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B-Heating Components
Combustion air inducer (B6), primary limit control (S10),
SureLight ignitor, burners, flame rollout switch (S47), gas
valve (GV1), combustion air prove switch (S18), and clam
shell heat exchangers are located in the heating compart
ment. The heating compartment can be accessed by re
moving the burner access panel.
1. Ignitor (Figure 10)
ML193UH units use a mini-nitride ignitor made from a pro
prietary ceramic material. Ignitor longevity is enhanced by
controlling the voltage to the ignitor. Units equipped with
control 103085 have a 120V ignitor. Units equipped with
control 100973 have a 95V ignitor. See figure 11 and table
7 for resistance and voltage checks.
2. Flame Sensor (Figure 10)
A flame sensor is located on the left side of the burner sup
port. The sensor is mounted on the front burner box plate
and the tip protrudes into the flame envelope of the left-
most burner. The sensor can be removed for service with
out removing any part of the burners. During operation,
flame is sensed by current passed through the flame and
sensing electrode. The ignition control allows the gas valve
to remain open as long as flame signal is sensed.
NOTE - The ML193UH furnace contains electronic
components that are polarity sensitive. Make sure that
the furnace is wired correctly and is properly grounded.
3. Flame Rollout Switches (Figure 10)
Flame rollout switches S47 are SPST N.C. high tempera
ture limits located on the top left and bottom right of the front
buner box plate. S47 is wired to the burner ignition control
A92. When either of the switches sense flame rollout (indi
cating a blockage in the combustion passages), the flame
rollout switch trips, and the ignition control immediately
closes the gas valve. Switch S47 in all ML193UH units is
factory preset to open at 210
_
F + 12
_
F (99
_
C + 6.7
_
C) on a
temperature rise. All flame rollout switches are manual re
set.
FIGURE 10
ML193UH Burner Box Assembly
IGNITOR
FLAME SENSOR
ROLLOUT SWITCHES
GAS VALVE
BURNERS
ORIFICES
FRONT BURNER BOX PLATE
