Lennox SL280UHNV - Proper Combustion; Rate Check; High Altitude

Page 40
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 ue will
cause unit to shut o󰀨 at pressure switch. Check ue
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 o󰀨 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 ow. 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 o all other gas
appliances connected to meter.
TABLE 17
GAS METER CLOCKING CHART
SL280N Unit
Seconds for One Revolution
Natural LP Propane
1 cu ft
Dial
2 cu ft
Dial
1 cu ft
Dial
2 cu ft
Dial
060 60 120 150 300
080 45 90 112 224
100 36 72 N/A N/A
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 eld-provided barbed
tting. Connect measuring device “+” connection to
barbed tting to measure manifold pressure.
Start unit and allow 5 minutes for unit to reach
steady state.
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 o󰀨 and remove manometer as soon as an
accurate reading has been obtained.
TABLE 18
Manifold in wc and LP/Propane Conversion Kit
Altitude
ft
Unit LP Kit
High
Fire Nat
Low
Fire
Nat
High
Fire LP
Low
Fire LP
0-4500
-060 20P40
3.0 - 3.8 1.3 - 1.7
3.4 - 3.8 1.5 - 1.9
-080 20P41
-100 N/A N/A N/A
4501 -
7500
-060 20P40
2.5 - 3.3 1.3 - 1.7
2.7 - 3.2 1.3 - 1.7
-080
N/A N/A N/A
-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 ow rate before check-
ing combustion. Table 19 shows acceptable combustion
for ALL SL280UHNV models. The maximum carbon
monoxide reading should not exceed 100 ppm.
TABLE 19
Firing Rate CO
2
% For Nat
CO
2
% For LP/
Propane
High Fire
6.0 - 7.8 7.5 - 9.0
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.
TABLE 20
Unit
Pressure Switch
4501 - 7500 ft
Natural LP/propane
-060 20K91 No Change
-080
No Change
N/A
-100 N/A