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7. Combustion Air Inducer (B6)
All units use a two-stage combustion air inducer to move
airthrough the burners and heat exchanger during heating
operation. The blower uses a 120VAC motor. The motor
operates during all heating operation and is controlled by
furnace / blower 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 in-
ducer 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 pressure switch connected to the combustion air induc-
er orifice plate is used to prove inducer operation. The
combustion air inducer orifice will be different for each
model. See TABLE 10 for orifice sizes. The switch mon-
itors air pressure in the inducer housing. During normal
operation, the pressure in the housing is negative. If pres-
sure 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 10
Unit
-070
-090
8. Combustion Air Inducer Pressure Switch (S18)
S18 is a dual combustion air pressure switch (first and
second stage) located on the combustion air inducer or-
ifice bracket. 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 proving switch
electrically connected to the furnace control. The purpose
of the switch is to prevent burner operation if the com-
bustion air inducer is not operating or if the flue becomes
obstructed.
On heat demand (first or second stage) the switch sens-
es that the combustion air inducer is operating. It closes
a circuit to the furnace control when pressure inside the
combustion 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 atmospheric pressure. If the flue becomes obstructed
during operation, the switch senses a loss of negative
pressure (pressure becomes more equal with atmospher-
ic 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.
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.
C.A.I. Orifice Size
1.406"
1.690"
Page 21
COMBUSTION AIR PRESSURE SWITCH
Tap
Bracket
High Fire
Switch
FIGURE 17
TABLE 11
Unit
Set Point High Heat
-070
0.55"
-090
0.55"
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 18.
Low Fire
Switch
Set Point Low Heat
0,25"
0.25"
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