Page 17
13.Combustion Air Inducer (B6)
All G40DF(X) 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
furnace control A92. The inducer also operates for 15 sec-
onds before burner ignition (pre-purge) and for 5 seconds
after the gas valve closes (post-purge).
A proving switch connected to the combustion air inducer ori-
fice plate (figure 17) is used to prove inducer operation. The
combustion air inducer orifice will be different for each model.
See table 13 for orifice sizes. The switch monitors air pres-
sure in the inducer housing. During normal operation, the
pressure in the housing is negative. If pressure becomes less
negative (signifying an obstruction) the proving switch opens.
When the proving switch opens, the furnace control (A92) im-
mediately closes the gas valve to prevent burner operation.
TABLE 13
G40DF(X) Unit
C.A.I. Orifice Size
−045
1.438"
−070
1.688"
−090
1.875"
−110
2.250"
−135
2.600"
FIGURE 17
Flue Box Gasket
Flue Collector Box
Gasket
Combustion
Air Inducer
Combustion Air
Orifice Plate
COMBUSTION AIR INDUCER ASSEMBLY
Prove Switch
*
−1 to −14
pressure
port location
Pressure Port
14.Combustion Air Inducer
Proving Switch (S18)
G40DF(X) series units are equipped with a combustion air
proving switch located on the combustion air inducer orifice
bracket. The switch is connected to the combustion air in-
ducer housing by means of a flexible silicone hose. It moni-
tors negative air pressure in the combustion air inducer
housing.
The switch is a single-pole single-throw proving switch elec-
trically connected to the furnace control. The purpose of the
switch is to prevent burner operation if the combustion air in-
ducer is not operating or if the flue becomes obstructed.
On start-up, the switch senses that the combustion air in-
ducer is operating. It closes a circuit to the furnace control
when pressure inside the combustion air inducer de-
creases to a certain set point. Set points vary depending on
unit size. See table 14. The pressure sensed by the switch
is negative relative to atmospheric pressure. If the flue be-
comes obstructed during operation, the switch senses a
loss of negative pressure (pressure becomes more equal
with atmospheric pressure) and opens the circuit to the fur-
nace control and gas valve. A bleed port on the switch al-
lows relatively dry air in the vestibule to purge switch tub-
ing, 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 by−
passed for any reason. If switch is closed or by−passed, the
control will not initiate ignition at start up.
TABLE 14
PROVE SWITCH SET POINTS
Unit
0 − 4500’
(0 − 1372m)
4501’ − 7500’
(1373 −
2286m)
7501’ −
10000’
(2286 −
3098m)
−045−1 to −14
.36" (89Pa)
.36" (89Pa)
.36" (89Pa)
−070−1 to −14
.36" (89Pa)
.36" (89Pa)
.36" (89Pa)
−090−1 to −14
.47" (116Pa)
.43" (106Pa)
.36" (89Pa)
−110−1 to −14
.51" (126Pa)
.43" (106Pa)
.36" (89Pa)
−135−1 to −14
.51" (126Pa)
.47" (116Pa)
.43" (106Pa)
ALL −15 and later
models
.0.31" (77Pa)
NOTE−Some units require a high altitude pressure
switch kit if installed above 4500 ft (1370 m).