Lennox International Inc Merit Series Gas Furnace Gas Pressure Adjustment, Proper Combustion

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 32 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 (ATCHGUARD" flame failure mode). The ignition control will then auto- matically repeat steps 1 through 6 after 60 minutes.

7 − To interrupt the 60−minute ATCHGUARD" period, move thermostat from to 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
ML180UH	Natural			LP
Unit	1 cu ft	2 cu ft		1 cu ft	2 cu ft
	Dial	Dial		Dial	DIAL
−045	80	160		200	400
−070	55	110		136	272
−090	41	82		102	204
−110	33	66		82	164
−135	27	54		68	136
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 10 below. If manifold pressure matches table 12 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.

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 12 for proper line pressure. Replace the threaded plug after mea- surements 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 pres- sure.

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 12.

NOTE − Shut unit off and remove manometer as soon as an accurate reading has been obtained. Take care to remove 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 50 ppm.

TABLE 11

ML180UH	CO2% For Nat	CO2% For L.P.
Unit
−045
−070
−090	7.2 − 7.8	7.5 − 9.0
−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 12 for manifold pressure and table 13 for pressure switch change and gas conversion kits.

IMPORTANT

For safety, shut unit off and remove manometer as soon as an accurate reading has been obtained. Take care to replace pressure tap plug.

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