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9. Gas Valve (Figure 6)
The ML180UHE(X) uses an internally redundant gas
valve to assure safety shut-off. If the gas valve must be
replaced, the same type valve must be used. 24VAC ter
-
minals and valve switch are located on the valve. All ter
-
minals on the gas valve are connected to wires from the
integrated control. 24V applied to the terminals energizes
the valve.
Inlet and outlet pressure taps are located on the valve. A
regulator adjustment screw is located on the valve. LPG
changeover kits are available from Lennox. Kits include
burner orifices and a gas valve regulator spring.
10. Combustion Air Inducer (B6)
All units use a combustion air inducer to move air through
the burners and heat exchanger during heating operation.
The blower uses a 120VAC motor. The motor operates
during all heating operation and is controlled by integrat
-
ed control A92. The inducer also operates for 15 seconds
before burner ignition (pre-purge) and for 5 seconds after
the gas valve closes (post-purge).
A pressure switch mounted on the combustion air inducer
orifice plate is used to prove inducer operation. The com
-
bustion air inducer orifice will be different for each model.
See table 7 for orifice sizes. The switch monitors air pres
-
sure in the inducer housing. During normal operation, the
pressure in the housing is negative. If pressure becomes
less negative (signifying any obstruction in the flue) the
pressure switch opens. When the pressure switch opens,
the integrated control (A92) immediately de-energizes the
gas valve to prevent burner operation.
TABLE 7
Model
C.A.I. Orifice Size
045E36A
1.045”
070E36B
1.316”
090E48B
1.531”
110E60C
1.690”
11. Combustion Air Inducer Pressure Switch (S18)
Units are equipped with a combustion air pressure switch
located on the combustion air inducer orifice bracket. The
switch is connected to the combustion air inducer housing
by means of a flexible silicone hose. It monitors negative
air pressure in the combustion air inducer housing.
The switch is a single-pole single-throw proving switch
electrically connected to the furnace control. The purpose
of the switch is to prevent burner operation if the com
-
bustion air inducer is not operating or if the flue becomes
obstructed.
On start-up, the switch senses that the combustion air
inducer is operating. It closes a circuit to the integrated
control when pressure inside the combustion air inducer
decreases to a certain set point. Set points vary depend
-
ing on unit size. See table 8. The pressure sensed by
the switch is negative relative to atmospheric pressure.If
the flue becomes obstructed during operation, the switch
senses a loss of negative pressure (pressure becomes
more equal with atmospheric pressure) and opens the cir
-
cuit to the integrated control and gas valve. A bleed port
on the switch allows relatively dry air in the vestibule to
purge switch tubing, to prevent condensate build up.
TABLE 8
Unit
inches wc
Make
Break + 0.05
045E36A
-0.75
-0.60
07036EB
-0.80
-0.65
090E48B
-0.75
-0.60
110E60C
-0.80
-0.65
The switch is factory set and is not field adjustable. It is a
safety shut-down control in the furnace and must not be
bypassed for any reason. If switch is closed or by-passed,
the integrated control will not initiate ignition at start up.
Troubleshooting
See figure 9 for measuring operating pressure and check
-
ing resistance in the pressure switch.
MULTI−METER
SET TO MEASURE OHMS
+
-
or
Field Provided
Tubing To CAI Port
To Pressure Switch
Remove tubing from CAI and
insert “Tee” and additional tubing.
High
Low
FIGURE 9
