Lennox LGT Series Unit Information page 27

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 (in-
dicating a blockage in the combustion air passages), the
flame rollout limit trips and the ignition control immediately
closes the gas valve.
Limit S47 is factory preset to open at 290F + 12F (143.3C
+ 6.7C) 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 nega-
tive pressure fall and switch actuation allows the ignition
sequence to continue (proves, by closing, that the com-
bustion 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 burn-
er while clearing the combustion chamber of exhaust gas-
es. See figure 19 for the inducer location. The inducer is
energized by the A55 Unit Controller via K13 relay.
S10
PRIMARY
LIMIT
SWITCH
GV1 GAS
VALVE
J15 P15
HEATING SECTION COMPONENTS
B6
PROVING SWITCH
COMBUSTION
AIR INDUCER
J19
P19
J29
S47 ROLLOUT
SWITCH
FIGURE 19
The inducer uses a 208/230V single-phase PSC motor
and a 4.81in. x 1.25in. (122mm x 32mm) blower wheel.
The motor operates at 3200RPM and is equipped with
auto-reset overload protection. Blower is supplied by var-
ious manufacturers. 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 disas-
sembled 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.
(reference)
S18 BLOWER
SENSOR
IGNITION
WIRE
Page 27
A3
IGNITION
CONTROL
WIRE