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C- Heating Components
1. Ignitor
The SureLight
ignitor is made of durable silicon nitride.
®
Ignitor longevity is enhanced by controlling voltage to the
ignitor. 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 15 for
ignitor location and FIGURE 16 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
support. See FIGURE 15. 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 burners. 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 sig-
nal 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
2.6 or greater
2.5 or less
3. Gas Valve
The valve (FIGURE 19 and FIGURE 20) is internally re-
dundant 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
terminals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A
regulator adjustment screw is located on the valve. LPG
change over kits are available from Lennox. Kits include
burner orifices and a gas valve.
Low
Drop Out
1.1
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
15. 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 immedi-
ately 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 manual-
ly reset. To manually reset a tripped switch, push the reset
button located on the control.
5. Burners
All units use inshot burners. Burners are factory set and
require no adjustment. Always operate the unit with the
burner box front panel in place. Each burner uses an ori-
fice (see table 20 for orifice size) that is precisely matched
to the burner 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- MAINTENANCE sub-section A- Heat Ex-
changer and Burners.
6. Primary Limit Control (S10)
The primary limit (S10) is located in the heating vestibule
panel. 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 automatically resets when unit temperature re-
turns to normal. The switch must reset within three min-
utes or the SureLight control will go into Watch guard for
one hour. The switch is factory set and cannot be adjust-
ed. 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 automati-
cally 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.
Page 42
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