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6.Flame Sensor (Figure 9)
A flame sensor is located on the left side of the burner sup-
port. See parts arrangement figure1. The sensor is
mounted on the flame rollout plate and the tip protrudes
into the flame envelope of the left−most burner. The sensor
can be removed for service without removing any part of
the burners. During operation, flame is sensed by current
passed through the flame and sensing electrode. The igni-
tion control allows the gas valve to remain open as long as
flame signal is sensed. See table 18 for flame sense signal.
FIGURE 9
5/16"
7. Ignitor (Figure 10)
Two type ignitors are used for the SureLight ignition sys-
tem; silicon nitride and mini nitride. The ignition control
finds the lowest ignitor temperature which will successfully
light the burner, thus increasing the life of the ignitor. Due to
this feature of the board, voltage cannot be measured. To
check ignitor, measure its resistance. See table 12 for ohm
value and ignitor control matchup.
TABLE 12
SureLight Control
Ignitor
Resistance
56L84, 10M93,
32M88, 69M08
Silicon Nitride
10.9 − 19.7
78M47 &
100973−01
Mini Nitride
50 − 450
NOTE − The G40DF(X) furnace contains electronic
components that are polarity sensitive. Make sure that
the furnace is wired correctly and is properly grounded.
8.Blower Motors and Capacitors
All G40DF(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 − Shafts on 1 HP motors have 2 flat sides and are
matched with blower wheels with 2 set screws.
FIGURE 10
B"
Ignitor Location
BURNERS FRONT VIEW
MEASUREMENT IS TO I.D.
OF RETENTION RING
BRACKET
Silicon nitride
Mini Nitride
A"
B"
.625
.406
.685
.306
A"
IGNITOR TYPE
9.Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit located on
top of the burner box. See parts arrangement figure 1. 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 con-
trol A92. When S47 senses rollout, the ignition control im-
mediately 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 com-
bustion air. The switch is factory set to open at 210° F and
cannot be adjusted. The switch can be manually reset. To
manually reset a tripped switch, push the reset button located
on the control.
10.Primary Limit Control (S10)
The primary limit (S10) on G40DF(X) units is located in the
heating vestibule panel. G40DF−3 and later models will have
a limit shield. See figure 11 and make note of limit orientation to
shield. When excess heat is sensed in the heat exchanger, the
limit will open. If the limit is open, the furnace control energizes
the supply air blower and closes the gas valve. The limit auto-
matically resets when unit temperature returns to normal. The
switch must reset within three minutes or SureLight board will
go into Watch guard for one hour. The setpoint is factory set
(printed on side of switch) and cannot be adjusted.
