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C- Heating Components
1. Ignitor
The SureLight
®
ignitor is made of durable silicon nitride. Ig
nitor longevity is enhanced by controlling voltage to the ig
nitor. The integrated control provides a regulated 120 volts
to the ignitor for a consistent ignition and long ignitor life.
Ohm value should be 39 to 70. See figure 13 for ignitor
location and figure 14 for ignitor check out.
NOTE -
The SL280UHNV(X) furnace contains electronic
components that are polarity sensitive. Make sure that the
furnace is wired correctly and is properly grounded.
2. Flame Sensor
A flame sensor is located on the top of air gas plenum. See
figure 13. The sensor can be removed for service without
removing the the burner. During operation, flame is sensed
by current passed through the flame and sensing elec
trode. The SureLight control allows the gas valve to remain
open as long as flame signal is sensed. To check flame
sense signal use the push-button found on the integrated
control and go to Field Test Mode. The menu will display the
flame signal. See table 15 for flame signal.
TABLE 15
Flame Signal in Microamps
Normal
Low
Drop Out
2.6 or greater
2.5 or less
1.1
3. Gas Valve
The valve (figure 17) is internally redundant to assure safe
ty shut-off. If the gas valve must be replaced, the same type
valve must be used.
24VAC terminals and gas control knob are located on the
valve. A wire harness connects the terminals from the gas
valve to the electronic ignition control. 24V applied to the termi
nals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A regu
lator adjustment screw is located on the valve.
LPG change over kits are available from Lennox. Kits include
burner orifices and a gas valve.
4. Thermal Switch (S47)
The auto-reset switch is located on the front of the intake
air elbow. The switch will safely shut the unit down if exces
sive temperatures are detected. See figure 13. When the
switch senses excessive temperature, the circuit breaks
and the ignition control immediately stops ignition and clos
es the gas valve.
5. Burner and Orifice
Burners are factory set and require no adjustment. Always
operate the unit with air gas plenum in place. The burner
has one orifice located between the gas valve and the air
intake assembly (Figure 13). To check or replace the orifice
remove the intake air screen, coupling and intake air elbow.
Using a 5/8” socket, go through the elbow and unscrew the
gas orifice. The burner uses an orifice (Table 16) that is pre
cisely matched to the burner input. The burner can be re
moved for service. If burner has been removed, it is critical
to replace all gaskets.
TABLE 16
Gas Orifices (Nat)
Unit Input
Orifice Size (0-4500 ft)
060
0.0595
080
0.0689
100
0.0810
6. Primary Limit Control (S10)
The primary limit (S10) is located in the heating vestibule pan
el. When excess heat is sensed in the heat exchanger, the
limit will open. If the limit is open, the furnace control energizes
the supply air blower and closes the gas valve. The limit auto
matically resets when unit temperature returns to normal. The
switch must reset within three minutes or the SureLight control
will go into Watch guard for one hour. The switch is factory
set and cannot be adjusted. The switch may have a different
set point for each unit model number. See Lennox Repair
Parts Handbook if limit switch must be replaced,
