Combustion Air Blower; Flame Rollout Switches; Primary Limit Control; Secondary Limit Controls - Lennox G24M SERIES Unit Information

C--Combustion Air Blower (B6)
All G24M units use a combustion air blower to move
air through the burners and heat exchanger during
heating operation. Some early model G24M units are
equipped with a blower that uses a PSC (Permanent
Split Capacitor) 120VAC motor. PSC motors use run
capacitors. Other early and late model G24M units are
equipped with a blower that uses a shaded pole 120V
motor. The motor operates during all heating opera-
tion and is controlled by furnace control A3. For G24M
units equipped with the Ram ignition system, the
blower will operate for 45 seconds before burner igni-
tion (pre-purge) and for 5 seconds after the gas valve
closes (post-purge). For G24M units equipped with the
SureLight ignition system, the blower will operate for
15 seconds before burner ignition (pre purge) and for 5
seconds after the 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 blow-
er housing. During normal operation, the pressure in the
housing is negative. If pressure becomes less negative
(signifying an obstruction) 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 lo-
cated 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 connected in series
with the ignition control A3. When S47 senses rollout,
the ignition control immediately stops ignition and
closes the gas valve. If unit is running and flame roll-
out is detected, the gas valve will close and ignition
control 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 reset a tripped switch,
push the reset button located on the control.
FIGURE 12
ROLLOUT SWITCH (S47)
MANUAL
RESET BUTTON
E--Primary Limit Control (S10)
LIMIT CONTROL FOR
G24M SERIES UNITS
THIS TYPE AUTO-RESET LIMIT
IS USED FOR THE PRIMARY LIMIT (S10) AND
RIGHT SECONDARY LIMIT (S21)
(see FIGURE 11)
The primary limit (S10) on G24M 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 automati-
cally resets when unit temperature returns to normal.
The switch is factory set and cannot be adjusted. The
switch may have different setpoints for each unit model
number. However, the setpoint will be printed on the
side of the limit.
F--Secondary Limit Controls (S21)
The secondary limit (S21) on G24M 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 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. Two limits are supplied in
each furnace and each limit is a different style (figures
13 and 14). The setpoint will be printed on the side of
the limit. If stick limit (figure 14) suffers from nuisance
trips anf the furnace is in the horizontal position, re-
place with limit kit no. 50L98.
G--Spark Electrode and Flame Sensor
Early Model G24M Units
Figure 15 shows the arrangement of flame sensor,
spark electrode and burners. The ignition control uses
direct spark to ignite the rightmost burner and the
burners cross-light to the left. The flame sensor uses
flame rectification
retention ring in the end of each burner is used to
maintain correct flame length and shape and to keep
the flame from lifting off the burner head.
Figure 16 shows the gap between tip of the electrodes
and the burner surface.
Page 14
FIGURE 13
SECONDARY LIMIT CONTROL (S21)
FOR G24M SERIES UNITS
THIS TYPE AUTO-RESET LIMIT IS
USED FOR THE LEFT SECONDARY
LIMIT (S21) (see FIGURE 11)
INSULATING C
OVER (s)
FIGURE 14
to sense combustion. A flame