Gas Pressure Adjustment

12− If the appliance will not operate, follow the instructions urning 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 position. Do not force. See figure 35 or 36.

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.

11 −Are flame rollout switches tripped? If flame rollout switches are tripped, call the service technician for in- spection.

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 20 below. If manifold pressure matches table 22 and rate is incorrect, check gas orifices for proper size and restriction. Remove temporary gas meter if installed.

NOTE − Shut unit off and remove manometer as soon as an accurate reading has been obtained. Take care to replace pressure tap plug.

TABLE 20

GAS METER CLOCKING CHART

		Seconds for One Revolution

SL280		Natural			LP
Unit		1 cu ft	2 cu ft		1 cu ft	2 cu ft
		Dial	Dial		Dial	DIAL

−045		80	160		200	400

−70		55	110		136	272

−90		41	82		102	204

−110		33	66		82	164

−135		27	54		68	136

	Natural−1000 btu/cu ft			LP−2500 btu/cu ft

Supply Line Pressure

White Rodgers Gas Valve

An inlet post located on the gas valve provides access to the supply pressure. See figure 35. Back out the 3/32 hex screw one turn, connect a piece of 5/16 tubing and connect to a manometer to measure supply pressure. See table 22 for supply line pressure.

Honeywell Gas Valve

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 22 for supply line pressure. Replace the threaded plug after mea- surements have been taken.

Manifold Pressure

NOTE − Pressure test adapter kit (10L34) is available from Lennox to facilitate manifold pressure measurement.

1 − Connect test gauge to manifold pressure post (figure 35) or tap (figure 36) on gas valve.

2 − Ignite unit on high fire and let run for 5 minutes to allow for steady state conditions.

3 − After allowing unit to stabilize for 5 minutes, record manifold pressure and compare to value given in table 22.

4 − If necessary, make adjustments. Figures 35 and 36 show location of high fire adjustment screw.

5 − If an adjustment is made on high fire, re−check man- ifold pressure on low fire. Do not adjust low fire man- ifold pressure. If low fire manifold pressure is more than 1/2" above or below value specified in table 22, replace valve.

Proper Combustion

Furnace should operate minimum 15 minutes with correct manifold pressure and gas flow rate before checking com- bustion. Table 21 shows acceptable combustion for ALL SL280UHV models. The maximum carbon monoxide read- ing should not exceed 50 ppm.

TABLE 21

Firing Rate	CO2% For Nat	CO2% For L.P.
High Fire	6.8 − 7.4	7.5 − 9.0

Low Fire	4.2 − 5.7	5.0 − 6.0

Page 52