508142-01
Issue 2047
Page 17 of 33
Combustion Air Inducer (B6)
All A80DF1E / 80G1DFE units use a combustion air inducer
to move air through the burners and heat exchanger during
heating operation. The blower uses a 120VAC motor.
The motor operates during all heating operation and is
controlled by integrated control A92. The inducer also
operates for 15 seconds before burner ignition (pre-purge)
and for 5 seconds after the gas valve closes (post-purge).
A pressure switch mounted on the combustion air inducer
orifice plate is used to prove inducer operation. The
combustion air inducer orifice will be different for each
model. The switch monitors air pressure in the inducer
housing. During normal operation, the pressure in the
housing is negative. If pressure becomes less negative
(signifying any obstruction in the flue) the pressure switch
opens. When the pressure switch opens, the integrated
control (A92) immediately de-energizes the gas valve to
prevent burner operation.
Combustion Air Inducer Pressure Switch (S18)
A80DF1E / 80G1DFE series units are equipped with a
combustion air pressure switch located on the combustion
air inducer orifice bracket. The switch is connected to the
combustion air inducer housing by means of a flexible
silicone hose. It monitors negative air pressure in the
combustion air inducer housing.
The switch is a single-pole single-throw proving switch
electrically connected to the furnace control. The
purpose of the switch is to prevent burner operation if
the combustion air inducer is not operating or if the flue
becomes obstructed.
On start-up, the switch senses that the combustion air
inducer is operating. It closes a circuit to the integrated
control when pressure inside the combustion air inducer
decreases to a certain set point. Set points vary depending
on unit size. See Table 7. The pressure sensed by the
switch is negative relative to atmospheric pressure. If
the flue becomes obstructed during operation, the switch
senses a loss of negative pressure (pressure becomes
more equal with atmospheric pressure) and opens the
circuit to the integrated control and gas valve. A bleed port
on the switch allows relatively dry air in the vestibule to
purge switch tubing, to prevent condensate build up.
The switch is factory set and is not field adjustable. It is a
safety shut-down control in the furnace and must not be
bypassed for any reason. If switch is closed or by-passed,
the integrated control will not initiate ignition at start up.
Unit
Break ±
0.05
(in. w.c.)
045
-0.60
070
-0.65
090
-0.60
110
-0.65
Table 7.
Troubleshooting
See Figure 9 for measuring operating pressure and
checking resistance in the pressure switch.
Figure 9.
MULTI−METER
SET TO MEASURE OHMS
+
-
or
Field Provided
Tubing To CAI Port
To Pressure Switch
Remove tubing from CAI and
insert “Tee” and additional tubing.
High
Low
