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b. Low Water Cutoff (LWCO): Automatically
interrupts main burner operation when surface of
boiler water falls to lowest permissible operating
level. Normal operation resumes when water
returns to normal level.
c. Blocked Vent Switch. Automatically interrupts
main burner operation when excessive vent
system blockage occurs. Control is a multiple
use device. If blocked vent switch is activated do
not attempt to place boiler in operation. Correct
source of blockage and reset blocked vent
switch.
d. Flame Roll-out Switch. Automatically interrupts
boiler operation when ames or excessive heat
are present in vestibule. Control is single use
device. If ame roll-out switch is activated do
not attempt to place boiler in operation. Correct
source of blockage and replace ame roll-out
switch.
e. Thermocouple: senses pilot ame and causes gas
valve to turn off main burner and pilot burner gas
ow should pilot burner ame extinguish.
H. Steam Boiler with Continuous Ignition (Standing Pilot)
and Probe Low Water Cutoff. See Figure 35.
1. Normal Operation
a. Thermostat calls for heat. Thermostat Isolation
Relay is energized, closing contacts.
b. Vent Damper (if used) opens as stated in Vent
Damper Sequence of Operation.
c. Gas Valve(s) is energized allowing main gas ow
and ignition of Main Burners.
d. After Thermostat is satised Gas Valve(s) is de-
energized, extinguishing main ame. Vent
Damper (if used) closes.
2.
Safety Shutdown
a. Limit: Automatically interrupts main burner
operation when steam pressure exceeds set point.
Maximum allowable pressure is 15 psi. Normal
operation resumes when system pressure falls
below set point.