508148-01
Issue 2128
Page 25 of 75
Primary Limit Control
The primary limit (S10) is located in the heating vestibule
panel. When excess heat is sensed in the heat exchanger,
the limit will open. If the limit is open, the furnace control
energizes the supply air blower and closes the gas valve.
The limit automatically resets when unit temperature
returns to normal. The switch must reset within three
minutes or the control will go into Watch guard for one
hour. The switch is factory set and cannot be adjusted. The
switch may have a different set point for each unit model
number.
Combustion Air Inducer and Cold End
Header Box
All A96US2V units use a two-stage combustion air inducer
to move air through the burners and heat exchanger 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 inducer
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
firststage heat, then switches to high speed for second
stage heat.
The combustion air inducer is installed on the cold end
header box. The cold end header box is a single piece made
of hard plastic. The box has an internal channel where the
combustion air inducer creates negative pressure at unit
start up. The channel contains an orifice used to regulate
flow created by the combustion air inducer. The box has
pressure taps for the combustion air inducer pressure
switch hoses. The pressure switch measures the pressure
differential across the combustion air inducer orifice or
difference in the channel and the box. If replacement is
necessary the gaskets used to seal the box to the vestibule
panel and the combustion air inducer to the box, must also
be replaced.
A proving switch connected to the combustion air inducer
orifice plate is used to prove inducer operation. The
combustion air inducer orifice will be different for each
model. See Table 15 for orifice sizes. The pressure switch
measures the pressure differential across the combustion
air inducer orifice. When the proving switch opens, the
furnace control (A92) immediately closes the gas valve to
prevent burner operation.
Combustion Air Inducer Pressure Switch
A96US2V series units are equipped with a dual combustion
air pressure switch (first and second stage) located on the
combustion air inducer orifice bracket. See Figure 14. 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 pressure
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 15.
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. A bleed port on the switch allows relatively
dry air in the vestibule to purge switch tubing, to prevent
condensate build up.
Figure 14.
3/16 Terminals
Low Fire Switch
High Fire Switch
1/4" Terminals
Tap (negative - )
Tap (po)
Unit
Set Point
Low Heat
Set Point
High Heat
-045
0.40
0.70
-070-3
0.50
0.90
-090-1
0.50
0.85
-090-2 and later
0.50
0.85
-110
0.50
0.90
-135
0.45
0.90
Table 15.
0 - 4500’
