Lennox EL296DFE Series Manual page 21

7. Combustion Air Inducer (B6) and Cold End Header
Box
All units use a two-speed combustion air inducer to move
air through the burners and heat exchanger during heating
operation. The blower uses a 120VAC motor. 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.
The combustion air inducer is installed on the cold end
header box (CEHB). The cold end header box is a sin-
gle piece made of hard plastic. The box has an internal
channel where the combustion air inducer creates neg-
ative pressure 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 mea-
sures the pressure differential across the cold end head-
er box orifice or difference in the channel and the box.
If replacement 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 pressure switch measures the pressure differential
across the CEHB orifice to prove inducer operation. The
CEHB orifice will be different for each model. See table
10 for orifice sizes. When the proving switch opens, the
furnace control (A92) immediately closes the gas valve to
prevent burner operation.
TABLE 10
Unit
-045
070-1, -2
-070-3 and later
-090
-110
8. Combustion Air Inducer Pressure Switch (S18)
(Figure 16)
Units are equipped with a dual combustion air pressure
switch (first and second stage) located on the CEHB. The
switch is connected to the cold end header box by means
of flexible silicone hoses. It monitors negative differential
pressure across the cold end header box orifice.
The switches are a single-pole single-throw pressure
switches electrically connected to the integrated control.
The purpose of the switches is to prevent burner operation
if the combustion air inducer is not operating, the inlet or
exhaust pipes or heat exchanger are restricted.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 across the CEHB or-
ifice becomes greater then the switch set point
CEHB Orifice Size
0.618
0.810
0.920
0.920
1.040
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Set points vary depending on unit size. See table 11. Both
pressures sensed by the switches are negative relative
to atmospheric pressure. If the inlet or exhaust pipes, or
the heat exchanger become obstructed, the switch sens-
es the loss of differential pressure and opens the circuit to
the furnace integrated 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 Inducer Pressure Switch
Tap (negative - )
Low Fire Switch
1/4 " Terminals
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
Unit Set Point Low Heat
-045 0.35
-070-1, -2 0.45
-070-3 & later 0.45
-090 0.50
-110 0.45
TABLE 12
4501 - 7500'
Unit Set Point Low Heat
-045 0.35
-070-1, -2 0.45
-070-3 & later 0.45
-090 0.50
-110 0.45
TABLE 13
7501 - 10,000'
Unit Set Point Low Heat
-045 0.35
-070-1, -2 0.41
-070-3 & later 0.40
-090 0.45
-110 0.41
Tap (positive +)
3/16 Terminals
High Fire Switch
0- 4500'
Set Point High Heat
0.60
0.90
0.81
0.90
0.90
Set Point High Heat
0.55
0.81
0.70
0.85
0.81
Set Point High Heat
0.50
0.74
0.65
0.81
0.74