Page 16
BURNER BOX PRESSURE
(Negative inches water gauge
−1.0
−0.2
0
−0.4
−0.6
−0.8
2.5
3.0
3.5
2.6
2.7
2.8
2.9
3.1
3.2
3.3
3.4
GAS V
A
L
VE OUTPUT
MANIFOLD PRESSURE (positive inches water column)
NORMAL OPERATION (Natural Gas Units)
measured on right side of burner box)
Gray area indicates normal operating range + 10% of manifold pressure
The purpose of this chart is to explain unit operation . Each unit may vary
depending on installation, altitude, intake/exhaust configuration and other
factors.
A
OPERATION AT THIS EXTREME
MAY INDICATE A BLOCKED
OUTLET OR OTHER PROBLEM
OPERATION AT THIS EXTREME
MAY INDICATE A BLOCKED
INLET OR OTHER PROBLEM
"
FIGURE 17
Figure 17 show how gas valve output changes as burner
box pressure changes. Generally, a lower burner box
pressure produces a leaner gas/air mixture and a higher
burner box pressure produces a richer mixture. A proce
dure showing how to check manifold pressure is shown on
page 22.
14.Differential Pressure Switch (S102) all
units, (S128) G32−75 units only
(Combustion Air Prove Switch)
All G32 series units are equipped with a differential pressure
switch S102 located on the vestibule panel.The G32−75 unit
only, is equipped with a second pressure switch S128. The
switches are connected to the combustion air blower hous
ing by means of a flexible silicon hose. A separate hose con
nects the pressure switch to the burner box and the gas
valve regulator. The silicon hose on S128 will tee into S102.
See figure18. The switch monitors air pressure in the com
bustion air blower housing and burner box.
Both switches are a singlepole singlethrow normally open
pressure switch electrically connected in series with the igni
tion control. The purpose of the switch is to prevent burner
operation if sufficient combustion air is not available.
On startup, the switch senses that the combustion air blow
er is operating. It closes a circuit to the ignition control when
the difference in pressure across the pressure switch in
creases above a particular setting. See table 12. The pres
sure sensed by the switch is relative to the pressure in the
burner box.
In order for the furnace to operate, the larger
negative must always be on the combustion air blower
side of the switch
.
If the flue or air inlet become obstructed
during operation, the switch senses a loss of pressure differ
ential (drops below set point) and opens the circuit to the
ignition control.
FIGURE 18
BURNER
BOX
GAS
VALVE
SENSING
HOSE
COMBUSTION
AIR BLOWER
BURNER
BOX
SENSING
HOSE
GAS
VALVE
TEE
HOSE
BARB
HIGH HEAT
DIFFERENTIAL
PRESSURE
SWITCH
(G32−75 ONLY)
TEE
DIFFERENTIAL PRESSURE SWITCH CIRCUITRY
(G32−75 SHOWN)
LOW HEAT
DIFFERENTIAL
PRESSURE
SWITCH
PRESSURE SWITCH
HOSE
The switch is factory set and is not adjustable. It is a safety
shutdown control and
MUST
not be bypassed.
Figure 19 shows an example of the pressure differential
(.20" W.C. shown) required to obtain unit operation. If the
switch does not successfully sense the required differen
tial, the switch cannot close and the furnace cannot oper
ate.
When measuring the pressure differential, readings should
be taken at the pressure switch.
TABLE 12
G32
model
Pressure
Switch
Pressure Switch Setting
in. negative W.C..
75,000
S102
.40
$
.05
75,000
btuh
S128
.20
$
.05
100,000
btuh
S102
.25
$
.05
125,000
btuh
S102
.20
$
.05