Lennox F8AUH series Service Manual page 9

3− Furnace Control (A3)
All F8AUH model units are equipped with an integrated
ignition blower control (50A55), which controls ignition,
safety circuits, blower operation, fan off timing and allows
for thermostat connection and troubleshooting. The sys-
tem consists of ignition control board (figure 6 with control
terminal designations in table 4) and a silicon carbide ig-
nitor (figure 7). The board features two LED lights for
troubleshooting and two accessory terminals rated at (4)
four amps. See table 3 for troubleshooting diagnostic
codes. Table 1 and 2 show jack plug terminal designa-
tions. Units equipped with this control board board can be
used with either electronic or electro−mechanical thermo-
stats without modification. Each time power is applied to
the furnace, the control board board performs a selfcheck
including energizing the combustion air blower for a peri-
od of 1 second.
TABLE 1
CONTROL BOARD J156 (J2) TERMINAL
DESIGNATIONS
PIN #
1 Combustion Air Blower Line
2
3
Combustion Air Blower Neutral
4
TABLE 2
CONTROL BOARD J58 (J1) TERMINAL
DESIGNATIONS
PIN #
1
2
3
4
5
6
7
8
9
10
11
12
FUNCTION
Ignitor Line
Ignitor Neutral
FUNCTION
High Limit Out
Not Used
24V
Not Used
Rollout Switch In
24V
High Limit In
Ground
Gas Valve In
Pressure Switch In
Rollout Switch Out
Gas Valve Out
a−Electronic Ignition (See Figure 5)
On a call for heat the 50A55 control board monitors the
combustion air blower pressure switch. The control will not
begin the heating cycle if the pressure switch is closed (by−
passed). Once the pressure switch is determined to be
open, the combustion air blower is energized. When the
differential in the pressure switch is great enough, the
pressure switch closes and a 15−second pre−purge be-
gins. If the pressure switch is not proven within 2−1/2 min-
utes, the control goes into Watchguard−Pressure Switch
mode for a 5−minute re−set period.
After the 15−second pre−purge period, the carbide ignitor
warms up for 11 to 21 seconds during which the gas valve
opens at 19 seconds for a 4−second trial for ignition. The
ignitor stays energized for the first second of the 4−second
trial. If ignition is not proved during the 4−second period,
the control will try four more times with an inter purge and
warm−up time between trials of 37 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.
b−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 flipping the dip
switches located on the integrated control. The unit is
shipped with a factory fan off setting of 90 seconds. Fan off
time will affect comfort and is adjustable to satisfy individu-
al applications. See figure 4.
FAN-OFF TIME ADJUSTMENT
60sec
.
To adjust fan−off timing, flip dip switch to desired setting.
Page 9
90sec 120sec
. .
FIGURE 4
180sec
.