Lennox LDT Series Information page 29

3-Burner Assembly (Figure 21)
The burners are controlled by the spark electrode, flame
sensing electrode, gas valve and combustion air blower.
The spark electrode, flame sensing electrode and gas
valve are directly controlled by ignition control. Ignition
control and combustion air blower is controlled by Unit
Controller A55.
Burners
All units use cluster type inshot burners (see figure 21).
Burners are factory set and do not require adjustment.
A peep hole with cover is furnished in the heating
access panel for flame viewing. Always operate the unit
with the access panel in place.
Burners can be removed as an assembly for service.
Burner maintenance and service is detailed in the
SERVICE CHECKS section of this manual.
TYPICAL GAS BURNER ASSEMBLY
FIGURE 21
ORIFICE WITH
PIPE THREADS
Tighten to 6.25 + .5 ft.lbs.
Do not over-tighten.
FIGURE 22
Orifice
Each burner uses an orifice (figure 22) which is
precisely matched to the burner input. Install only the
orifices with the same threads. The orifice is
threaded into the burner manifold. The burner is
supported by the orifice and will easily slide off for
service.
NOTE-Do not use thread sealing compound on the
orifices. Using thread sealing compound may plug the
orifices.
Natural gas orifice size is on nameplate. The LP gas
orifice size is on the label provided in the LP kit.
NOTE- In primary and secondary high temperature
limits S10 and S99 the ignition circuits in both gas heat
sections one and two are immediately de-energized
when terminals 1-3 open and the indoor blower motor is
immediately energized when terminals 1-2 close. This
is the primary and secondary safety shut-down function
of the unit.
4-Primary High Temperature Limits S10 &
S99
S10 is the primary high temperature limit for gas heat
section one and S99 is the primary high temperature limit
for gas heat section two.
In LDT156/240 units, S10 and S99 are located on the drip
shield behind the blower housing. In this location S10 and
S99 also serve as secondary limits. See figure 23.
Primary limit S10 is wired to the Unit Controller A55 which
energizes burner 1 control (A3), while primary limit S99 is
wired to the A55 Unit Controller which energizes burner 2
control (A12). Its N.C. contacts open to de-energize the
ignition control when excessive temperature is reached in
the blower compartment. At the same time, the N.O.
contacts of S10 and S99 close energizing the blower relay
coil K3 through control A55. If either limit trips the blower will
be energized. Limits settings are factory set and cannot be
adjusted. If limit must be replaced same type and set point
must be used. See Repair Parts Handbook.
S10 and S99 Location
S10 AND S99 ON
DRIP SHIELD BE­
HIND BLOWER
HOUSING
5-Flame Rollout Limits S47, S69
Flame rollout limits S47 on first heat section and S69 on
second heat section are SPST N.C. high temperature limits
located just above the burner air intake opening in the
burner enclosures (see figure20). Both switches are wired
to the A55 Unit Controller. When S47 or S69 senses flame
rollout (indicating a blockage in the combustion air
passages), the corresponding flame rollout limit trips and
the ignition control immediately closes the gas valve.
Limit S47 and S69 in standard heat units are factory preset
to open at 290_F + 12_F (121.1_C + 6.7_C) on a
temperature rise, while on high heat units both limits open
at 270_F + 12_F (132.2_C + 6.7_C) on a temperature rise.
All flame rollout limits are manual reset.
6-Combustion Air Prove Switches S18, S45
Prove switches S18 (first heat section) and S45 (second
heat section) are located in the compressor compartment.
Each has its own control box. Both are identical SPST N.O.
switches and monitor combustion air inducer operation.
Switch S18 and S45 are wired to the A55 Unit Controller.
Page 29
CONDENSER
DIVIDER
PANEL
GAS HEAT
SECTION 2
GAS HEAT
SECTION 1
FIGURE 23