Dual Pressure Switch; Single Pressure Switch - Lennox G71MPP SERIES Unit Information

negative and positive taps
(same on other side)
The CAI 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 CAI. The box has pressure taps for the CAI pres-
sure switch hoses.
The dual pressure switches measure the pressure differ-
ential across the CAI orifice (difference in the channel
and cold end header box). The single pressure switch
measures differential across the burner box and chan-
nel. A window is provided on the bottom right hand side of
the cold end header box to indicate orifice size. See fig-
ure 15. See table 30 for orifice size per unit. If replace-
ment is necessary the gaskets used to seal the box to
the vestbule panel and the CAI to the box, must also
be replaced.
TABLE 30
G71MPP Unit
−070
−090
−110
−135
orifice
FIGURE 15
C.A.I. Orifice Size
0.938"
1.063"
1.250"
1.625"
Page 35
Duel Pressure Switch
1 − Remove thermostat demand and allow unit to cycle off.
2 − Install a tee in the negative (−) line and a tee in the positive (+)
line running from the lower pressure switch to the cold end
header box.
3 − Install a manometer with hose from the negative (−) side of
the manometer to the tee installed in the negative (−) line
and with hose from the positive (+) side of the manometer
to the tee in the positive (+) line.
NOTE − Both sides of the cold end header box are negative.
However the (+) port reads less negative pressure than the (−)
port.
4 − Operate unit and observe manometer reading.
Readings will change as heat exchanger warms.
a. Take one reading immediately after start-up.
b. Take a second reading after unit has reached steady
state (approximately 5 minutes). This will be the pressure
differential.
The pressure differential should be greater
than those listed in tables 31 and 32.
5 − Remove thermostat demand and allow to cycle off.
+
−
6 − Remove manometer and tee's. Reinstall combustion air
sensing hoses to the pressure switch.
Single Pressure Switch with Pressure Conditioning Device
7 − Repeat steps 1 through 6 for the single pressure switch
with pressure conditioning device. Tee into both hoses from
the pressure switch to the gas valve and install manometer.
Readings should be greater than those shown in table 33.
TABLE 31

Dual Pressure Switch

0' to 7500'
Set Point
G71MPP Unit
High Fire
−070
−090
−110
−135
TABLE 32*
Dual Pressure Switch
7501' to 10,000'
Set Point
G71MPP Unit
High Fire
−070
−090
−110
−135
*Unit may require conversion kit at this altitude. See High Altitude
table.
TABLE 33

Single Pressure Switch

G71MPP Unit
−070
−090
−110
−135
Set Point
Low Fire
1.37" 0.30"
1.22" 0.25"
1.32" 0.20
0.83" 0.15"
Set Point
Low Fire
1.22" 0.30"
1.02" 0.25"
1.12" 0.20"
0.63" 0.15"
Set Point
High Fire
.27"