Lennox EL297DFV Series Unit Information page 37

7. Combustion Air Inducer (B6) and
Cold End Header Box
All EL297DFV units use a two-stage combustion air induc-
er 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 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 induc-
er pressure switch hoses. The pressure switch measures
the pressure differential across the combustion air inducer
orifice or difference 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
orifice plate is used to prove inducer operation. The com-
bustion air inducer orifice will be different for each model.
See TABLE 14 for orifice sizes. The pressure switch mea-
sures the pressure differential across the combustion air
inducer orifice. When the proving switch opens, the fur-
nace control (A92) immediately closes the gas valve to
prevent burner operation.
TABLE 14
EL297DFV Unit
-045
-070
-090
-110
8. Combustion Air Inducer
Pressure Switch (S18)
EL297DFV(X) series units are equipped with a dual com-
bustion air pressure switch (first and second stage) lo-
cated on the combustion air inducer orifice bracket. See
FIGURE 16. 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 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 flue be-
comes obstructed.
C.A.I. Orifice Size
0.650
0.810
0.920
1.070
Page 37
On heat demand (first 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 15,
TABLE 16 and TABLE 17. The pressure sensed by the
switch is negative relative to atmospheric pressure. If
the flue 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 condensate build up.
COMBUSTION AIR PRESSURE SWITCH
Tap (negative - )
Low Fire Switch
1/4 " Terminals
FIGURE 16
TABLE 15
0 - 4500 ft
Unit Set Point High Heat
-045
-070
-090
-110
TABLE 16
4501 - 7500 ft
Unit Set Point High Heat
-045
-070
-090
-110
TABLE 17
7500 ft - 10,000 ft
Unit Set Point High Heat
-045
-070
-090
-110
Tap (positive +)
3/16 Terminals
High Fire Switch
Set Point Low Heat
0.74 0.41
0.90 0.50
0.90 0.45
1.00 0.55
Set Point Low
Heat
0.74 0.41
0.85 0.50
0.75 0.45
0.95 0.55
Set Point Low Heat
0.70 0.40
0.75 0.45
0.70 0.40
0.90 0.50