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8. Combustion Air Inducer (B6)
All units use a two-stage combustion air inducer to move
air through the burners and heat exchanger during heat-
ing operation. The blower uses a 120VAC motor. The mo-
tor operates during all heating operation and is controlled
by furnace / blower control A92. The inducer also operates
for 15 seconds before burner ignition (pre-purge) and for
5 seconds after the gas valve closes (post-purge). The in-
ducer operates on low speed during first-stage heat, then
switches to high speed for second stage heat.
NOTE - Each furnace model uses a unique CAI. Refer to
Lennox Repair Parts listing for correct inducer for replace-
ment.
A pressure switch connected to the combustion air induc-
er orifice plate is used to prove inducer operation. The
combustion air inducer orifice will be different for each
model. See TABLE 15 for orifice sizes. The switch mon-
itors air pressure in the inducer housing. During normal
operation, the pressure in the housing is negative. If pres-
sure becomes less negative (signifying an obstruction) the
proving switch opens. When the proving switch opens, the
furnace control (A92) immediately closes the gas valve to
prevent burner operation.
TABLE 15
Unit
-070
-090
-110
-135
9. Combustion Air Inducer Pressure Switch (S18)
S18 is a dual combustion air pressure switch (first and
second stage) located on the combustion air inducer ori-
fice bracket. See FIGURE 17. The switch is connected to
the combustion air inducer housing by means of a flexible
silicone hose. It monitors negative air pressure in the com-
bustion air inducer housing.
The switches are a single-pole single-throw proving switch
electrically connected to the furnace control. The purpose
of the switch is to prevent burner operation if the com-
bustion air inducer is not operating or if the flue becomes
obstructed.
On heat demand (first or second stage) the switch sens-
es that the combustion air inducer is operating. It closes
a circuit to the furnace control when pressure inside the
combustion air inducer decreases to a certain set point.
Set points vary depending on unit size.
C.A.I. Orifice Size
1.344
1.531
1.812
1.875
Page 45
See TABLE 16. The pressure sensed by the switch is neg-
ative relative to atmospheric pressure. If the flue becomes
obstructed during operation, the switch senses a loss of
negative pressure (pressure becomes more equal with at-
mospheric pressure) and opens the circuit to the furnace
control and gas valve. A bleed port on the switch allows
relatively dry air in the vestibule to purge switch tubing, to
prevent condensate build up.
NOTE - The switch is factory set and is not field adjust-
able. It is a safety shut-down control in the furnace and
must not be by-passed for any reason. If switch is closed
or by-passed, the control will not initiate ignition at start up.
COMBUSTION AIR PRESSURE SWITCH
Tap
Bracket
High Fire
Switch
FIGURE 17
TABLE 16
Set Point Low
Unit
-070V36A-01
and later
-090V36B
-090V60C -01
and later
-090V48B-01,
-02, -03
-090V48B-04
and later
-110V60C-01
and later
135V60D-01 and
later
Low Fire
Switch
Set Point High
Heat
Heat
0.25
0.60
0.30
0.68
0.30
0.68
0.35
0.68
0.30
0.68
0.35
0.75
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