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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 SL280UHV(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 left side of the burner sup
port. See figure 13. The sensor is tip protrudes into the
flame envelope of the left-most burner. The sensor can be
removed for service without removing any part of the burn
ers. During operation, flame is sensed by current passed
through the flame and sensing electrode. 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 14 for flame signal.
TABLE 14
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 or 18) is internally redundant to assure
safety 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. Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit located on
top of the burner box, one on each side.- See figure 13. The
limit is a N.C. SPST manual‐reset limit connected in series
with the secondary limit S21. When S47 senses rollout, the
circuit breaks and the ignition control immediately stops
ignition and closes the gas valve. Rollout can be caused by
a blocked heat exchanger, flue or lack of combustion air.
The switch is factory set to trip (open) at 210°F and cannot be
adjusted. The switch can be manually reset. To manually re
set a tripped switch, push the reset button located on the con
trol.
5. Burners
All units use inshot burners. Burners are factory set and re
quire no adjustment. Always operate the unit with the burner
box front panel in place. Each burner uses an orifice (see
table 19 for orifice size) that is precisely matched to the burn
er input. Burners can be removed as a one piece assembly
for service. If burner assembly has been removed, it is critical
to align center of each burner to the center of the clamshell
when re-installing. See more detail in Section VI- MAINTE
NANCE sub-section E- Heat Exchanger and Burners.
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,
7. Secondary Limit Controls (S21)
The secondary limit (S21) is located in the blower compart
ment on the back side of the blower housing. SL280UHV units
require two secondary limits. When excess heat is sensed in
the blower compartment, the limit will open. If the limit is open,
the furnace control energizes the supply air blower and closes
the gas valve. The limit automatically resets when unit temper
ature 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.
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