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7. Combustion Air Inducer (B6) and
Cold End Header Box
All EL296UHE units use a two-speed combustion air induc
er to move air through the burners and heat exchanger dur
ing heating operation. The blower uses a 120VAC motor.
The motor operates during all heating operation and is con
trolled 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 single
piece made of hard plastic. The box has an internal chan
nel where the combustion air inducer creates negative
pressure at unit start up. The channel contains an orifice
used to regulate flow created by the combustion air induc
er. The box has pressure taps for the combustion air induc
er pressure switch hoses. The pressure switch measures
the pressure differential across the cold end header box
orifice or difference 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 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 fur
nace control (A92) immediately closes the gas valve to pre
vent burner operation.
TABLE 10
EL296UHE Unit
CEHB Orifice Size
-045 0.618
-070 0.810
-090 0.920
-110 1.040
-135 1.235
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 operation
if the combustion air inducer is not operating, the inlet or ex
haust 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 cir
cuit to the integrated control when pressure across the
CEHB orifice becomes greater then the switch set point.
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 senses the
loss of differential pressure and opens the circuit to the fur
nace 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.
3/16 Terminals
Low Fire Switch
High Fire Switch
1/4
" Terminals
Tap (negative - )
Tap (positive +)
FIGURE 16
Combustion Air Inducer Pressure Switch
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 11 0- 4500'
Unit Set Point Low Heat Set Point High Heat
-045 0.40 0.70
-070 0.50 0.90
-090 0.50 0.90
-110 0.50 0.90
-135 0.45 0.90
TABLE 12 4501 - 7500'
Unit Set Point Low Heat Set Point High Heat
-045 0.35 0.55
-070 0.45 0.81
-090 0.50 0.85
-110 0.45 0.81
-135 0.45 0.85
TABLE 13 7501 - 10.000'
Unit Set Point Low Heat Set Point High Heat
-045 0.35 0.50
-070 0.41 0.74
-090 0.45 0.81
-110 0.41 0.74
-135 0.41 0.74
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