Lennox SL280UHNV - Page 32

Page 32
7. Combustion Air Inducer (B6)
All SL280UHNV 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
rst 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 (rst 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 ue becomes obstructed.
On heat demand (rst 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 SL280UHNV models.
COMBUSTION AIR PRESSURE SWITCH
Positive +
to air intake pipe
Negative -
to air intake
elbow
FIGURE 16
The pressure sensed by the switch is negative relative
to atmospheric pressure. If the ue 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 eld 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 Pressure Switch Or Tubing
-01 THROUGH -04 MODELS
-05 MODELS
FIGURE 17