14
c
.
Blocked Vent Switch. Automatically interrupts
main burner operation when excessive vent
system blockage occurs. Vent Damper remains
open with call for heat. If blocked vent switch
is activated do not attempt to place boiler in
operation. The source of blockage must be
corrected by trained and skilled personnel from a
qualified service agency before resetting switch.
d
.
Flame Roll-out Switch. Automatically interrupts
boiler operation when flames or excessive heat
are present in vestibule. Vent Damper remains
open with call for heat. Control is single use
device. If flame roll-out switch is activated do
not attempt to place boiler in operation. The
source of blockage must be corrected and the
flame roll-out switch replaced by trained and
skilled personnel from a qualified service agency.
e
.
Thermocouple: senses pilot flame and causes gas
valve to turn off main burner and pilot burner gas
flow should pilot burner flame extinguish. Vent
Damper remains open with call for heat.
H.
Sequence of Operation, Continuous Ignition
(Standing Pilot).
1. Normal Operation
a. Thermostat calls for heat. Vent Damper opens.
b. Gas Valve energized allowing main gas flow and
ignition of Main Burners.
c
.
Call for heat ends. Gas Valve de-energized,
extinguishing main flame. Vent Damper closes.
2. Safety Shutdown
a
.
Low Water Cutoff. Automatically interrupts main
burner operation when surface of boiler water
falls to lowest permissible operating level. Vent
damper closes. Normal operation resumes when
water returns to normal level.
b. Limit: Automatically interrupts main burner
operation when steam pressure exceeds set point.
Maximum allowable pressure is 15 psi. Vent
Damper closes. Normal operation resumes when
steam pressure falls below set point.
Figure 14: Wiring Diagram, Continuous Ignition (Standing Pilot) and Hydrolevel Model 400
or McDonnell & Miller PS-802-24 Cutoff
