Troubleshooting Guide: Millivolt Gas Control System - Lennox MPB33CNM Care And Operation Instructions Manual

LENNOX HEARTH PRODUCTS • MERIT PLUS

TROUBLESHOOTING GUIDE: MILLIVOLT GAS CONTROL SYSTEM

NOTE: Before troubleshooting the gas control system, ensure external gas shut off valve, located at gas supply inlet, (and wall switch, if
applicable), is in the ON position.
NOTE: Valve system troubleshooting should only be performed by a qualifi ed service technician.
SYMPTOM POSSIBLE CAUSES
Spark igniter will Defective igniter (no spark at electrode).
not light pilot after
repeated pressing of
Defective or misaligned electrode at pilot
igniter button.
(spark at electrode).
Gas supply pressure errant.
Pilot orifi ce plugged.
Pilot will not stay lit Defective pilot generator (thermocouple).
after following the
lighting instructions.
Pilot burning, no gas Limit Switch defective.
to burner, valve knob
ON, wall switch ON.
Wall switch or wires defective.
Thermopile may not be generating suffi cient
millivolts.
Plugged burner orifi ce.
ON/OFF Switch and *Remote Switch are in the
ON position resulting in excessive resistance.
Frequent pilot/burner Pilot fl ame may be too low or blowing (high)
outage problem. causing the pilot/valve safety to drop out.
Table 15: Millivolt Gas Control System
® B-VENT GAS FIREPLACES (MPB33/35/40/45) • CARE AND OPERATION INSTRUCTIONS
CORRECTIVE ACTION
Check for spark at electrode and pilot; if no spark and electrode wire is
properly connected, replace Igniter.
Using a match, light pilot. If pilot lights, turn off pilot and trigger the
igniter button again. If pilot lights, an improper gas mixture caused
the bad lighting and a longer purge period is recommended. If pilot
will not light – check gap at electrode (igniter rod) and pilot – should
be 1/8 in. to have a strong spark. If gap measures 1/8 in., replace pilot
(see Figure 14 on Page 17).
Check inlet gas pressure. It should be within the limits as marked on
the rating plate.
Clean or replace pilot orifi ce.
Check pilot fl ame, it must impinge on thermocouple (see Figure 14 on
Page 17). Clean and/or adjust pilot for maximum fl ame impingement
on thermocouple. Ensure that the connection between the valve and
thermocouple are tight and secure.
Isolate the "limit" switch. Disconnect the wires from the
ON/OFF switch and the valve (label wires for reattachment). Test for
continuity with a multimeter. If continuity is not indicated, switch is
defective and must be replaced.
Check wall switch and wires for proper connections. Jumper wire
across terminals at wall switch, if burner comes on, replace defective
wall switch. If okay, jumper wires across wall switch wires at valve, if
burner comes on, wires are faulty or connections are bad.
Check thermopile with millivolt meter. Take reading at thermopile
terminals of gas valve. Should read 325 millivolts minimum with
optional wall switch OFF. Replace faulty thermopile if reading is below
specifi ed minimum.
Check burner orifi ce for stoppage and remove.
When turning on the burner using a *remote switch, ensure that the
unit mounted (optional) ON/OFF switch is in the OFF position. If both
switches are in the ON position, it may result in excessive resistance
(and millivolt drainage) and the burner may not come on.
Clean and/or adjust pilot fl ame for maximum fl ame impingement on
thermocouple (see Figure 14 on Page 17).
NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE.
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