Lennox SL280DFNV Series Unit Information page 27

7. Combustion Air Inducer (B6)
All SL280DFNV units use a two-stage combustion air in-
ducer to move air through the burners and heat exchang-
er during heating operation. The blower uses a 120VAC
motor. The motor operates during all heating operation
and is controlled by integrated control control A92. The in-
ducer also operates for 15 seconds before burner ignition
(prepurge) and for 5 seconds after the gas valve closes
(postpurge). The inducer operates on low speed during
firststage heat, then switches to high speed for second
stage heat.
8. Combustion Air Inducer Prove Switch (S18)
SL280DFNV 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 sin-
gle-pole single-throw proving switch electrically connect-
ed 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
circuit to the integrated control when pressure inside the
combustion 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 SL280DFNV models.
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 pressure dif-
ference and opens the circuit to the furnace control and
gas valve. If replacing switch or switch tubing make note
of all connections and see FIGURE 16.
NOTE - The switch is factory set and is not field adjust-
able. It is a safety shut-down control in the furnace and
must not be by-passed for any reason. If switch is closed
or bypassed, the control will not initiate ignition at start up.
Replacing the Pressure Switch or Tubing
FIGURE 16
Page 27
COMBUSTION AIR PRESSURE SWITCH
Positive +
to air intake pipe
FIGURE 15
Negative -
to air intake
elbow