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8. Combustion Air Inducer (B6) & Pressure Switch
(S18)
All SLP98UHV units are equipped with a combustion air in-
ducer (B6) and
dual
pressure switch assembly (high fire and
low fire). The pressure switch (figure 14) 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.
WARNING
The pressure switch is a safety shut−down control
in the furnace and must not be jumpered for any
reason other than 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 14
bracket
low fire
high fire
Pressure Switch (S18)