Page 14
31-5000729 Rev. 0
Combustion Air Inducer (B6) & Cold End Header Box
All NF96DS 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 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 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.
Combustion Air Pressure Switch (Figure 10)
NF96DS 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
insure proper flow through the heat exchanger.
The switch is a SPST N.O. pressure switch electrically
connected 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
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 combustion air
inducer orifice exceeds 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 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 differential drops below the set point. See
Table 5.
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 6.
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.
Unit
Altitude (ft.)
0 - 4500
Set Point “w.c.
045
-0.65”
070
-0.90”
090
-0.90”
NOTE:
Set point is factory set and non-adjustable
Table 5.
