Lennox F8AUH series Service Manual page 12

5−Blower Motors and Capacitors
All F8AUH units use direct drive blower motors. All motors
used are 120V permanent split capacitor motors to ensure
maximum efficiency. See table 5 for ratings.
TABLE 5
F8AUH BLOWER RATINGS 120V 1PH
BLOWER MOTOR
F8AUH24−045/060/075
F8AUH36-060/075
F8AUH48-075/100/120
F8UH60−100/120
To Remove Blower From Unit: Remove Bolts and
Wiring Jackplugs. Then Slide Out Front of Unit.
FIGURE 9
6−Combustion Air Blower (B6)
All F8AUH units use a combustion air blower to move air
through the burners and heat exchanger during heating
operation. The blower uses a 120VAC motor. The motor
operates during all heating operation and is controlled by
furnace control A3. The blower also operates for 15 se-
conds before burner ignition (pre-purge) and for 5 seconds
after the gas valve closes (post-purge)
A pressure switch connected to the combustion air blower
housing is used to prove combustion air blower operation.
The switch monitors air pressure in the blower housing. Dur-
ing normal operation, the pressure in the housing is nega-
tive. If pressure becomes less negative (signifying an ob-
struction) the pressure switch opens. When the pressure
switch opens, the furnace control (A3) immediately closes
the gas valve to prevent burner operation.
HP CAP
1/4 5MFD 370V
1/3 5MFD 370V
1/2 7.5MFD 370V
3/4 40MFD 370V
SECONDARY
LIMITS (S21)
Top
Back
Left
Right
Front
Bottom
BLOWER
MOTOR
MOTOR
CAPACITOR
7−Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit located on
top of the 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 con-
nected in series with the ignition control A3. When S47
senses rollout, the ignition control immediately stops igni-
tion and closes the gas valve. If unit is running and flame
rollout is detected, the gas valve will close and ignition con-
trol will be disabled. Rollout can be caused by a blocked
flue or lack of combustion air. The switch is factory set and
cannot be adjusted. The setpoint will be printed on the side of
the limit. The switch can be manually reset. To manually re-
set a tripped switch, push the reset button located on the
control.
8−Primary Limit Control (S10) Figure 12
The primary limit (S10) on F8AUH units is located in the
middle of the heating vestibule panel. When excess heat is
sensed in the heat exchanger, the limit will open. If the limit is
tripped, the furnace control energizes the supply air blower
and closes the gas valve. The limit automatically resets when
unit temperature returns to normal. The switch is factory set
and cannot be adjusted. The switch may have a different
setpoint for each unit model number. However, the setpoint
will be printed on the side of the limit.
9−Secondary Limit Controls (S21) Figure 12
The secondary limit (S21) on F8AUH units is located in the
blower compartment in the back side of the blower housing.
When excess heat is sensed in the blower compartment, the
limit will open. If the limit is tripped, the furnace control ener-
gizes the supply air blower and closes the gas valve. The limit
automatically resets when unit temperature returns to normal.
The switch is factory set and cannot be adjusted. F8AUH 2
ton units will be equipped with type limit in figure 11 and
F8AUH 3 ton and greater will have type limit in figure 12 .
LIMIT CONTROL FOR
F8AUH2 UNITS
Page 12
ROLLOUT SWITCH (S47)
MANUAL
RESET BUTTON
FIGURE 10
FIGURE 11