Lennox EL296UH090XV48C - Combustion Air Inducer Pressure Switch (S18)

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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 A92. The inducer also
operates for 15 seconds before burner ignition (prepurge)
and for 5 seconds after the gas valve closes (postpurge).
The inducer operates on low speed during rst 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 orice used
to regulate ow created by the combustion air inducer.
The box has pressure taps for the combustion air induc-
er pressure switch hoses. The pressure switch measures
the pressure di󰀨erential across the combustion air inducer
orice or di󰀨erence in the channel and the box. If replace-
ment is necessary the gaskets used to seal the box to
the vestibule panel and the combustion air inducer to
the box, must also be replaced.
A proving switch connected to the combustion air inducer
orice plate is used to prove inducer operation. The com-
bustion air inducer orice will be di󰀨erent for each model.
See TABLE 17 for orice sizes. The pressure switch mea-
sures the pressure di󰀨erential across the combustion air
inducer orice. When the proving switch opens, the fur-
nace control (A92) immediately closes the gas valve to
prevent burner operation.
TABLE 17
EL296UHV Unit C.A.I. Orice Size
-045 all - numbers 0.618
-070-1 through -6 0.810
-070-7 & later 0.920
-090-1, -2, -3, -4
-090-5 and later
0.920
0.905
-110-1, -2, -3, -4
-110-5 and later
1.040
1.070
-135 all - numbers 1.235
8. Combustion Air Inducer
Pressure Switch (S18)
EL296UHV(X) series units are equipped with a dual com-
bustion air pressure switch (rst and second stage) lo-
cated on the combustion air inducer orice bracket. See
FIGURE 17. The switch is connected to the combustion
air inducer housing by means of a exible silicone hose.
It monitors negative air pressure in the combustion air in-
ducer 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 ue be-
comes obstructed.
On heat demand (rst 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 18.
The pressure sensed by the switch is negative relative
to atmospheric pressure. If the ue becomes obstructed
during operation, the switch senses a loss of negative
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 conden-
sate build up.
COMBUSTION AIR PRESSURE SWITCH
High Fire Switch
Low Fire Switch
Positive Port
Negative Port
FIGURE 17
NOTE - The switch is factory set and is not eld 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 18
0 - 4500’
Unit Set Point Low Heat Set Point High Heat
-045 0.35 0.60
-070-1 through -6 0.45 .090
-070-7 & later 0.50 .085
-090 0.50 0.90
-110 0.45 0.90
-135 0.45 0.90
TABLE 19
4501 - 7500’
Unit Set Point Low Heat Set Point High Heat
-045 0.35 0.55
-070-1 through -6 0.45 .081
-070-7 & later 0.45 .075
-090 0.50 0.85
-110 0.45 0.81
-135 0.45 0.85