Lennox LGA Series Unit Information page 26

PRIMARY LIMIT LOCATION
BLOWER
OPENING
S10
S130
FIGURE 19
5-Secondary High Temperature Limit S21
S21 is the secondary high temperature limit for gas
heat. The secondary limit S21 is located on the blower
transition. Secondary limit S21 is also wired to the main
control panel A55. The secondary limit functions in the
same manner as the primary limit, but is factory set to
actuate at different temperatures. The N.O. contacts of
S21 is connected to the blower relay coil K3 through
control A55. If the limit trips the burners and combustion
air blower will de-energize but the blower will remain en-
ergized. All limits used are SPDT N.C. auto-reset limits.
Limit S21 in standard and high heat units are factory pre-
set to open at 175_F + 5_F (79_C + 2.8_C) on a tempera-
ture rise and automatically reset at 135_F +7_F (57_C +
3.9_C) on a temperature fall.
6-Flame Rollout Limit S47
Flame rollout limit S47 is a SPST N.C. high temperature
limit located just above the burner air intake opening in the
burner enclosure (see figure 13). S47 is wired to the main con-
trol panel A55. When S47 senses flame rollout (indicating
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 200_F + 12_F (93_C +
6.7_C) on a temperature rise. All flame rollout limits are
manual reset.
7-Combustion Air Prove Switch S18
The combustion air prove switch S18 is a SPST N.O. pressure
switch located in the vestibule area. The switch is used to
monitor combustion air blower operation. Switch S18 is wired
to the main control panel A55. The switch actuates at 0.31"
W.C. negative pressure + 0.05" W.C. (77Pa+ 12.4 Pa) on a
pressure fall which allows power to the ignition control
(proves, by closing, that the combustion air blower is oper-
ating before allowing the ignition control to energize). The
combustion air prove switch is factory set and not adjust-
able.
8-Combustion Air Inducer B6
S131
Combustion air inducer B6 provides fresh air to the burners
while clearing the combustion chamber of exhaust gases.
The inducer begins operating immediately upon receiving a
thermostat demand and is de-energized immediately when
thermostat demand is satisfied.
BLOWER
The combustion air inducer uses a 208/230V single-phase
OPENING
PSC motor and a 4.72in. x 1.31in. (120mm x 33.3mm) blower
wheel. The motor operates at 3200RPM and is equipped with
auto-reset overload protection. Blowers are supplied by vari-
ous manufacturers. Ratings may vary by manufacturer. Spe-
cific blower electrical ratings can be found on the unit rating
plate.
All combustion air inducer motors are sealed and can-
not be oiled. The blower cannot be adjusted but can be
disassembled for cleaning.
9-Combustion Air Motor Capacitor C3
The combustion air blower motor in all LGA units re-
quires a run capacitor. Capacitor C3 is connected to
combustion air blower B6. All capacitors are rated at 3
MFD and 370VAC.
10-Gas Valve GV1
Gas valve GV1 is a one or two-stage redundant gas valve
used in all LGA units. Single stage units are equipped with
valves manufactured by Honeywell. Two-stage
fire) units, are equipped with valves manufactured by Honey-
well or White Rodgers. For both valves first stage (low fire) is
quick opening (on and off in less than 3 seconds). Second
stage on the White Rodgers is slow opening (on to high fire
pressure in 40 seconds and off to low fire pressure in 30 sec-
onds). Second stage on the Honeywell valve is quick opening.
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 oper-
ator is energized directly from A55. The WhiteRodgers two
stage valve is adjustabel for high fire only. Low fire is not ad-
justable. The Honeywell two stage valve is adjustable for both
high fire and low fire. A manual shut-off knob is provided on
the valve for shut-off. Manual shut-off knob immediately
closes both stages without delay. Figures 20 and 21 show
gas valve components. Tables 5 and 6 show factory set
gas valve manifold pressures for LGA series units.
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