Flame Sensor; Ignitor; Electronic Ignition; Fan-Off Time Adjustment - Lennox G40UH(X) Unit Information

Electronic Ignition (See Figure 7)
On a call for heat the ignition board monitors the combus-
tion air inducer prove switch. The control board will not be-
gin the heating cycle if the prove switch is closed (by−
passed). Once the prove switch is determined to be open,
the combustion air inducer is energized. When the differen-
tial in the prove switch is great enough, the prove switch
closes and a 15−second pre−purge begins. If the prove
switch is not proven within 2−1/2 minutes, the control board
goes into Watchguard−Pressure Switch mode for a 5−min-
ute re−set period.
After the 15−second pre−purge period, the ignitor warms up
for 20 seconds during which the gas valve opens at 19 sec-
onds for a 4−second trial for ignition. The ignitor remains
energized for the first 3 seconds during the 4 second trial. If
ignition is not proved during the 4−second period, the con-
trol will try four more times with an inter purge and warm−up
time between trials of 35 seconds. After a total of five trials
for ignition (including the initial trial), the control goes into
Watchguard−Flame Failure mode. After a 60−minute reset
period, the control will begin the ignition sequence again.
The control board has an added feature of ignitor power
regulation to maintain consistent lighting and longer ignitor
life under all line voltage conditions.
Fan Time Control
The fan on time of 45 seconds is not adjustable. Fan off
time (time that the blower operates after the heat demand
has been satisfied) can be adjusted by moving the jumper
to a different setting. The unit is shipped with a factory fan
off setting of 90 seconds. For customized comfort, monitor
the supply air temperature once the heat demand is satis-
fied. Note the supply air temperature at the instant the
blower is de−energized.
Adjust the fan−off delay to achieve a supply air temperature
between 90° − 110° at the instant the blower is de−ener-
gized. (Longer delay times allow for lower air temperature,
shorter delay times allow for higher air temperature). See
figure 9.
Board 100973−01 only has a 45 second fan off delay after
cooling demand has been met. This delay is factory set and
not adjustable.

FAN-OFF TIME ADJUSTMENT

To adjust fan−off timing, reposition jumper across pins to
achieve desired setting.
FIGURE 9
5.Flame Sensor (Figure 10)
A flame sensor is located on the left side of the burner sup-
port. See figure 10. The sensor is mounted on the flame rol-
lout plate and the tip protrudes into the flame envelope of
the left−most burner. The sensor can be removed for ser-
vice without removing any part of the burners. During op-
eration, flame is sensed by current passed through the
flame and sensing electrode. The ignition control allows
the gas valve to remain open as long as flame signal is
sensed.
6. Ignitor (Figure 11)
The SureLight ignitor used on G40UH units, is made of du-
rable silicon nitride. The board finds the lowest ignitor tem-
perature which will successfully light the burner, thus in-
creasing the life of the ignitor. Due to this feature of the
board, voltage cannot be measured. The check ignitor,
measure its resistance. A value of 10.9 to 19.7 ohms indi-
cates a good ignitor.
The mini−nitride ignitor used on G40UH units (equipped
with control 78M47 or 100973−01) is made from a propri-
etary ceramic material. Ignitor longevity is enhanced by
controlling the voltage to the ignitor. The check ignitor,
measure its resistance. A value of 50 to 450 ohms indicates
a good ignitor.
NOTE − The G40UH(X) furnace contains electronic
components that are polarity sensitive. Make sure that
the furnace is wired correctly and is properly grounded.
5/16"
Page 20
FIGURE 10