Lennox G32V Series Service Literature page 36

7.Differential Pressure Switch (S102) all
units, (S128) G32V−75 units only
(Combustion Air Prove Switch)
All G32V series units are equipped with a proving switch
S102 located on the vestibule panel.The G32V−75 unit only,
is equipped with a second prove switch S128. The switches
are connected to the combustion air inducer housing by
means of a flexible silicon hose. A separate hose connects
the prove switch to the burner box and the gas valve regula-
tor. The silicon hose on S128 will tee into S102. See fig-
ure31. The switch monitors air pressure in the combustion
air inducer housing and burner box.
Both switches are a single-pole single-throw normally open
pressure switch electrically connected in series with the igni-
tion control. The purpose of the switch is to prevent burner
operation if the combustion air inducer is not operating. The
switch also guards against insufficient combustion air, due to
restrictions in the intake and exhaust piping.
On start-up, the switch senses that the combustion air induc-
er is operating. It closes a circuit to the ignition control when
the difference in pressure across the prove switch increases
above a particular setting. See table 30. The pressure
sensed by the switch is relative to the pressure in the burner
box. In order for the furnace to operate, the larger nega-
tive must always be on the combustion air inducer side
of the switch. If the flue or air inlet become obstructed dur-
ing operation, the switch senses a loss of pressure differen-
tial (drops below set point) and opens the circuit to the igni-
tion control.
The switch is factory set and is not adjustable. It is a safety
shut-down control and MUST not be bypassed.
Figure 32 shows an example of the pressure differential
(.25" 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 prove switch.
G32V Pressure
model Switch
S102
75,000 75,000
btuh
S128
100,000
S102
btuh
125,000
S102
btuh
DIFFERENTIAL PRESSURE SWITCH CIRCUITRY
(G32V3−75 SHOWN)
TEE
S102 PROVE
SWITCH
TEE
S128 PROVE
SWITCH
(G32V3−75
ONLY)
COMBUSTION
AIR BLOWER
Page 36
TABLE 30
Pressure Switch Setting
in. negative W.C..
.40 $ .05
.20 $ .05
.25 $ .05
.20 $ .05
BURNER
BOX
HOSE
BARB
BURNER
SENSING
HOSE
GAS
VALVE
SENSING
HOSE
VALVE
PRESSURE SWITCH
HOSE
FIGURE 31
BOX
GAS