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FIGURE 6
Primary Limit Location and Heat Exchanger
Install limit face down
4. Gas Valve (Figure 7)
The ML193UHE uses an internally redundant 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 terminals on the gas valve are con
nected 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
LP/Propane changeover kit is available.
GAS VALVE SHOWN IN ON POSITION
MANIFOLD
PRESSURE
OUTLET
PORT
INLET
PRESSURE
PORT
MANIFOLD PRESSURE
ADJUSTMENT SCREW
(under barbed fitting)
FIGURE 7
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 (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 igni
tion control allows the gas valve to remain open as long as
flame signal is sensed.
NOTE - The ML193UHE is polarity sensitive. Make sure
that the furnace is wired correctly and is properly grounded.
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 control to the sensor during unit operation.
Current passes from the sensor through the flame to ground to
complete a safety circuit.
To Measure Flame Signal - Integrated Control:
Use a digital readout meter capable of reading DC mi
croamps. See figure 8 for flame signal check.
1 - Set the meter to the DC amps scale.
2 - Turn off supply voltage to control.
3 - Remove sensor wire from integrated control.
4 - Connect (-) lead to flame sensor wire.
5 - Connect (+) lead to Terminal FS on integrated control.
6 - Turn supply voltage on and close thermostat contacts to
cycle system.
7 - When main burners are in operation for two minutes, take
reading.
ML193UHE units use a nitride ignitor made from a propri
etary ceramic material. To check ignitor, measure its resis
tance and voltage. A value of 39 to 70 ohms indicates a
good ignitor. Voltage to the ignitor should be 102 - 132VAC.
See figure 9 for resistance and voltage checks.