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7. Combustion Air Inducer (B6)
All units use a two-stage combustion air inducer to move air
through the burners and heat exchanger during heating op
eration. The blower uses a 120VAC motor. The motor oper
ates during all heating operation and is controlled by fur
nace / blower control A92. The inducer also operates for 15
seconds before burner ignition (pre‐purge) and for 5 sec
onds 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 re
placement.
A pressure switch connected to the combustion air inducer
orifice plate is used to prove inducer operation. The combus
tion air inducer orifice will be different for each model. See
table 10 for orifice sizes. The switch monitors air pressure 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) immedi
ately closes the gas valve to prevent burner operation.
TABLE 10
EL280DFE Unit
C.A.I. Orifice Size
-070
1.406”
-090
1.690”
8. Combustion Air Inducer
Pressure Switch (S18)
S18 is a dual combustion air pressure switch (first and sec
ond stage) located on the combustion air inducer orifice
bracket. The switch is connected to the combustion air in
ducer housing by means of a flexible silicone hose. It moni
tors negative air pressure in the combustion 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 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 furnace control when pressure inside the com
bustion air inducer decreases to a certain set point.
Set points vary depending on unit size. See table 11. 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.
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.
FIGURE 16
COMBUSTION AIR PRESSURE SWITCH
Tap
High Fire
Switch
Low Fire
Switch
Bracket
TABLE 11
EL280DFE Unit
Set Point High
Heat
Set Point Low Heat
-070
0.55”
0.25”
-090
0.55”
0.25'
II-PLACEMENT AND INSTALLATION
Make sure unit is installed in accordance with installation
instructions and applicable codes.
III-START‐UP
A-Preliminary and Seasonal Checks
1 - Inspect electrical wiring, both field and factory installed
for loose connections. Tighten as required.
2 - Check voltage at disconnect switch. Voltage must be
within range listed on the nameplate. If not, consult the
power company and have voltage condition corrected
before starting unit.
B-Heating Start‐Up
WARNING
Shock and burn hazard.
EL280DFE units are equipped with a hot surface
ignition system. Do not attempt to light manually.
1 -
STOP
! Read the safety information at the beginning of
this section.
2 - Set the thermostat to the lowest setting.
3 - Turn off all electrical power to the unit.
4 - This furnace is equipped with an ignition device which
automatically lights the burners. Do
not
try to light the
burners by hand.
5 - Remove the upper access panel.
6 - Turn switch on gas valve to
OFF
. Do not force. See
figure 17.
