Lennox ML193UHE Series Unit Information page 11

Rollout Switch(s)
Flame Sensor
B-Heating Components
Combustion air inducer (B6), primary limit control (S10),
ignitor, burners, flame rollout switch (S47), gas valve
(GV1), combustion air pressure switch (S18), and heat
exchangers are located in the heating compartment. The
heating compartment can be accessed by removing the
outer access panel.
1. Flame Rollout Switches (FIGURE 6)
Flame rollout switches S47 are SPST N.C. high tempera-
ture limits located on the left and right of the front buner
box plate. S47 is wired to the burner ignition control A92.
When either of the switches sense flame rollout (indicating
a blockage in the combustion passages), the flame rollout
switch trips, and the ignition control immediately closes
the gas valve. Switch S47 in all ML196UHE units is factory
preset to open at 210 F + 12 F (99 C + 6.7 C) on a tempera-
ture rise. All flame rollout switches are manual reset. See
table 4 flash code 8 for troubleshooting.
2. Heat Exchanger (FIGURE 7)
ML193UHE units use an aluminized steel primary and
stainless steel secondary heat exchanger assembly. Heat
is transferred to the air stream from all surfaces of the heat
exchanger. The shape of the heat exchanger ensures
maximum efficiency.
Burner Box Front
Burner Box Assembly
Gas Valve
FIGURE 6
The combustion air inducer pulls fresh air through the
burner box. This air is mixed with gas in the burners. The
gas / air mixture is then burned at the entrance of each
clamshell. Combustion gases are then pulled through the
primary and secondary heat exchangers and exhausted
out the exhaust vent pipe.
3. Primary Limit Control (FIGURE 7)
Primary limit (S10) used on ML193UHE units is located in
the heating vestibule panel. When excess heat is sensed
in the heat exchanger, the limit will open. Once the limit
opens, the furnace control energizes the supply air blower
and de-energizes the gas valve. The limit automatically re-
sets when unit temperature returns to normal. The switch
is factory set and cannot be adjusted. For limit replace-
ment remove wires from limit terminals, remove mounting
screws, rotate limit switch 90 degrees and slowly remove
from the vestibule panel. Install replacement limit with
same care. See TABLE 4 or TABLE 5 flash code 7 for
troubleshooting if limit switch opens during operation.
Page 11
Burner Assembly
Ignitor