Proper Combustion; Rate Check; High Altitude - Lennox SL280UHNV Installation Instructions Manual

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.
Supply Line Pressure
A port on the inlet side of the gas valve 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 measurements
have been taken.
IMPORTANT
For safety, shut unit off and remove manometer as soon
as an accurate reading has been obtained. Take care to
replace pressure tap plug.
Proper 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 17 below.
NOTE- To obtain accurate reading, shut off all other gas
appliances connected to meter.
TABLE 17
gAS METER CLOCKINg CHART
Seconds for One Revolution
Natural
SL280N Unit
1 cu ft
Dial
060 60
080 45
100 36
Natural-1000 btu/cu ft - Propane - 2500btu/cu ft

Rate Check

Manifold Pressure
To correctly measure manifold pressure, follow the steps
below. .
1 - Remove the threaded plug from the outlet side of
the gas valve and install a field-provided barbed
fitting. Connect measuring device "+" connection to
barbed fitting to measure manifold pressure.
Start unit and allow 5 minutes for unit to reach
steady state.
LP Propane
2 cu ft 1 cu ft 2 cu ft
Dial Dial Dial
120 150 300
90 112 224
72 N/A N/A
2 - After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to value given in
Table 18. Make manifold adjustment if necessary
3 - Shut unit off and remove manometer as soon as an
accurate reading has been obtained.
Manifold in wc and LP/Propane Conversion Kit
Altitude
Unit LP Kit
ft
-060 20P40
0-4500 -080 20P41
-100
-060 20P40
4501 -
-080
7500
-100

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 19 shows acceptable combustion
for ALL SL280UHNV models. The maximum carbon
monoxide reading should not exceed 100 ppm.
Firing Rate
High Fire
Low Fire

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 20
lists pressure switch requirements at high altitude. The
combustion air pressure switch is factory-set and requires
no adjustment.
Unit
-060
-080
-100
Page 40
TABLE 18
Low
High
Fire
Fire Nat Fire LP
Nat
3.4 - 3.8
3.0 - 3.8 1.3 - 1.7
N/A
2.7 - 3.2
2.5 - 3.3 1.3 - 1.7
N/A
TABLE 19
CO
CO % For Nat
2
6.0 - 7.8
TABLE 20
Pressure Switch
4501 - 7500 ft
Natural
20K91
No Change
High Low
Fire LP
1.5 - 1.9
N/A N/A
1.3 - 1.7
N/A N/A
% For LP/
2
Propane
7.5 - 9.0
LP/propane
No Change
N/A
N/A