Lennox ML193DFE Series Unit Information page 14

7. Combustion Air Inducer (B6)
& Cold End Header Box
All ML193DFE 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 operation and is
controlled by burner ignition control A3. Blower operates
continuously while there is a call for heat. The burner ig-
nition 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 pres-
sure across the CAI 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 CAI
to the box, must also be replaced.
TABLE 6
ML193DFE Unit
-045
-070
-090
8. Combustion Air Pressure Switch (FIGURE 12)
ML193DFE 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.
C.A.I. Orifice Size in.
0.700
1.025
1.120
Page 14
Pressure Switch
Negative Port
FIGURE 12
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 13.
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 7
ML196DFE
0-4500
Unit
Set Point
-045
-070 -0.65
-090
*Set point is factory set and non-adjustable
Terminals
Positive Port
Altitude ft
7501-
4501-7500
10,000
Set Point Set Point
-0.60 -0.55