Gas Pressure Adjustment; Proper Combustion; Table 21 - Lennox SIGNATURE SL280UHV Installation Instructions Manual

12− If the appliance will not operate, follow the instructions
Turning Off Gas to Unit" and call your service techni-
cian 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 figure 35 or 36.
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 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.
11 −Are flame rollout switches tripped? If flame rollout
switches are tripped, call the service technician for in-
spection.

Gas Pressure Adjustment

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 20 below. If manifold pressure matches table 22
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 replace
pressure tap plug.
GAS METER CLOCKING CHART
SL280
Unit
1 cu ft
Dial
−045 80
−70 55
−90 41
−110 33
−135 27
Natural−1000 btu/cu ft
Supply Line Pressure
White Rodgers Gas Valve
An inlet post located on the gas valve provides access to
the supply pressure. See figure 35. 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 22
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 threaded
plug, install a field−provided barbed fitting and connect a
manometer to measure supply pressure. See table 22 for
supply line pressure. Replace the threaded plug after mea-
surements 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 (figure
35) or tap (figure 36) on gas valve.
2 − Ignite unit on high fire 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
22.
4 − If necessary, make adjustments. Figures 35 and 36
show location of high fire adjustment screw.
5 − If an adjustment is made on high fire, re−check man-
ifold pressure on low fire. Do not adjust low fire man-
ifold pressure. If low fire manifold pressure is more
than 1/2" above or below value specified in table 22,
replace valve.

Proper Combustion

Furnace should operate minimum 15 minutes with correct
manifold pressure and gas flow rate before checking com-
bustion. Table 21 shows acceptable combustion for ALL
SL280UHV models. The maximum carbon monoxide read-
ing should not exceed 50 ppm.
Firing Rate
High Fire
Low Fire
Page 52
TABLE 20
Seconds for One Revolution
Natural
2 cu ft 1 cu ft
Dial Dial
160 200
110 136
82 102
66 82
54 68
LP−2500 btu/cu ft

TABLE 21

CO % CO
2 For Nat
6.8 − 7.4
4.2 − 5.7
LP
2 cu ft
DIAL
400
272
204
164
136
%
2 For L.P.
7.5 − 9.0
5.0 − 6.0