Lennox ML193DFE Series Unit Information page 11

1. Flame Rollout Switches (FIGURE 7)
Flame rollout switches S47 are SPST N.C. high tempera-
turelimits located on the top left and bottom right of the
front burner box plate. S47 is wired to the burner ignition
control A92. When either of the switches sense flame roll-
out (indicating a blockage in the combustion passages),
the flame rollout switch trips, and the ignition control im-
mediately closes the gas valve. Switch S47 in all ML193D-
FE units is factory preset to open at 250 F + 12 F (121 C +
6.7 C) on a temperature rise. All flame rollout switches are
manual reset.
2. Heat Exchanger (FIGURE 8)
ML193DFE 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.
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 8)
Primary limit (S10) used on ML193DFE units is locat-
ed 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 auto-
matically resets when unit temperature returns to normal.
The switch is factory set and cannot be adjusted. For limit
replacement remove wires from limit terminals and rotate
limit switch 90 degrees. Slowly remove from the vestibule
panel. Install replacement limit with same care.
Primary Limit Location and Heat Exchanger
Install limit face down
FIGURE 8
4. Gas Valve (GV1)
The ML193DFE uses an internally redundant to valve to
assure safety shut-off. If the gas valve must be replaced,
the same type valve must be used. 24VAC terminals and
gas control switch are located on top of the valve. All ter-
minals on the gas valve are connected to wires from the
ignition control. 24V applied to the terminals opens the
valve. Inlet and outlet pressure taps are located on the
valve. A manifold adjustment screw is also located on the
valve. An LPG changeover kit is available.
MANIFOLD PRESSURE
ADJUSTMENT SCREW
(under barbed fitting)
INLET
PRESSURE
PORT
GAS VALVE SHOWN IN ON POSITION
5. Flame Sensor (FIGURE 8)
A flame sensor is located on the left side of the burner
support. The sensor is mounted on the front burner box
plate and the tip protrudes into the flame envelope of the
leftmost burner. The sensor can be removed for service
(use steel wool only to clean) without removing any part of
the burners. During operation, flame is sensed by current
passed through the flame and sensing electrode. The ig-
nition control allows the gas valve to remain open as long
as flame signal is sensed.
NOTE - The ML193DFE is polarity sensitive. Make sure
that the furnace is wired correctly and is properly ground-
ed.
A microamp DC meter is needed to check the flame signal
on the integrated control. Flame (microamp) signal is an
electrical current which passes from the integrated con-
trol to the sensor during unit operation. Current passes
from the sensor through the flame to ground to complete
a safety circuit. See FIGURE 10 for sensor flame signal
check.
6. Ignitor (FIGURE 7)
ML193DFE units use a mini-nitride ignitor made from a
proprietary ceramic material. Ignitor longevity is enhanced
by controlling the voltage to the ignitor. See FIGURE 11 for
resistance and voltage checks.
Page 11
MANIFOLD
PRESSURE
FIGURE 9
OUTLET
PORT