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12 - If the appliance will not operate, follow the
instructions “Turning Off Gas to Unit” and call your
service technician or gas supplier.
Turning Off Gas to Unit
1 - Set the thermostat to the lowest setting.
2 - Turn off all electrical power to the unit if service is to
be performed.
3 - Remove the upper access panel.
4 - Move gas valve switch to OFF position. Do not
force. See gure 34 or 35.
5 - Replace the upper access panel.
Failure To Operate
If the unit fails to operate, check the following:
1 - Is the thermostat calling for heat?
2 - Are access panels securely in place?
3 - Is the main disconnect switch closed?
4 - Is there a blown fuse or tripped circuit breaker?
5 - Is the lter dirty or plugged? Dirty or plugged lters
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 ue will
cause unit to shut off at pressure switch. Check ue
and outlet for blockages.
11 - Are ame rollout switches tripped? If ame rollout
switches are tripped, call the service technician for
inspection.
Gas Pressure Adjustment
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. If manifold pressure matches table 19
and rate is incorrect, check gas orices 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 17
GAS METERING CLOCKING CHART
SL280
Unit
Natural 1000 btu/cu ft LP 2500 btu cu/cu ft
Seconds For One Revolution
1 cu ft
dial
2 cu fr
dial
1 cu ft
Dial
2 cu ft
Dial
-045 80 160 200 400
-070 55 110 136 272
-090 41 82 102 204
-110 33 66 82 164
-135 27 54 68 136
Supply Line Pressure
White Rodgers Gas Valve
An inlet post located on the gas valve provides access
to the supply pressure. See gure 34. Back out the 3/32
hex screw one turn, connect a piece of 5/16 tubing and
connect to a manometer to measure supply pressure. See
table 19 for supply line pressure.
Honeywell Gas Valve
A threaded plug on the inlet side of the gas valve provides
access to the supply pressure tap. Remove the thread-
ed plug, install a eld-provided barbed tting and connect
a manometer to measure supply pressure. See table 19
for supply line pressure. Replace the threaded plug after
measurements have been taken.
Manifold Pressure
NOTE - Pressure test adapter kit (10L34) is available from
Lennox to facilitate manifold pressure measurement.
1 - Connect test gauge to manifold pressure post
(gure 34) or tap (gure 35) on gas valve.
2 - Ignite unit on high re and let run for 5 minutes to
allow for steady state conditions.
3 - After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to value given in
table 19.
4 - If necessary, make adjustments. Figures 34 and 35
show location of high re adjustment screw.
5 - If an adjustment is made on high re, re-check
manifold pressure on low re. Do not adjust low re
manifold pressure. If low re manifold pressure is
more than 1/2” above or below value specied in
table 19, replace valve.
6 - Shut unit off and remove manometer as soon as an
accurate reading has been obtained. Take care to
replace pressure tap plug if removed.
7 - Start unit and perform leak check. Seal leaks if
found.
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 Combustion
Furnace should operate minimum 15 minutes with correct
manifold pressure and gas ow rate before checking com-
bustion. Table 18 shows acceptable combustion for ALL
SL280UHV models. The maximum carbon monoxide
reading should not exceed 100 ppm.
TABLE 18
Firing Rate CO
2
% For Nat CO
2
% For LP
High Fire 6.8 - 7.4 7.5 - 9.0
Low Fire 4.2 - 5.7 5.0 - 6.0
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