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Placing the furnace into operation:
ML193DF units are equipped with an automatic hot sur-
face ignition system. Do not attempt to manually light
burners on this furnace. Each time the thermostat calls
for heat, the burners will automatically light. The ignitor
does not get hot when there is no call for heat on these
units.
Priming Condensate Trap
The condensate trap should be primed with water prior to
start−up to ensure proper condensate drainage. Either
pour 10 fl. oz. (300 ml) of water into the trap, or follow
these steps to prime the trap:
1 − Follow the lighting instructions to place the unit into op-
eration.
2 − Set the thermostat to initiate a heating demand.
3 − Allow the burners to fire for approximately 3 minutes.
4 − Adjust the thermostat to deactivate the heating de-
mand.
5 − Wait for the combustion air inducer to stop. Set the
thermostat to initiate a heating demand and again al-
low the burners to fire for approximately 3 minutes.
6 − Adjust the thermostat to deactivate the heating de-
mand and wait for the combustion air inducer to stop.
At this point, the trap should be primed with sufficient
water to ensure proper condensate drain operation.
WARNING
If you do not follow these instructions exactly, a fire
or explosion may result causing property damage,
personal injury or death.
Gas Valve Operation (Figure 54)
1 − STOP! Read the safety information at the beginning of
this section.
2 − Set the thermostat to the lowest setting.
3 − Turn off all electrical power to the unit.
4 − This furnace is equipped with an ignition device which
automatically lights the burners. Do not try to light the
burners by hand.
5 − Remove the upper access panel.
6 − Move gas valve switch to OFF. See figure 54.
7 − Wait five minutes to clear out any gas. If you then smell
gas, STOP! Immediately call your gas supplier from a
neighbor’s phone. Follow the gas supplier’s instruc-
tions. If you do not smell gas go to next step.
8 − Move gas valve switch to ON. See figure 54.
9 − Replace the upper access panel.
10− Turn on all electrical power to to the unit.
11− Set the thermostat to desired setting.
NOTE − When unit is initially started, steps 1 through 11
may need to be repeated to purge air from gas line.
GAS VALVE SHOWN IN ON POSITION
MANIFOLD
PRESSURE
OUTLET
INLET
PRESSURE
PORT
MANIFOLD
PRESSURE
ADJUSTMENT
SCREW
FIGURE 54
12− If the appliance will not operate, follow the instructions
Turning Off Gas to Unit" and call your service techni-
cian or gas supplier.
Turning Off Gas to Unit
1 − Set the thermostat to the lowest setting.
2 − Turn off all electrical power to the unit if service is to be
performed.
3 − Remove the upper access panel.
4 − Move gas valve switch to OFF.
5 − Replace the upper access panel.
Failure To Operate
If the unit fails to operate, check the following:
1 − Is the thermostat calling for heat?
2 − Are access panels securely in place?
3 − Is the main disconnect switch closed?
4 − Is there a blown fuse or tripped breaker?
5 − Is the filter dirty or plugged? Dirty or plugged filters will
cause the limit control to shut the unit off.
6 − Is gas turned on at the meter?
7 − Is the manual main shut-off valve open?
8 − Is the internal manual shut-off valve open?
9 − Is the unit ignition system in lockout? If the unit locks out
again, inspect the unit for blockages.
Heating Sequence Of Operation
1 − When thermostat calls for heat, combustion air inducer
starts.
2 − Combustion air pressure switch proves blower opera-
tion. Switch is factory−set and requires no adjustment.
3 − After a 15−second prepurge, the hot surface ignitor en-
ergizes.
4 − After a 20−second ignitor warm−up period, the gas
valve solenoid opens. A 4−second Trial for Ignition peri-
od begins."
5 − Gas is ignited, flame sensor proves the flame, and the
combustion process continues.
6 − If flame is not detected after first ignition trial, the igni-
tion control will repeat steps 3 and 4 four more times
before locking out the gas valve (WATCHGUARD"
flame failure mode). The ignition control will then auto-
matically repeat steps 1 through 6 after 60 minutes.
To interrupt the 60−minute WATCHGUARD" period,
move thermostat from Heat" to OFF" then back to
Heat". Heating sequence then restarts at step 1.