Lennox F8AUH series Service Manual page 15

When checking piping connections for gas leaks, use pre-
ferred means. Kitchen detergents can cause harmful corro-
sion on various metals used in gas piping. Use of a specialty
Gas Leak Detector is strongly recommended.
Do not use matches, candles, flame or any other source of
ignition to check for gas leaks.
D−Testing Gas Supply Pressure
When testing supply gas pressure, connect test gauge to
inlet pressure tap (field provided). See figure 16. Check
gas line pressure with unit firing at maximum rate. Low
pressure may result in erratic operation or underfire. High
pressure can result in permanent damage to gas valve or
overfire. For F8AUH−45,60 and 75 BTUH natural gas
units, operating pressure at the unit must be a minimum
4.5"W.C. For the F8AUH−100 and 120 BTUH units, the op-
erating pressure must be a minimum of 5.0"W.C. For L.P.
gas units, operating pressure at unit gas connection must
be a minimum of 11.0" W.C.
On multiple unit installations, each unit should be checked
separately, with and without other units operating. Supply
pressure must fall within range listed in previous para-
graph.
E−Check Manifold Pressure
After line pressure has been checked and adjusted, check
manifold pressure. Move pressure gauge to outlet pres-
sure tap located on unit gas valve (GV1). Checks of man-
ifold pressure are made as verification of proper regulator
adjustment. Manifold pressure for the F8AUH can be mea-
sured at any time the gas valve is open and is supplying gas
to the unit. Normal manifold pressure for natural gas units is
3.5 in. W.C. For LP/propane gas the correct manifold pres-
sure is 9.5 in. W.C.
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.
TABLE 6
GAS VALVE REGULATION
Unit (Fuel) Operating Pressure (outlet) in. W.C.
Natural
L.P
.
The gas valve is factory set and should not require adjustment.
All gas valves are factory regulated. See table 6. See specifica
tions section of this manual for High Altitude manifold pressure
settings.
3.5 +0 −0.3
9.5 + 0.5
Manifold Adjustment Procedure:
1 − Connect a test gauge to outlet pressure tap on gas
valve. Start unit and allow 5 minutes for unit to reach
steady state.
2 − 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
mostly blue with some orange streaks.
3 − After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to values given in
table 6.
NOTE−Shut unit off and remove manometer as soon as
an accurate reading has been obtained. Take care to
replace 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 7 below. Ad
just manifold pressure on gas valve to match time needed.
NOTE− To obtain accurate reading, shut off all other
gas appliances connected to meter.
GAS METER CLOCKING CHART
FAUH
Unit U i
1 cu ft
Dial
−45 80
−60 60
−75 48
−100 36
−120 30
Natural−1000 btu/cu ft
For safety, shut unit off and remove manometer as
soon as an accurate reading has been obtained.
Take care to replace pressure tap plug.
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.
Page 15
TABLE 7
Seconds for One Revolution
Natural
2 cu ft 1 cu ft
Dial Dial
160 200
120 150
96 120
72 90
60 75
LP−2500 btu/cu ft
IMPORTANT
LP
2 cu ft
DIAL
400
300
240
180
150