter 4 Sequence of Operation
4-2 750-177
The controls used vary depending upon the fuel oil or gas and the specific requirement of applicable regulatory bodies.
Refer to the boiler wiring diagram to determine the actual controls provided. The circuits and controls normally used in
the circuits follow and are referred to in the following sequence of operation.
Limit Circuit:
• Burner switch (BS)
• Operating limit control (OLC) - pressure or temperature
• High limit control (HLC) - pressure or temperature
• Low-water cutoff (LWCO)
• Gas-oil selector switch (GOS) - (Combination burner only)
• Low gas pressures switch (LGPS)
• High gas pressure switch (HGPS)
Fuel valve interlock circuit
• Main gas valve auxiliary switch (MGVAS)
• Oil valve auxiliary switch (OVAS)
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)
ABC
D
EFG
8
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