Lennox MERIT Series Installation Instructions Manual page 40

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
operation. Switch is factory-set and requires no
adjustment.
3 - After a 15-second prepurge, the hot surface ignitor
energizes.
4 - After a 20-second ignitor warm-up period, the gas
valve solenoid opens. A 4-second Trial for Ignition
period 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
ignition 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 automatically 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.
Gas Pressure Adjustment
Gas Flow (Approximate)
TABLE 12
GAS METER CLOCKING CHART
Seconds for One Revolution
ML193 Natural
Unit
1 cu ft
Dial
-045 80
-070 55
-090 41
-110 33
Natural-1000 btu/cu ft LP-2500 btu/cu ft
LP
2 cu ft 1 cu ft 2 cu ft
Dial Dial Dial
160 200
110 136
82 102
66 82
NOTE - To obtain accurate reading, shut off all other gas
appliances connected to meter.
Furnace should operate at least 5 minutes before check-
ing gas flow. Determine time in seconds for two revolu-
tions of gas through the meter. (Two revolutions assures a
more accurate time). Divide by two and compare to time
in table 12. If manifold pressure matches table 13 and rate
is incorrect, check gas orifices for proper size and restric-
tion. Remove temporary gas meter if installed.
Supply Pressure Measurement
A threaded plug on the inlet side of the gas valve provides
access to the supply pressure tap. Remove the threaded
plug, install a field-provided barbed fitting and connect a
manometer to measure supply pressure. See table 13 for
proper supply line pressure. Replace the threaded plug
after measurements have been taken.
Supply Line and Manifold Pressure (inches w.c.)
Unit
All
All
NOTE - A natural to L.P. propane gas changeover kit is
necessary to convert this unit. Refer to the changeover kit
installation instruction for the conversion procedure.
Manifold Pressure Measurement
1 - Remove the threaded plug from the outlet side of
the gas valve and install a field-provided barbed
fitting. Connect to a manometer to measure manifold
pressure.
2 - Start unit and allow 5 minutes for unit to reach steady
state.
3 - While waiting for the unit to stabilize, notice the
flame. Flame should be stable and should not lift
from burner. Natural gas should burn blue.
4 - After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to value given in
table 13.
5 - Shut unit off and remove manometer as soon as an
accurate reading has been obtained. Take care to
replace pressure tap plug.
6 - Start unit and perform leak check. Seal leaks if found.
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Page 40
TABLE 13
Manifold
Fuel
Pressure
Nat. Gas 3.5
L.P. Gas 10.0
Line Pressure
Min - Max
4.5 - 10.5
11.0 - 13.0