Lennox SLP99UHV Series Unit Information page 41

8. Combustion Air Inducer (B6) & Pressure Switch
(S18)
All SLP99UHV units are equipped with a combustion air
inducer (B6) and dual pressure switch assembly (high fire
and low fire). The pressure switch (FIGURE 17) serves
four functions. First it establishes calibration points for
the vent calibration routine. The combustion air inducer's
speed at a given firing rate is a function of the vent system
resistance. The calibration routine establishes the inducer
speed required to make low and high fire switches for a
given vent pipe installation and interpolates the speeds
required to achieve all intermediate rates between these
two points. The setting for lowfire switch on the assembly
is such that it does not normally enter into the vent cali-
bration routine.
Second, the switch proves combustion air inducer oper-
ation by sensing a vacuum energizing the control circuit
and allowing ignition. The low fire pressure switch pro-
vides this function.
Third, the switch interrupts the combustion process in the
event vent outlet or combustion air intake blockage. Final-
ly, the switch interrupts the combustion process if the con-
densate drainage system becomes blocked to the point
the condensate level builds up in the cold end header box/
secondary coil or vent system.
If the switch assembly is to be replaced, replace the en-
tire assembly. Individual switch components cannot be
replaced.
Combustion Air Inducer Specifications
• Three phase AC induction motor
• Sealed ball bearings
• VFD controlled by the furnace control board
• Operates at 60 to 180Hz
• Voltage range 33 - 110 VAC
• Amp draw 0.75 - 1.0
• Windings resistance 16.74 / 14.26
• Speed range 1000 - 5400 RPM
Combustion Air Inducer Troubleshooting
• Is the CAI mounted correctly?
• Any air or condensate leaks?
• Does the motor and wheel turn freely?
• Any missing or broken mounting screws?
• Perform a visual inspection
• Clean as needed
• Measure the resistance between each of the wind-
ings. All readings should read 16.74 / 14.26 and be
approximately 10% of each other.
• Check each winding to ground to confirm there is
no continuity / not grounded
The pressure switch is a safety shut-down control
in the furnace and must not be jumpered for any
reason other than troubleshooting.
Pressure Switch Troubleshooting
To troubleshoot the pressure switch, add a temporary
jumper. The unit will not fire with the switch jumpered.
Therefore, the pressure switch must be bypassed after
the combustion air inducer is activated. This will determine
if the pressure switch and furnace are operating properly.
However, this may not indicate if the sealed combustion
system is operating properly.
Pressure Switch (S18)
low fire
Page 41
WARNING
FIGURE 17
bracket
high fire