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7. Combustion Air Inducer (B6)
All SL280UHNV units use a two-stage combustion air in
ducer to move air through the burners and heat exchanger
during heating operation. The blower uses a 120VAC mo
tor. The motor operates during all heating operation 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 second stage
heat.
8. Combustion Air Inducer Prove Switch (S18)
SL280UHNV series units are equipped with a single com
bustion air proving switch (first and second stage) located
near the gas valve. See figure 15. The switch is a single-pole
single-throw proving switch electrically connected to the in
tegrated control. The purpose of the switch is to prevent
burner operation if the combustion air inducer is not operat
ing 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. The set
point is 0.20” for low heat and high heat and the same for all
SL280UHNV models. 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 pressure difference and opens the circuit to the fur
nace control and gas valve. If replacing switch or switch
tubing make note of all connections and see figure 16.
FIGURE 15
COMBUSTION AIR PRESSURE SWITCH
Po
to air intake pipe
Negative -
to air intake
elbow
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.
FIGURE 16
Replacing the Pressure Switch or Tubing
