Gas Pressure Measurement; Proper Combustion; High Altitude - Lennox MERIT Series Installation Instructions Manual

11 - Are flame rollout switches tripped? If flame rollout
switches are tripped, call the service technician for
inspection.
Heating Sequence Of Operation (follow steps below
or see Figure 30 for more detail)
1 - When thermostat calls for heat, combustion air
blower 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.
7 - 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 10
GAS METER CLOCKING CHART
Seconds For One Revolution
ML180UHE Natuarl
Unit
1 cu ft
Dial
-045 80
-070 55
-090 41
-110 33
-135 27
Natural-1000 btu/cu ft LP-2500 btu/cu ft
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 8. If manifold pressure matches table 10 and rate is
incorrect, check gas orifices for proper size and restriction.
Remove temporary gas meter if installed.
NOTE - To obtain accurate reading, shut off all other gas
appliances connected to meter.
LP/Propane
2 cu ft 1 cu ft 2 cu ft
Dial Dial Dial
160 200 400
110 136 272
82 102 204
66 82 164
54 68 136
Supply Pressure Measurement
A threaded plug on the inlet side of the gas valve provides
access to the supply pressure tap. Remove the thread-
ed plug, install a field-provided barbed fitting and connect
a manometer to measure supply pressure. See table 10
for proper line pressure. Replace the threaded plug after
measurements have been taken.
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, observe 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 10.
NOTE - Shut unit off and remove manometer as soon as
an accurate reading has been obtained. Take care to re-
move barbed fitting and replace threaded plug.

Proper Combustion

Furnace should operate a minimum 15 minutes with cor-
rect manifold pressure and gas flow rate before checking
combustion. Take combustion sample beyond the flue out-
let and compare to the tables below. The maximum car-
bon monoxide reading should not exceed 100 ppm.
ML180UHE Unit
-045
-070
-090
-110
-135

High Altitude

The manifold pressure may require adjustment and com-
bustion air pressure switch may need replacing to ensure
proper combustion at higher altitudes. Refer to table 10
for manifold pressure and table 11 for pressure switch
change and gas conversion kits.
For safety, shut unit off and remove manometer as soon
as an accurate reading has been obtained. Take care to
replace pressure tap plug.
Page 29
TABLE 11
CO % Nat
2
7.2 - 7.8
IMPORTANT
CO % LP
2
7.5 - 9.0