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Manifold operating pressure for the G51MP can be measured
at any time the gas valve is open and is supplying gas to the
unit.
TABLE 17
GAS VALVE MANIFOLD PRESSURE W.C.
Natural 3.5 + 0.3
L.P. 10.0 + 0.7
IMPORTANT
For safety, connect a shut-off valve between the
manometer and the gas tap to permit shut off of
gas pressure to the manometer.
Manifold Pressure Measurement and Adjustment
1 − Connect test gauge to outlet tap on gas valve.
2 − Disconnect pressure sensing hose from the gas valve.
Plug end of hose using tape or equivalent. Leave hose
barb on gas valve open to atmosphere.
3 − Start unit and allow 5 minutes for unit to reach steady
state.
4 − While waiting for the unit to stabilize, notice the flame.
Flame should be stable and should not lift from burner.
Natural gas should burn blue. L.P. gas should burn most-
ly blue with some orange streaks.
5 − After allowing unit to stabilize for 5 minutes, adjust the
manifold pressure on the gas valve if necessary (see
table 17).
6 − When test is complete, reconnect sensing hose to gas
valve barb.
NOTE−Shut unit off and remove manometer as soon as
an accurate reading has been obtained. Take care to re-
place pressure tap plug.
F− Proper Gas Flow (Approximate)
Furnace should operate at least 5 minutes before checking
gas flow. Determine time in seconds for two revolutions of
gas through the meter. (Two revolutions assures a more
accurate time.) Divide by two and compare to time in
table 18 below. Adjust manifold pressure on gas valve to
match time needed.
NOTE− To obtain accurate reading, shut off all other gas
appliances connected to meter.
TABLE 18
GAS METER CLOCKING CHART
1 cu ft Dial 2 cu ft Dial 1 cu ft Dial 2 cu ft DIAL
−45 80 160 200 400
−70 51 102 129 258
−90 40 80 100 200
−110 33 66 82 164
−135 27 53 67 134
Natural−1000 btu/cu ft LP−2500 btu/cu ft
G− Proper Combustion
Furnace should operate at least 15 minutes with correct
manifold pressure and gas flow rate before checking com-
bustions. Take sample beyond the flue outlet and compare
to table 19. The maximum carbon monoxide reading
should not exceed 100 ppm.
TABLE 19
Unit Btuh
CO
2
%
For
Nat
CO
2
%
For
L.P
−045 6.3 − 7.3 7.4 − 8.4
−070 6.5 − 7.5 7.6 − 8.6
−090
−110
7.0 − 8.0 8.2 − 9.2
−135
H− Condensate pH Range
The condensate is mildly acidic and can be measured with
pH indicators. The pH scale is a measurement of acidity and
alkalinity. The following scale shows the relative pH of some
common liquids as compared with condensate of G51MP
units. The concentration of the acidity of all these fluids in-
cluding the condensate is very low and harmless.
pH RANGE
0
1
2
3
4
5
6
7
8
9
10
11
Vinegar
Wine
Orange Juice
Tomato Juice
Tap Water
G51 Condensate
pH Range
Increasing
Acidity
Increasing
Alkalinity
I−Flame Signal
A transducer (Part #78H5401 available from Lennox Repair
Parts) is required to measure flame signal if meter used will not
read a low microAmp signal. Seefigure45. The transducer
converts microAmps to volts on a 1:1 conversion. See table 20
for flame signal. A digital readout meter must be used. The
transducer plugs into most meters.
To Measure Flame Signal:
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.
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