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8. Combustion Air Inducer (B6) & Pressure Switch
(S18)
All SLP99UHV units are equipped with a combustion air in
ducer (B6) and dual pressure switch assembly (high fire and
low fire). The pressure switch (figure 17) serves four func
tions. First it establishes calibration points for the vent cal
ibration routine. The combustion air inducer's speed at a giv
en 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 installa
tion and interpolates the speeds required to achieve all inter
mediate rates between these two points. The setting for low-
fire switch on the assembly is such that it does not normally
enter into the vent calibration routine.
Second, the switch proves combustion air inducer operation
by sensing a vacuum energizing the control circuit and allow
ing ignition. The low fire pressure switch provides this func
tion.
Third, the switch interrupts the combustion process in the
event vent outlet or combustion air intake blockage.
Finally, the switch interrupts the combustion process if the
condensate 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 entire
assembly. Individual switch components cannot be re
placed.
Combustion Air Inducer Specifications
S Three phase AC induction motor
S Sealed ball bearings
S VFD controlled by the furnace control board
S Operates at 60 to 180Hz
S Voltage range 33 - 110 VAC
S Amp draw 0.75 - 1.0
S Windings resistance 16.74 / 14.26
S Speed range 1000 - 5400 RPM
Combustion Air Inducer Troubleshooting
S Is the CAI mounted correctly?
S Any air or condensate leaks?
S Does the motor and wheel turn freely?
S Any missing or broken mounting screws?
S Perform a visual inspection
S Clean as needed
S Measure the resistance between each of the windings.
All readings should read 16.74 / 14.26 and be approxi
mately 10% of each other.
S Check each winding to ground to confirm there is no
continuity / not grounded
WARNING
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 in
ducer is activated. This will determine if the pressure switch
and furnace are operating properly. However, this may not indi
cate if the sealed combustion system is operating properly.
FIGURE 17
bracket
low fire
high fire
Pressure Switch (S18)
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