Lennox iComfort SL280UH070V36A Manual page 41

8. Combustion Air Inducer (B6)
All SL280UHV(X) units use a two-stage combustion air in­
ducer to move air through the burners and heat exchanger
during heating operation. The blower uses a 120VAC mo­
tor. The motor operates during all heating operation and is
controlled by integrated control 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 inducer 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 proving switch connected to the combustion air inducer
orifice plate is used to prove inducer operation. The com­
bustion air inducer orifice will be different for each model.
See table 15 for orifice sizes. The switch monitors air pres­
sure in the inducer housing. During normal operation, the
pressure in the housing is negative. If pressure 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
SL280UHV(X) Unit
-070
-090
-110
-135
9. Combustion Air Inducer
Prove Switch (S18)
SL280UHV(X) series units are equipped with a dual com­
bustion air proving switch (first and second stage) located
on the combustion air inducer orifice bracket. See figure
15. The switch is connected to the combustion air inducer
housing by means of a flexible silicone hose. It monitors
negative air pressure in the combustion air inducer hous­
ing.
The switches are a single‐pole single‐throw proving switch
electrically connected to the integrated control. The pur­
pose of the switch is to prevent burner operation if the com­
bustion air inducer is not operating or if the flue becomes
obstructed.
C.A.I. Orifice Size
1.344
1.531
1.812
1.875
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On heat demand (first or second stage) the switch senses
that the combustion air inducer is operating. It closes a cir­
cuit to the integrated control when pressure inside the com­
bustion air inducer decreases to a certain set point.
Set points vary depending on unit size. See table 16. The
pressure sensed by the switch is negative relative to atmo­
spheric pressure. If the flue becomes obstructed during op­
eration, the switch senses a loss of negative pressure
(pressure becomes more equal with atmospheric pres­
sure) 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.
COMBUSTION AIR PRESSURE SWITCH
Tap
Bracket
High Fire
Switch
FIGURE 15
NOTE - The switch is factory set and is not field adjustable.
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.
TABLE 16
SL280UHV(X) Set Point Low
Unit Heat
-070V36A-01
0.25
and later
-090V36B
-090V60C 0.30
-01 and later
-090V48B-01,
0.30
-02, -03
-090V48B-04
0.35
and later
-110V60C-01
0.30
and later
-135V60D-01
0.35
and later
Low Fire
Switch
Set Point High Heat
0.60
0.68
0.68
0.68
0.68
0.75