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31-5000731 Rev. 0
Combustion Air Inducer Pressure Switch (S18)
NF80UV series units are equipped with a dual combustion
air pressure switch (first and second stage) located on
the combustion air inducer orifice bracket. The switch is
connected to the combustion air inducer housing by means
of a flexible silicone hose. It monitors negative air pressure
in the combustion air inducer housing.
The switches are a single-pole single-throw proving
switch electrically connected 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.
Set points vary depending on unit size. See Table 6.
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 negative
pressure (pressure becomes more equal with atmospheric
pressure) and opens the circuit to the furnace control and
gas valve.
Unit
Set Point
Low Heat
Set Point
High Heat
-070
0.25
0.60
-090
0.35
0.68
-110
0.30
0.68
Table 6.
Pressure Switch Setting 0 - 4500’
Troubleshooting
See Figure 17 for measuring operating pressure and
checking resistance in the pressure switch.
MULTI
−
METER
SET TO MEASURE OHMS
+
-
or
Field Provided
Tubing To CAI Port
To Pressure Switch
Remove tubing from CAI and
insert “Tee” and additional tubing.
High
Low
Figure 17.