Lennox 80MGF Series Manual page 17

B−Blower Motors and Capacitors
All 80MGF units use direct drive blower motors. All motors
used are 120V permanent split capacitor motors to ensure
maximum efficiency. See table 11 for ratings.
TABLE 11
80MGF BLOWER RATINGS 120V 1PH
BLOWER MOTOR
80MGF2
80MGF3
80MGF3/4
80MGF4
80MGF4/5
80MGF5/6
SUPPLY AIR BLOWER
AND SECONDARY LIMITS
To Remove Blower From Unit: Remove Bolts and
Wiring Jackplugs. Then Slide Out Front of Unit.
FIGURE 13
C−Combustion Air Blower (B6)
All 80MGF units use a combustion air blower to move air
through the burners and heat exchanger during heating
operation. The blowers will use either a shaded pole 120V
motor or a C−frame 120V motor. Shaded pole and C−frame
motors do not use run capacitors. The motor operates dur-
ing all heating operation and is controlled by furnace con-
trol A3. The motor also operates for a period before burner
ignition (pre−purge). The pre−purge time is 45 seconds for
80MGF units equipped with the RAM control and 15 sec-
onds for units equipped with the EGC control. With either
the RAM, EGC or SureLight control board, the motor will
operate 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-
HP CAP
1/4 5MFD 370V
1/3 5MFD 370V
1/2 7.5MFD 370V
1/2 7.5MFD 370V
3/4 40MFD 370V
3/4 40MFD 370V
SECONDARY
LIMITS (S21)
Top
Back
Left
Right
Front
Bottom
BLOWER
MOTOR
MOTOR
CAPACITOR
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.
D−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.
E−Primary Limit Control (S10)
LIMIT CONTROL FOR
80MGF SERIES UNITS
THIS TYPE AUTO-RESET LIMIT
IS USED FOR THE PRIMARY LIMIT (S10) AND
RIGHT SECONDARY LIMIT (S21)
(see FIGURE 13)
The primary limit (S10) on 80MGF 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. There are two types of
primary limit. See figures 15 and 16.
Page 17
ROLLOUT SWITCH (S47)
MANUAL
RESET BUTTON
FIGURE 14
FIGURE 15