Lennox EL296UHVX Series Unit Information page 48

7. Combustion Air Inducer (B6) and
Cold End Header Box
All EL296UHV(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 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.
The combustion air inducer is installed on the cold end
header box. The cold end header box is a single piece
made of hard plastic. The box has an internal channel
where the combustion air inducer creates negative pres­
sure at unit start up. The channel contains an orifice used
to regulate flow created by the combustion air inducer.
The box has pressure taps for the combustion air inducer
pressure switch hoses. The pressure switch measures
the pressure differential across the combustion air induc­
er orifice or difference in the channel and the box. If re­
placement is necessary the gaskets used to seal the
box to the vestibule panel and the combustion air in­
ducer to the box, must also be replaced.
A proving switch connected to the combustion air inducer ori­
fice plate is used to prove inducer operation. The combustion
air inducer orifice will be different for each model. See table
17 for orifice sizes. The pressure switch measures the pres­
sure differential across the combustion air inducer orifice.
When the proving switch opens, the furnace control (A92) im­
mediately closes the gas valve to prevent burner operation.
TABLE 17
EL296UHV(X) Unit
-045 all - numbers
-070-1 through -6
-070-7 & later
-090-1, -2, -3, -4
-090-5 and later
-110-1, -2, -3, -4
-110-5 and later
-135 all - numbers
8. Combustion Air Inducer
Pressure Switch (S18)
EL296UHV(X) series units are equipped with a dual com­
bustion air pressure switch (first and second stage) located
on the combustion air inducer orifice bracket. See figure 16.
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 pressure 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
0.618
0.810
0.920
0.920
0.905
1.040
1.070
1.235
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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 18. 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
High Fire Switch
Negative Port
FIGURE 16
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 18 0- 4500'
Unit Set Point Low Heat
-045 0.35
-070-1 through -6 0.45
-070-7 & later 0.50
-090 0.50
-110 0.45
-135 0.45
TABLE 19 4501 - 7500'
Unit Set Point Low Heat
-045 0.35
-070-1 through -6 0.45
-070-7 & later 0.45
-090 0.50
-110 0.45
-135 0.45
TABLE 20 7500 - 10.000'
Unit Set Point Low Heat
-045 0.35
-070-1 through -6 0.41
-070-7 & later 0.40
-090 0.45
-110 0.41
-135 0.41
Low Fire Switch
Positive Port
Set Point High Heat
0.60
0.90
0.85
0.90
0.90
0.90
Set Point High Heat
0.55
0.81
0.75
0.85
0.81
0.85
Set Point High Heat
0.50
0.74
0.70
0.81
0.74
0.74