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7. Combustion Air Inducer (B6)
All SL297UHNV units use a two-stage combustion air in
ducer (figure 15) to move air through the burners and heat
exchanger during heating operation. The blower uses a
120VAC motor. The motor operates during all heating op
eration and is controlled by integrated control 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). The inducer operates on low speed
during first-stage heat, then switches to high speed for sec
ond stage heat.
8. Combustion Air Inducer
Prove Switch (S18)
SL297UHNV series units are equipped with a dual combus
tion air proving switch (first and second stage) located on
the combustion air inducer housing. See figures 15 and 16.
The switch is connected to the combustion air inducer hous
ing by means of a flexible silicone hose. It monitors negative
air pressure in the combustion air inducer housing.
The switches are a single‐pole single‐throw proving switch
electrically connected to the integrated 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 heat demand (first or second stage) the switch senses
that the combustion air inducer is operating. It closes a cir
cuit to the integrated control when pressure inside the com
bustion air inducer decreases to a certain set point.
Set points vary depending on unit size. See table 17. The
pressure sensed by the switch is negative relative to atmo
spheric pressure. If the flue becomes obstructed during op
eration, the switch senses a loss of negative pressure
(pressure becomes more equal with atmospheric pres
sure) and opens the circuit to the furnace 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.
FIGURE 15
COMBUSTION AIR INDUCER AND PROVE SWITCH
Prove Switch
Combustion Air Inducer
FIGURE 16
COMBUSTION AIR PRESSURE SWITCH
3/16 Terminals
Low Fire Switch
High Fire Switch
1/4" Terminals
Tap (negative - )
Tap (po)
NOTE -
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 17 0- 4500'
Unit
Set Point Low Heat
Set Point High Heat
-040
0.15”
0.50”
-060
-080
