Lennox EL280UH110P60C Unit Information page 17

7. Combustion Air Inducer (B6)
All EL280UH(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 op-
erates 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 inducer
orifice plate is used to prove inducer operation. The combus-
tion air inducer orifice will be different for each model. See
table 6 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 in the flue) the proving switch
opens. When the proving switch opens, the furnace control
(A92) immediately closes the gas valve to prevent burner op-
eration.
TABLE 6
EL280UH(X) Unit
−070
−09036B
−090(X)48B
−110
−135
8. Combustion Air Inducer
pressure Switch (S18)
EL280UH(X) series units are equipped with a dual combus-
tion air pressure switch (first and second stage) located on
the combustion air inducer orifice bracket. See figure 10.
The switch is connected to the combustion air inducer hous-
ing by means of a flexible silicone hose. It monitors negative
air pressure in the combustion air inducer housing.
The switches are a single-pole single-throw proving 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 becomes obstructed.
C.A.I. Orifice Size (in)
1.375
1.625
1.690
1.875
1.940
Page 17
On heat demand (first or second stage) the switch senses
that the combustion air inducer is operating. It closes a cir-
cuit to the integrated 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-
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.
COMBUSTION AIR PRESSURE SWITCH
Tap
3/16" terminals
Bracket
High Fire
Switch
FIGURE 10
NOTE − The switch is factory set and is not field adjustable.
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.
TABLE 7
0 − 4500 ft
Set Point Low
EL280UH(X) Unit
Heat (in w.c.)
−070 0.35
−090P36B 0.35
−090P48B 0.30
−090XP48B 0.35
−110P48C/60C 0.30
−110XP60C 0.35
−135 0.35
NOTE − See table 11 for high altitude pressure switch kits.
1/4 terminals
Low Fire
Switch
Set Point High Heat
(in w.c.)
0.68
0.68
0.68
0.68
0.68
0.68
0.75