Gas Pressure Adjustment; High Altitude; Proper Combustion; Table 4 - Lennox ML180UHA Installation Instructions Manual

4 - Move lever on gas valve to OFF. See figure 27.
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 circuit 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 lock out? If the unit locks out
again, call the service technician to inspect the unit for
blockages.
10 - Is pressure switch closed? Obstructed flue will cause
unit to shut off at pressure switch. Check flue and outlet
for blockages. If no blockage is found call the service
technician.
11 - Are flame rollout switches tripped? If flame rollout
switches are tripped, call the service technician for in­
spection.
Heating Sequence Of Operation
(follow steps below or see Figure 26 for more detail)
1 - When thermostat calls for heat, combustion air blower
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.
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
NOTE - The manufacturer recommended gas orifice size is
1.60 mm for natural gas and 0.86 mm for L.P.
To check for proper flow to combustion chamber, deter­
mine MJ/Hr input from appliance rating plate. Divide this in­
put rating by the MJ/Hr per cubic meter of available gas.
Result is the required number of cubic meter per hour. De­
termine the flow of gas through the gas meter for two min­
utes and multiply by 30 to get the hourly flow of gas.
IMPORTANT
The White Rodgers 36G24 gas valve (figure 27) is
equipped with pressure posts for measuring supply
and manifold pressures. The posts provide built-in
7.9mm (5/16") hose connections and have an inte­
gral 2.4mm (3/32") Allen-head screw. Rotate the
screw counterclockwise one full turn to permit pres­
sure measurement. Reseat the screw (rotate one full
turn clockwise) after measurements have been taken
to prevent gas leakage.
Use pressure measurement kit 69M1701 to connect
7.9mm (5/16") pressure post on gas valve to 6.4mm (1/4")
hose off the manometer.
Line Pressure Measurement
1 - Check the gas line pressure with the unit firing at maxi­
mum rate. See table 5 for maximum and minimum line
pressures.
After the line pressure has been checked and adjusted,
check the manifold pressure per table 5. See figure 27 for
the location of the manifold pressure adjustment screw.
Manifold Pressure Measurement
1 - Connect the outlet side of the gas 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
5.
5 - Shut unit off and remove manometer as soon as an ac­
curate reading has been obtained. Take care to re­
place pressure tap plug.
6 - Start unit and perform leak check. Seal leaks if found.

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.
Unit
-070
-090
-110
-135
Page 20

TABLE 4

CO % CO %
2 For Nat 2
6.3 - 7.8 7.0 - 9.0
For L.P.