FIGURE 20
TRANSDUCER
(PART #78H5401)
FIGURE 22
STATIC PRESSURE TEST
Page 27
G−Flame Signal
A microamp DC meter is needed to check the flame signal
on the ignition control.
Flame (microamp) signal is an electrical current which passes
from the furnace control through the sensor during unit opera-
tion. Current passes from the sensor through the flame to
ground to complete a safety circuit.
To Measure Flame Signal − Ignition Control:
A transducer (Part
#78H5401 available from
Lennox Repair Parts) is
required to measure flame
signal if meter used will not
read a low micro amp sig-
nal. Seefigure20. The
transducer converts mi-
croamps to volts on a 1:1 conversion. See table15 for flame
signal . A digital readout meter must be used. The transduc-
er plugs into most meters. See figure 21 for proper use of
transducer.
1 − Set the volt meter to the DC voltage scale. Insert
transducer into the VDC and common inputs. Ob-
serve correct polarities. Failure to do so results in
negative (−) values.
2 − Turn off supply voltage to control.
3 − Disconnect ignition control flame sensor wire from the
flame sensor.
4 − Connect (−) lead of the transducer to flame sensor.
5 − Connect (+) lead of transducer to the ignition control sen-
sor wire.
6 − Turn supply voltage on and close thermostat contacts to
cycle system.
7 − When main burners are in operation for two minutes, take
reading. Remember 1 DC volt = 1 DC microamp.
TABLE 15
Flame Signal in Microamps
Control
Normal
Low
Drop Out
56L84,
10M93
0.18
0.16 − 0.17
0.15
78M47
100973−01
1.5
0.5 − 1.4
0.4
SET DIAL TO
MEASURE VDC
(+)
(−)
(−) TO
FLAME
SENSOR
NOTE−MUST USE DIGITAL METER
RED COLLAR
INDICATES
POSITIVE
LEAD
(+) TO
IGNITION
CONTROL
SENSOR
WIRE
FIGURE 21
WARNING
Fire and explosion hazard.
These instructions MUST be followed exactly.
Can cause a fire or explosion resulting in property
damage, personal injury or loss of life.
V−TYPICAL OPERATING CHARACTERISTICS
A−Blower Operation and Adjustment
NOTE− The following is a generalized procedure and
does not apply to all thermostat controls.
1 − Blower operation is dependent on thermostat control
system.
2 − Generally, blower operation is set at thermostat sub-
base fan switch. With fan switch in ON position, blower
operates continuously. With fan switch in AUTO position,
blower cycles with demand or runs continuously while
heating or cooling circuit cycles.
3 − Depending on the type of indoor thermostat, blower
and entire unit will be off when the system switch is in
OFF position.
B−Temperature Rise
Temperature rise for G40UH(X) units depends on unit in-
put, blower speed, blower horsepower and static pressure
as marked on the unit rating plate. The blower speed must
be set for unit operation within the range of TEMP. RISE
°
F" listed on the unit rating plate.
To Measure Temperature Rise:
1 − Place plenum thermometers in the supply and return air
plenums. Locate supply air thermometer in the first hori-
zontal run of the plenum where it will not pick up radiant
heat from the heat exchanger.
2 − Set thermostat to highest setting.
3 − After plenum thermometers have reached their high-
est and steadiest readings, subtract the two readings.
The difference should be in the range listed on the unit
rating Plate. If the temperature is too low, decrease
blower speed. If temperature is too high, first check the
firing rate. Provided the firing rate is acceptable, in-
crease blower speed to reduce temperature. To
change blower speed taps see the Blower Speed Taps
section in this manual.
C−External Static Pressure
1 − Tap locations shown in figure 22.
2 − Punch a 1/4" diameter hole
in supply and return air ple-
nums. Insert manometer
hose flush with inside edge
of hole or insulation. Seal
around the hose with perma-
gum. Connect the zero end
of the manometer to the dis-
charge (supply) side of the system. On ducted sys-
tems, connect the other end of manometer to the return
duct as above. For systems with non−ducted returns,
leave the other end of the manometer open to the at-
mosphere.
