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7. Combustion Air Inducer (B6) and
Cold End Header Box
All EL297UHV(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 A92. The inducer also
operates for 15 seconds before burner ignition (prepurge)
and for 5 seconds after the gas valve closes (postpurge).
The inducer operates on low speed during rst 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 orice used
to regulate ow created by the combustion air inducer.
The box has pressure taps for the combustion air induc-
er pressure switch hoses. The pressure switch measures
the pressure dierential across the combustion air inducer
orice or dierence in the channel and the box. If replace-
ment is necessary the gaskets used to seal the box to
the vestibule panel and the combustion air inducer to
the box, must also be replaced.
A proving switch connected to the combustion air inducer
orice plate is used to prove inducer operation. The com-
bustion air inducer orice will be dierent for each model.
See TABLE 15 for orice sizes. The pressure switch mea-
sures the pressure dierential across the combustion air
inducer orice. When the proving switch opens, the fur-
nace control (A92) immediately closes the gas valve to
prevent burner operation.
TABLE 15
EL297UHV Unit C.A.I. Orice Size
-045 0.650
-070 0.810
-090 0.920
-110 1.070
-135 1.300
8. Combustion Air Inducer
Pressure Switch (S18)
EL297UHV(X) series units are equipped with a dual com-
bustion air pressure switch (rst and second stage) lo-
cated on the combustion air inducer orice bracket. See
FIGURE 17. The switch is connected to the combustion
air inducer housing by means of a exible silicone hose.
It monitors negative air pressure in the combustion air in-
ducer housing.
The switches are a single-pole single-throw pressure
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 ue be-
comes obstructed.
On heat demand (rst or second stage) the switch senses
that the combustion air inducer is operating. It closes a
circuit to the integrated control when pressure inside the
combustion air inducer decreases to a certain set point.
Set points vary depending on unit size. See TABLE 16
0 - 4500’ The pressure sensed by the switch is nega-
tive relative to atmospheric pressure. If the ue becomes
obstructed during operation, the switch senses a loss of
negative pressure becomes more equal with atmospheric
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 con-
densate build up.
NOTE - The switch is factory set and is not eld 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 bypassed, the control will not initiate ignition at start up.
COMBUSTION AIR PRESSURE SWITCH
High Fire Switch
Low Fire Switch
Positive Port
Negative Port
FIGURE 17
TABLE 16 0 - 4500’
Unit Set Point High Heat Set Point Low Heat
-045 0.80 0.50
-070 0.95 0.55
-090 0.90 0.50
-110 1.00 0.55
-135 0.85 0.45
TABLE 17 4501 - 7500’
Unit Set Point High Heat Set Point Low Heat
-045 0.74 0.41
-070 0.85 0.50
-090 0.75 0.45
-110 0.95 0.55
-135 0.80 0.40
TABLE 18 7500 - 10,000’
Unit Set Point High Heat Set Point Low Heat
-045 0.70 0.40
-070 0.75 0.45
-090 0.70 0.40
-110 0.90 0.50
-135 0.75 0.35
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