Combustion Air Inducer; Cai & Cold End Header Box Assembly - Lennox G61MPV Service Manual

Measuring pressure differential Figures 18 and 19
The differential pressure is the difference in pressure mea-
sured across the cold end header box orifice.
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 one of the prove switches 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 nega-
tive (−) 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.
CAI & COLD END HEADER BOX ASSEMBLY
(−045 SHOWN)
_
+
prove switches orifice size
cold end header box
FIGURE 18
CAI & COLD END HEADER BOX ASSEMBLY
(−070, −071, −090, −091, −110, −111, −135 SHOWN)
_
+
negative and positive taps
(same on other side)
FIGURE 19
Install tee's in the
negative line and
positive line then
connect hoses to
manometer.
_
+
Install tee's in the
negative line and
positive line then
connect hoses to
manometer.
orifice size
prove switches
Page 45
4 − Operate unit and observe manometer reading. Read-
ings 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 pres-
sure differential.
The pressure differential should be greater
than those listed in tables 26 through 31.
5 − Remove thermostat demand and allow to cycle off.
6 − Remove manometer and tee's. Reinstall combustion air
sensing hoses to the prove switch.
7 − Repeat steps 1 through 6 for the other prove switch.
14. Combustion Air Inducer (B6)
All G61MPV units use a two−speed combustion air induc-
er (CAI) to move air through the burners and heat ex-
changer during heating operation. The inducer uses a
PSC 120VAC motor. The motor operates during all heating
operation and is controlled by the ignition control A92. In-
ducer operates continuously while there is a call for heat.
The burner ignition control will not proceed with the ignition
sequence until combustion air inducer operation is sensed
by the proving switches.
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 prove
switch hoses.
The prove switches measure the pressures across the
CAI orifice or difference in the channel and the box. A
window is provided on the bottom right hand side of the
box to indicate orifice size. See figure 19. See table 25 for
orifice size per unit. If replacement is necessary the
gaskets used to seal the box to the vestbule panel
and the CAI to the box, must also be replaced.