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6-Flame Roll-out Limit S47
Flame roll-out limit S47 is a SPST N.C. high temperature
limit located as shown in figure 19. S47 is wired to the A55
Unit Controller. When S47 senses flame roll-out (indicating
a blockage in the combustion air passages), the flame roll-
out limit trips and the ignition control immediately closes the
gas valve.
Limit S47 is factory preset to open at 290_F +
12_F
(143.3_C +
6.7_C) on a temperature rise. All flame roll-out
limits are manual reset.
7-Combustion Air Prove Switch S18
S18 is a SPST N.O. switch which monitors combustion air
inducer operation. See figure 19 for location. Switch S18 is
wired to the A55 Unit Controller.
The switch closes on a negative pressure fall. This negative
pressure fall and switch actuation allows the ignition se
quence to continue (proves, by closing, that the combus
tion air inducer is operating before allowing the gas valve to
open.) The combustion air prove switch is factory set and
not adjustable. The switch will automatically open on a
pressure rise (less negative pressure). S18 closes at 0.25 +
5 in.w.c. (62.3 + 12.4 Pa) and opens at 0.10 + 5 in.w.c. (24.8
+
12.4Pa)
8-Combustion Air Inducer B6
The combustion air inducer provides fresh air to the burner
while clearing the combustion chamber of exhaust gases.
See figure 19 for the inducer location. The inducer is ener
gized by the A55 Unit Controller via K13 relay.
The inducer uses a 208/230V single‐phase PSC motor and
a 4.81in. x 1.25in. (122mm x 32mm) blower wheel. The mo
tor operates at 3200RPM and is equipped with auto‐reset
overload protection. Blower is supplied by various manu
facturers. Ratings may vary by manufacturer. Specific
blower electrical ratings can be found on the unit rating
plate.
All combustion air blower motors are sealed and cannot be
oiled. The blower cannot be adjusted but can be disassem
bled for cleaning.
9-Combustion Air Motor Capacitor C3
Combustion air inducer B6 requires a run capacitor rated at
3 MFD and 370VAC.
10-Gas Valves GV1
Gas valve GV1 is a two‐stage redundant valve.. On first
stage (low fire) is quick opening (on and off in less than 3
seconds). Second stage is slow opening (on to high fire
pressure in 40 seconds and off to low fire pressure in 30
seconds). On a call for first stage heat (low fire), the valve is
energized by the ignition control simultaneously with the
spark electrode. On a call for second stage heat (high fire),
the second stage operator is energized directly from A55.
The valve is adjustable for high fire only. Low fire is not ad
justable. A manual shut-off knob is provided on the valve for
shut-off. Manual shut‐off knob immediately closes both
stages without delay. Figure 20 shows gas valve compo
nents. Table 7 shows factory gas valve regulation for LGT
units.
HEATING SECTION COMPONENTS
(reference)
FIGURE 19
IGNITION
WIRE
J15 P15
J29
B6
COMBUSTION
AIR INDUCER
GV1 GAS
VALVE
S18 BLOWER
PROVING SWITCH
S10
PRIMARY
LIMIT
SWITCH
A3
IGNITION
CONTROL
SENSOR
WIRE
J19
P19
S47 ROLLOUT
SWITCH
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