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10. Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit located on
top of the burner box (135 and 155 kBtuh units− rollout
switches are located on the inner sides of burner box).
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 ignition 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 10. The switch can be manually reset.
To manually reset a tripped switch, push the reset button lo-
cated on the control.
TABLE 10
BTUH INPUT
Rollout Switch Setpoint
−045
−070
250° F
−090
250° F
−110
−135
200° F
−155
200° F
11. Primary Limit Control (S10)
The primary limit (S10) on G50UH(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, see Lennox Repair Parts
handbook.
12. Secondary Limit Controls (S21)
The secondary limit (S21) on G50UH(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 11.
Table 11 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. G50UH−1 through −6
units use a surface type limit factory set to openat 125°F.
G50UH−7 and later units use an air stream limit factory set to
open at 135°. The secondary limit cannot be adjusted.
TABLE 11
G50UH
UNIT
Quantity
LeftSide
Right Side
24A−045
0
No
No
24A−070
2
Yes
Yes
36A−070
2
Yes
Yes
36B−090
1
No
Yes
36C−110
1
No
Yes
48B−070
2
Yes
Yes
48B−090
2
Yes
Yes
48C−110
1
No
Yes
48C−135
2
Yes
Yes
60C−110
1
No
Yes
60D−135
2
Yes
Yes
60D−155
2
Yes
Yes
