Start Up - Lennox EL280DF Series Unit Information

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 op­
erates 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 com­
bustion air inducer orifice will be different for each model.
See table 6 for orifice sizes. The switch monitors air pres­
sure 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) immediately closes the gas valve to prevent burner
operation.
TABLE 6
EL280DF Unit
-070
-090
-110
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 combus­
tion air inducer is not operating or if the flue becomes ob­
structed.
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 7. 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
C.A.I. Orifice Size
1.406"
1.690"
1.844"
Page 14
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.
COMBUSTION AIR PRESSURE SWITCH
Tap
Bracket
High Fire
Switch
FIGURE 10
TABLE 7
Set Point High
EL280DF Unit
-070
-090
-110
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.
EL280DF 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.
Low Fire
Switch
Set Point Low
Heat Heat
0.55" 0.25"
0.55" 0.25'
0.60" 0.25"