Unit Sputter Starts And Dies - Lennox G21Q3-40 Unit Information

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IS GAS SUPPLY NATURAL OR L.P.?
L.P.
HAS L.P. CONVERSION KIT
BEEN INSTALLED?
IF NOT, INSTALL KIT.
IS GAS SUPPLY ON?
1. CHECK SUPPLY VALVE.
2. CHECK STOP VALVE AT UNIT.
3. CHECK MANUAL KNOB ON
GAS VALVE IN UNIT.
AIR IN GAS PIPING?
BLEED AIR BY RUNNING UNIT THROUGH SEVERAL
TRIES FOR IGNITION. BREAK & REMAKE THERMOSTAT
DEMAND TO RESTART IGNITION SEQUENCE AT 2 TO 3
MINUTE INTERVALS.
CHECK FOR LEAKS IN GAS SUPPLY
PIPING & REPAIR IF NEEDED.
CAUTION DO NOT USE MATCH OR FLAME
TO CHECK FOR GAS LEAKS.
CORRECT
GAS PRES
SURE.
IS GAS VALVE ACTUALLY OPENING?
DETERMINE BY CHECKING FOR MANIFOLD
PRESSURE DURING TRIALS FOR IGNITION.
REPLACE GAS VALVE.
IS FLAME SIGNAL PRESENT AND COR
CHECK WITH MICROAMP METER
NO
CHECK SENSOR WIRE
IS AIR FLAPPER VALVE OPERATING NORMALLY?
REPLACE IF DETERI
ORATED
& RETEST UNIT
REMOVE & CHECK SPARK PLUG (USE LENNOX
PART #20H43 OR 3/4" SPARK PLUG SOCKET).
1. WAS PLUG TIGHT WHEN REMOVED?
2. ARE CRACKS PRESENT IN PORCELAIN?
3. REPLACE SENSOR IF
CRACKED?
IS GAS FLAPPER OPERATING NORMALLY?
REFER TO CHECKING GAS FLAPPER VALVE
YES
IS GAS ORIFICE CORRECT?
REFER TO CHECKING GAS ORIFICE"
REPLACE ORIFICE
IF
NECESSARY

UNIT SPUTTER STARTS AND DIES

GAS
CHECKS
NATURAL
NO YES
NO YES
RECT?
YES
(LACK OF AIR?) REFER TO
CHECKING AIR INTAKE FLAPPER VALVE"
YES NO
REPLACE AIR FLAPPER VALVE OR MATERIAL
NO
REPLACE GAS FLAPPER VALVE
CHECK FOR LEAKS IN UNIT MANIFOLD PIPING.
LEAKS
REPAIR LEAKS
& RETEST.
VISUALLY CHECK OUTSIDE TERMINATIONS OF
INTAKE & EXHAUST PVC PIPING FOR OBSTRUCTIONS.
VISUALLY CHECK CONDENSATE DRAIN FOR OBSTRUCTIONS.
REFER TO INSTALLATION INSTRUCTIONS: ARE IN
TAKE & EXHAUST LINES PROPERLY SIZED & AP
PLIED WITHIN LENGTH, DIAMETER & ELBOW LIMITS?
NO
CORRECT PIPING
ERRORS & RETEST.
(USE CARE TO PREVENT DAMAGE TO COVER GASKET).
8
9
IS 120VAC PRESENT BETWEEN
JP72−6 AND COMMON?
YES NO
IS 120VAC PRESENT
BETWEEN
JP72−6 AND JP72−5?
CONNECTIONS
YES NO
REPLACE
PRIMARY CONTROL.
IS DIFFERENTIAL PRESSURE SWITCH CUTTING OUT?
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NO LEAKS
YES
TURN OFF GAS TO UNIT.
REMOVE AIR INTAKE CHAMBER COVER
CHECK PURGE BLOWER FOR BINDING
OR MECHANICAL DAMAGE.
BREAK & REMAKE THERMOSTAT DEMAND
TO INITIATE CONTROL SEQUENCE.
IS120VAC PRESENT ACROSS PURGE
BLOWER MOTOR TERMINALS?
NO YES
DOES PURGE BLOWER RUN?
YES NO
REPLACE
PURGE BLOWER.
CHECKOUT
WIRING
& REPAIR.
MAKE OHM METER CHECK ACROSS
TERMINALS IMMEDIATELY AFTER
UNIT SHUTS OFF. METER READS
INFINITE( )OHMS WHEN SWITCH
CUTS UNIT OUT.
NO YES
CHECK FOR PARTIAL BLOCKAGE
OF ALL PVC PIPING & CONDENSATE
LINE.
IF NO BLOCKAGE EXISTS
REPLACE DIFFERENTIAL PRESSURE
SWITCH.
AIR
CHECKS