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7. Combustion Air Inducer (B6) and
Cold End Header Box
All ML296UHV(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.
The combustion air inducer is installed on the cold end
header box. The cold end header box is a single piece
made of hard plastic. The box has an internal channel
where the combustion air inducer creates negative pres
sure at unit start up. The channel contains an orifice used
to regulate flow created by the combustion air inducer.
The box has pressure taps for the combustion air inducer
pressure switch hoses. The pressure switch measures
the pressure differential across the combustion air induc
er orifice or difference in the channel and the box.
If re
placement is necessary the gaskets used to seal the
box to the vestibule panel and the combustion air in
ducer to the box, must also be replaced.
A proving switch connected to the combustion air inducer ori
fice plate is used to prove inducer operation. The combustion
air inducer orifice will be different for each model. See table 7
for orifice sizes. The pressure switch measures the pressure
differential across the combustion air inducer orifice. When
the proving switch opens, the furnace control (A92) immedi
ately closes the gas valve to prevent burner operation.
TABLE 7
ML296UHV(X) Unit
C.A.I. Orifice Size
-045
0.618
-070
0.920
-090
0.920
-110
1.040
8. Combustion Air Inducer
Pressure Switch (S18)
ML296UHV(X) series units are equipped with a dual com
bustion air pressure switch (first and second stage) located
on the combustion air inducer orifice bracket. See figure 13.
The switch is connected to the combustion air inducer hous
ing by means of a flexible silicone hose. It monitors negative
air pressure in the combustion air inducer housing.
The switches are a single‐pole single‐throw proving switch
electrically connected to the integrated control. The purpose
of the switch is to prevent burner operation if the combustion
air inducer is not operating or if the flue becomes obstructed.
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 8. 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.
FIGURE 13
COMBUSTION AIR PRESSURE SWITCH
3/16 Terminals
Low Fire Switch
High Fire Switch
1/4" Terminals
Tap (negative - )
Tap (po)
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 8 0 - 4500'
ML296UHV(X)
Unit
Set Point Low
Heat
Set Point High Heat
-045
0.35
0.60
-070
0.50
0.85
-090
0.50
0.90
-110
0.45
0.90
TABLE 9 4501 - 7500'
ML296UHV(X)
Unit
Set Point Low
Heat
Set Point High Heat
-045
0.35
0.55
-070
0.45
0.75
-090
0.50
0.85
-110
0.45
0.81
TABLE 10 7501 - 10,000'
ML296UHV(X)
Unit
Set Point Low
Heat
Set Point High Heat
-045
0.35
0.50
-070
0.40
0.70
-090
0.45
0.81
-110
0.41
0.74
