Flame Rollout Switches; Blower Motors And Capacitors; Primary Limit Control; Secondary Limit Controls - Lennox G40UH(X) Unit Information

8.Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit located on
top of the burner box (135 and 155 Btuh units− rollout
switches are located on inner sides of burner box). See
heating components figure 3. Each furnace is equipped
with two identical switches. One switch is located over the
leftmost burner and the other switch is located over the
rightmost burner. The limit is a N.C. SPST manual-reset
limit connected in series with the ignition control A92. When
S47 senses rollout, the ignition control immediately stops
ignition and closes the gas valve. If unit is running and
flame rollout is detected, the gas valve will close and igni-
tion control will be disabled. Rollout can be caused by a
blocked heat exchanger, flue or lack of combustion air. The
switch is factory set and cannot be adjusted. See table 9. The
switch can be manually reset. To manually reset a tripped
switch, push the reset button located on the control.
TABLE 9
BTUH INPUT
−045
−070
−090
−110
−135
−155
SUPPLY AIR BLOWER
AND SECONDARY LIMIT (S)
CAPACITOR
To Remove Blower From Unit: Disconnect Power, Remove Control
Box, Remove Bolts and Unplug Motor Wires From Control Board.
Then Slide Out Front of Unit.
FIGURE 14

9.Blower Motors and Capacitors

All G40UH(X) units use direct drive blower motors. All mo-
tors are 120V permanent split capacitor motors to ensure
maximum efficiency. Ratings for capacitors will be on motor
nameplate.
NOTE − The shaft on 1 HP motors have 2 flat sides and
are matched with blower wheels with 2 set screws.
Rollout Switch Setpoint
250° F 250° F
200° F 200° F
Left Side
Right Side
SECONDARY
LIMIT (S)
MOTOR/BLOWER
ASSEMBLY
10.Primary Limit Control (S10)
The primary limit (S10) on G40UH(X) units is located in the
heating vestibule panel. When excess heat is sensed in the
heat exchanger, the limit will open. If the limit is open, the fur-
nace control energizes the supply air blower and closes the
gas valve. The limit automatically resets when unit tempera-
ture returns to normal. The switch must reset within three min-
utes or SureLight board will go into Watch guard for one hour.
The switch is factory set and cannot be adjusted. The switch
may have a different setpoint for each unit model number. If
limit switch must be replaced, refer to Lennox Repair Parts
handbook.
11.Secondary Limit Controls (S21)
The secondary limit (S21) on G40UH(X) (−24A−045 will not
have a secondary limit) units is located in the blower compart-
ment in the back side of the blower housing. See figure 14.
Table 10 and the blower speed chart on the wiring diagram
show secondary limit quantities per unit. When excess heat is
sensed in the blower compartment, the limit will open. If the
limit is open, the furnace control energizes the supply air blow-
er and closes the gas valve. The limit automatically resets
when unit temperature returns to normal. G40UH−1 through −6
units use a surface type limit factory set to open at 125°F.
G40UH−7 units and later, use an airstream type limit factory
set to open at 135°. The secondary limit cannot be adjusted.
G40UH
Quantity
UNIT
24A−045 0
24A−070 2
36A−045 2
36A−070 2
36B−090 1
36C−110 1
48B−070 2
48B−090 2
48C−110 1
48C−135 2
60C−110 1
60D−135 2
60D−155 2
Page 22
TABLE 10
LeftSide Right Side
No No
Yes Yes
Yes Yes
Yes Yes
No Yes
No Yes
Yes Yes
Yes Yes
No Yes
Yes Yes
No Yes
Yes Yes
Yes Yes