Lennox EL280UHE Series Service Literature page 23

7. Combustion Air Inducer (B6)
All EL280UHE(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 (pre-purge)
and for 5 seconds 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 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 monitors
air pressure in the inducer housing. During normal oper-
ation, the pressure in the housing is negative. If pressure
becomes less negative (signifying an obstruction in the
flue) the proving switch opens. When the proving switch
opens, the furnace control (A92) immediately closes the
gas valve to prevent burner operation.
TABLE 10
EL280UHE(X) Unit
-045E36A
-070E36A
-090E36B
-090E48B
-110E48C
-110E60C
8. Combustion Air Inducer pressure Switch (S18)
EL280UHE(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 proving switch
electrically connected to the integrated control. The pur-
pose 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 (in)
1.063
1.375
1.625
1.690
1.875
1.875
Page 23
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 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.
COMBUSTION AIR PRESSURE SWITCH
Tap
Bracket
High Fire
Switch
FIGURE 16
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 bypassed, the control will not initiate ignition at start up.
TABLE 11
0 - 4500 FT
EL280UHE(X) Set Point Low
Unit
Heat (in w.c.)
-045E36A
-070E36A
-090E36B
-090E48B
-110E48C
-110E60C
NOTE - See table 15 for high altitude pressure switch kits.
Low Fire
Switch
Set Point High
Heat (in w.c.)
0.30
0.35
0.68
0.30