Lennox EL296UHE Series Unit Information page 21

7. Combustion Air Inducer (B6) and Cold End Header
Box
All EL296UHE units use a two-speed 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.
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 all - numbers
-070-1 & -2
-070-3 & later
-090-1
-090-2 and later
-110-1
-110-2 and later
-135 all - numbers
8. Combustion Air Inducer Pressure Switch (S18)
(Figure 16)
EL296UHE series units are equipped with a dual combus-
tion 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 nega-
tive 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 oper-
ation 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 orifice becomes greater then the switch set point.
CEHB Orifice Size
0.618
0.810
0.920
0.920
0.905
1.040
1.070
1.235
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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
-045
-070-1 & -2
-070-3 & later
-090
-110
-135
TABLE 12
4501 - 7500'
Unit Set Point Low
-045
-070-1 & -2
-070-3 & later
-090
-110
-135
TABLE 13
7501 - 10,000'
Unit Set Point Low
-045
-070-1 & -2
-070-3 & later
-090
-110
-135
Tap (positive +)
3/16 Terminals
High Fire Switch
0- 4500'
Set Point High
0.40 0.70
0.90
0.85
0.50
0.90
0.90
0.45 0.90
Set Point High
0.35 0.55
0.45 0.81
0.45 0.75
0.50 0.85
0.45 0.81
0.45 0.85
Set Point High
0.35 0.50
0.41 0.74
0.40 0.70
0.45 0.81
0.41 0.74
0.41 0.74