Lennox ML196DF045XE36B, Unit Information

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Page 14
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 heat -
ing operation. The blower uses a shaded pole 120VAC
motor. The motor operates during all heating operation
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 se -
quence 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 creat -
ed by the CAI. The box has pressure taps for the CAI
pressure 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 gaskets
used to seal the box to the vestibule panel and the CAI to
the box, must also be replaced.
TABLE 6
ML196DFE Unit C.A.I. Orifice Size in.
-045 0.618
-070 0.810
-090 0.905
-110 1.100
8. Combustion Air Pressure Switch (FIGURE 11)
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.
Pressure Switch
24VAC
TERMINALS
BRACKET
TAP
TAP
FIGURE 11
On start-up, the switch senses that the combustion air in -
ducer is operating. It closes a circuit to the ignition control
when the difference in pressure across the CAI orifice ex -
ceeds a non-adjustable factory setting. If the switch does
not successfully sense the required differential, the switch
cannot 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 differential across the switch. The prove switch
opens if the differential drops below the set point. See TA -
BLE 7.
Checks of pressure differential can aid in troubleshoot
-
ing. When measuring the pressure differential, readings
should be taken at the pressure switch. See FIGURE 12
and TABLE 8. Lack of differential usually indicates prob -
lems in the intake or exhaust piping, but may indicate
problems in the heat exchanger, condensing coil, header
boxes, combustion inducer or other components.
TABLE 7
ML196DFE
Unit
Altitude ft
0-4500 4501-7500
7501-
10,000
Set Point Set Point 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