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7. Combustion Air Inducer (B6)
& Cold End Header Box
All ML196DFE 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 burner ignition control A92.
Blower operates continuously while there is a call for heat.
The burner ignition control will not proceed with the ignition
sequence until combustion air inducer operation is sensed
by the proving switches.
The CAI 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 pressure at unit start up. The
channel contains an orifice used to regulate flow created
by the CAI. The box has pressure taps for the CAI pres
sure switch hoses. The pressure switch measure the
pressure across the CAI orifice or difference in the chan
nel and the box. If replacement is necessary the gas
kets used to seal the box to the vestibule panel and
the CAI to the box, must also be replaced.
TABLE 5
ML196DFE Unit
C.A.I. Orifice Size
-045 0.618”
-070 0.810”
-090 0.905”
-110 1.100”
8. Combustion Air Pressure Switch
(Figure 10)
ML196DFE series units are equipped with a differential
pressure switch located on the cold end header box. The
switches monitor across the CAI orifice to insure proper flow
through the heat exchanger.
The switch is a SPST N.O. prove 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
24VAC
TERMINALS
BRACKET
TAP
TAP
On start‐up, the switch senses that the combustion air inducer
is operating. It closes a circuit to the ignition control when
the difference in pressure across the CAI orifice exceeds a
non-adjustable factory setting. If the switch does not suc
cessfully sense the required differential, the switch can
not close and the furnace cannot operate. If the flue or
air inlet become obstructed during operation, the switch
senses a loss of pressure differential and opens the circuit to
the ignition control. If the condensate line is blocked, water
will back up into the header box and reduce the pressure dif
ferential across the switch. The prove switch opens if the dif
ferential 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 11 and table 7. Lack
of differential usually indicates problems in the intake or ex
haust piping, but may indicate problems in the heat ex
changer, condensing coil, header boxes, combustion
inducer or other components.
TABLE 6
ML196DFE
Unit
Altitude ft
0 - 4500 4501 - 7500 7501 - 10000
Set Point SetPoint Set Point
-045
-0.65 -0.60 -0.55
-070
-0.90 -0.80 -0.70
-090
-0.90 -0.85 -0.65
-110
-1.00 -0.85 -0.65
*Set point is factory set and non-adjustable
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