Gas Pressure Measurement; Proper Combustion - Lennox SL280DFNV Installation Instructions Manual

5 - Is the filter dirty or plugged? Dirty or plugged filters
will cause the limit control to shut the unit off.
6 - Is gas turned on at the meter?
7 - Is the manual main shut-off valve open?
8 - Is the internal manual shut-off valve open?
9 - Is the unit ignition system in lock out? If the unit locks
out again, call the service technician to inspect the
unit for blockages.
10 - Is pressure switch closed? Obstructed flue will
cause unit to shut off at pressure switch. Check flue
and outlet for blockages.
Gas Pressure Adjustment
IMPORTANT
If you do not follow these instructions exactly,
a fire or explosion may result causing property
damage,personal injury or death.
Gas Flow (Approximate)
Furnace should operate at least 5 minutes before check-
ing gas flow. Determine time in seconds for two revolu-
tions of gas through the meter. (Two revolutions assures a
more accurate time.) Divide by two and compare to time
in table 19 below. If manifold pressure matches table 20
and rate is incorrect, check gas orifices for proper size and
restriction. Remove temporary gas meter if installed.
NOTE - Shut unit off and remove manometer as soon as
an accurate reading has been obtained. Take care to re-
place pressure tap plug.
TABLE 19
GAS METERING CLOCKING CHART
Natural 1000 btu/cu ft
SL280
Seconds For One Revolution
Unit
1 cu ft dial 2 cu fr dial 1 cu ft Dial 2 cu ft Dial
-060 60
-080 45
Supply Line Pressure
A port on the inlet side of the gas valve (figure 26) provides
access to the supply pressure tap. Loosen the screws and
connect a manometer to measure supply pressure. The
minimum supply line pressure is 4.5" - 10.5"w.c. for natural
and 11.0 - 13.0" wc for LP/Propane. Tighten after mea-
surements have been taken.
Manifold Pressure
1 - Remove the threaded plug from the outlet side of
the gas valve (figure 26) and install a field-provided
barbed fitting. Connect measuring device "+"
connection to barbed fitting to measure manifold
pressure.
LP 2500 btu cu/cu ft
120 150 300
90 112 224
2- Start unit and allow 5 minutes for unit to reach steady
state.
3 - After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to value given in
Table 20.
4 - Shut unit off and remove manometer as soon as an
accurate reading has been obtained.
DO NOT ADJUST GAS VALVE
See unit service manual for troubleshooting if manifold
pressure and combustion sample do not meet
specification.
Manifold in wc and LP/Propane Conversion Kit
Altitude
Unit LP Kit
ft
-060 20P40
0-4500
-080 20P41
-060 20P40
4501 -
7500
-080

Proper Combustion

Restart unit and check for any gas leaks. Seal any leaks if
found. Furnace should operate minimum 15 minutes with
correct manifold pressure and gas flow rate before check-
ing combustion. Table 21 shows acceptable combustion
for all models.
Firing Rate
High Fire
Low Fire
The carbon monoxide reading should not exceed 100 ppm.
High Altitude
Units may be installed at altitudes up to 7,500 ft. above
sea level. See Table 18 for de-rate manifold values. Units
installed at altitude of 4501 - 7,500 feet require a pressure
switch change which can be ordered separately. Table 22
lists pressure switch requirements at high altitude. The
combustion air pressure switch is factory-set and requires
no adjustment.
Unit
-060
-080
Page 38
IMPORTANT
TABLE 20
Low
High
Fire
Fire Nat Fire LP
Nat
3.0 - 3.8 1.3 - 1.7 3.4 - 3.8
2.7 - 3.2
2.5 - 3.3 1.3 - 1.7
N/A
TABLE 21
CO2% Nat
6.0 - 7.8
TABLE 22
Pressure Switch
4501 - 7500 ft
Natural
20K91
No Change
High Low
Fire LP
1.5 - 1.9
1.3 - 1.7
N/A N/A
CO2% LP
7.5 - 9.0
LP/propane
No Change
N/A