Lennox MERIT ML180UHE Installation Instructions Manual page 40

Troubleshooting: Heating Sequence of Operation (Continued)
NORMAL HEATING MODE
15-SECOND COMBUSTION AIR INDUCER PREPURGE
INITIATED BY CLOSED PRESSURE SWITCH.
YES
IGNITOR WARM-UP −− 20 SECONDS.
YES
4-SECOND TRIAL FOR IGNITION.
GAS VALVE OPENS. IGNITOR ENERGIZED FOR
FIRST 3 SECONDS OF THE TRIAL.
YES
FLAME STABILIZATION PERIOD.
4 SECONDS
FLAME RECTIFICATION CURRENT
CHECK. CAN FLAME BE PROVEN WITHIN
4 SECONDS AFTER GAS VALVE OPENS?
(0.5 microamps)
YES
FLAME PRESENT?
YES
FLAME SIGNAL 1.5 MICROAMPS OR GREATER?
YES
INDOOR BLOWER ON
AFTER 30−SECOND DELAY
YES
PRIMARY LIMIT SWITCH CLOSED?
YES
ROLLOUT SWITCH CLOSED?
YES
COMBUSTION AIR PRESSURE
SWITCH CLOSED?
YES
THERMOSTAT DEMAND SATISFIED.
YES
LED SLOW FLASH.
YES
COMB. AIR INDUCER CONTINUES 5-SECOND
POST PURGE AFTER T'STAT DEMAND IS SATISFIED.
INDOOR AIR BLOWER COMPLETES SELECTED OFF"
DELAY BEFORE SHUTTING OFF.
HEATING SEQUENCE CONTINUED
NO
IS VOLTAGE ABOVE 70 VOLTS?
GAS VALVE OFF. COMBUSTION AIR INDUCER ON.
HAS CONTROL FAILED TO SENSE FLAME FOR
NO
FIVE CONSECUTIVE TRIES DURING A SINGLE
LED SIGNAL WATCHGUARD FAILURE CODE
IS 60-MINUTE RESET PERIOD COMPLETE?
NO
NO
LED FLASHES CODE 4 − LOW FLAME
(Does not affect operation of control)
LED FLASHES CODE 7 − PRIMARY LIMIT OPEN
COMBUSTION AIR INDUCER DE−ENERGIZED.
NO
INDOOR BLOWER ON UNTIL SWITCH CLOSES.
HAS PRIMARY LIMIT RESET
TIME EXCEEDED 3 MINUTES?
NO
NO
HAS PRESSURE SWITCH OPENED 5 TIMES
GAS VALVE DE−ENERGIZED. COMBUSTION AIR
INDUCER ON. INDOOR BLOWER OFF WITH
DELAY. HAS CAB PRESSURE SWITCH CLOSED
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ABNORMAL HEATING MODE
YES
INDOOR BLOWER OFF.
HEAT DEMAND?
WATCHGUARD MODE. GAS VALVE OFF.
COMBUSTION AIR INDUCER OFF.
INDOOR BLOWER OFF WITH DELAY
HAS CONTROL RESET IGNITION
SEQUENCE FOUR TIMES?
SIGNAL.
GAS VALVE DE−ENERGIZED.
LIMIT SWITCH CLOSED?
YES
LED FLASHES CODE 8 − ROLLOT SWITCH OPEN.
GAS VALVE POWER OFF. COMBUSTION AIR INDUCER POWER
ON. INDOOR BLOWER ON SEQUENCE HOLDS UNTIL ROLLOUT
SWITCH IS RESET AND MAIN POWER IS INTERRUPTED OR
THERMOSTAT IS CYCLED OFF/ON FOR 1 SEC. MINIMUM.
IN THE SAME HEAT DEMAND?
NO
WITHIN 2.5 MINUTES?
NO
5-MINUTE PRESSURE SWITCH
WATCHGUARD MODE.
LED FLASHES CODE 13 −
LOW LINE VOLTAGE.
NO
ONCE VOLTAGE IS ABOVE
75 VOLTS, HEATING
SEQUENCE RESTARTS.
NO
YES
YES
NO
YES
LED FLASHES CODE
7 − PRIMARY LIMIT
OPEN. GAS VALVE,
COMB. AIR INDUCER
AND INDOOR BLOW-
ER OFF. LEDs SIG-
NAL LIMIT SWITCH
OPEN UNTIL MAIN
POWER IS
INTERRUPTED OR
T'STAT IS CYCLED
OFF/ON FOR 1 SEC.
MINIMUM. 60−MINUTE
WATCHGUARD PE-
RIOD STARTS AT
YES
TIME LIMIT CLOSES.
IS 60−MIN. PERIOD
COMPLETE?
NO
LED FLASHES
CODE 10
1 HR PRESSURE
YES
SWITCH
WATCHGUARD
MODE
YES