Page 16
8. Combustion Air Inducer (B6)
& Cold End Header Box
All ML193DF units use a combustion air inducer to move air
through the burners and heat exchanger during heating op
eration. The blower uses a shaded pole 120VAC motor.
The motor operates during all heating operation and is con
trolled by burner ignition control A3. Blower operates con
tinuously 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 in
ducer creates negative pressure at unit start up. The chan
nel contains an orifice used to regulate flow created 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 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 9
ML193DF Unit
C.A.I. Orifice Size
-045
0.618
-070
0.891
-090
1.031
-110
1.297
9. Combustion Air Pressure Switch
(Figure 15)
ML193DF series units are equipped with a differential pres
sure 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 15
Pressure Switch
24VAC
TERMINALS
BRACKET
TAP
TAP
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, wa
ter 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 table 10.
Checks of pressure differential can aid in troubleshooting.
When measuring the pressure differential, readings should
be taken at the pressure switch. See figure 16. Lack of dif
ferential usually indicates problems in the intake or exhaust
piping, but may indicate problems in the heat exchanger,
condensing coil, header boxes, combustion inducer or oth
er components.
TABLE 10
ML193DF
Altitude ft
0 - 4500
4501 -
7500
7501 - 10000
Set Point
SetPoint
Set Point
-045
-0.65
-0.60
-0.55
-070
-090
-110
*Set point is factory set and non-adjustable
