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7. Combustion Air Inducer (B6)
& Cold End Header Box
All ML193UHE units use a combustion air inducer to
move air through the burners and heat exchanger during
heating operation. The blower uses a shaded pole
120VAC motor. The motor operates during all heating op
eration and is controlled by integrated control A92. Blower
operates continuously while there is a call for heat. The in
tegrated control will not proceed with the ignition sequence
until combustion air inducer operation is sensed by the prov
ing switches.
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 across the combustion air inducer 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.
TABLE 5
ML193UHE Unit
Combustion Air Inducer
Orifice Size
-030
0.525”
-045
0.700”
-070
1.025”
-090
1.200”
-110
1.406”
8. Combustion Air Pressure Switch
ML193UHE series units are equipped with a differential
pressure switch located on the cold end header box. The
switch monitors across the combustion air inducer orifice to in
sure proper flow through the heat exchanger.
The switch is a SPST N.O. pressure switch electrically con
nected to the integrated control. The purpose of the switch is
to prevent burner operation if the combustion air inducer is not
moving enough air for proper combustion.
FIGURE 10
Pressure Switch
Positive Port
Negative Port
Terminals
On start‐up, the switch monitors whether the combustion air
inducer is operating. It closes a circuit to the integrated
control when the difference in pressure across the com
bustion air inducer orifice exceeds a non-adjustable factory
setting. If the switch does not successfully sense the re
quired differential, the switch cannot close and the fur
nace cannot operate. If the flue or air inlet become ob
structed during operation, the switch senses a loss of
pressure differential and opens the circuit to the integrated
control. If the condensate line is blocked, water will back up
into the header box and reduce the pressure differential
across the switch. The pressure switch opens if the differ
ential drops below the set point. See table 6.
Checks of pressure differential can aid in troubleshooting.
When measuring the pressure differential, readings should be
taken at the pressure switch. See figure 11and table 7. Lack of
differential usually indicates problems in the intake or exhaust
piping, but may indicate problems in the heat exchanger,
condensing coil, header boxes, combustion inducer or
other components.
TABLE 6
Unit
Altitude ft.
0 - 4500
4501 - 7500
7501 - 10000
Set Point “w.c.
Set Point “w.c
Set Point “w.c.
-030
-0.65
-0.60
-0.55
-045
-0.65
-0.60
-0.55
-070
-0.65
-0.60
-0.55
-090
-0.65
-0.60
-0.55
-110
-0.65
-0.60
-0.55
*Set point is factory set and non-adjustable
