A.O. Smith VB/VW-500 - Manual (Page 34)

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for a heat request (determined by the Thermostat or controlling
probe inputs). All outputs are off in this state except that if the
Post-Circulate time is set to continuous, the pump will be on. When
the heat request is received, the system moves to the Pre-
Circulate state.
• Pre-Circulate:
The yellow LED is turned off and the green "Running" LED is
turned on. The green LED will remain on for all other states
except the fault states. Cold purging clears out any combustion
gas that may be in the combustion chamber. When purging is
complete the system moves to the Heat State.
• Heat Stage
The system will command the FCB micros to start their heat
sequence. will be activated in order based on an algorithm
that determines how much heat is needed. The system will
remain in this state until the heat request is satisfied, the Tstat
is opened, or a fault occurs.
• Soft Fault State: (See "Fault Description" section for list of soft
and Auto Reset faults.)
The pump remains on for the selected post-circulate time to
cycle the hot water out of the boiler. The FCB is commanded
to shut down and the Alarm output is turned on. The green
LED turns off and the red "Service" turns on. The MCB remains
in this state until one of the following occurs:
• One hour passes (automatic restart after one hour)
• If Communications error system will automatically
restart if communications re-establishes.
• If user presses Select key while current error screen is
displayed (Hard reset).
• If high-limit error - the outlet temperature drops below the
high limit trip point minus the high limit differential. (outlet
water temperature drops to safe level). The fault is logged in
the error history when the fault state is exited.
• Hard Fault State: (See "Fault Description" section for list of
soft and Auto Reset faults.) The pump remains on for the
selected post-circulate time to cycle the hot water out of the
boiler. The FCB is commanded to shut down and the Alarm
output is turned on. The green LED turns off and the red
"Service" turns on and off (flashes). The only way to exit this
state is for the user to press the Select key while the current
error screen is displayed. The fault is logged in the error
history when the fault state is exited.
While the MCB is in the Heating mode the FCB moves from Idle,
to Pre-Purge, to Heat Igniter, to Check for Flame, and then to
Heating. The process waits in this state until the call for heat is
satisfied, the thermostat input is opened, or a fault occurs. When
heat is satisfied, the sequence continues to Post-Purge and then
back to Idle. If three tries for ignition has been selected on the
MCB dipswitch and flame is not detected at the appropriate time,
then the sequence moves to the Inter-Purge state for 15 seconds
and the FCB heat sequence is repeated. If flame is still not detected
after the third try the process declares a fault, jumps out of sequence
and goes to the error state. Other types of faults detected at any
time will also cause a jump to the error state.
Description of FCB control states:
•
Idle State:
When any error is declared during idle state-The system will
remain in this state until the MCB request a heating cycle, or a
cold purge.
• Pre-Purge State:
The blower is turned on. After 10 to 34 seconds (34 seconds
for cold purging) the system switches to the Heat Igniter State.
• Heat Igniter State:
The sequence of operation is as follows: "The igniter relay is
energized and the heat up period begins. After 18-20 seconds,
if the igniter has reached a minimum of 2.8 amps, the gas valve
safety circuit is then activated. If in this amount of time, the
igniter fails to reach a minimum amp draw of 2.8 amps, the unit
will re-cycle up to 3 times before locking out on igniter hardware
failure. If the igniter does reach the minimum required amp
draw of 2.8 amps, the gas valve circuit is energized, and the
system advances to Flame Check status.
• Check for Flame State:
The gas valve is turned on. After 1.5 seconds, the system
checks that the gas valve relay is on. If it is not, the ignition
trial is considered to have failed and the system advances to
the Inter-Purge state, if any ignition trials remain, otherwise an
error is declared.
The flame sensor is checked. If flame is detected, the igniter
is turned off. The state machine then advances to the Heating
State. If after 5 seconds, the flame is still not sensed then the
ignition trial is considered to have failed. The state machine
advances to Inter-Purge if any ignition trials remain, otherwise
an error is declared.
• Heating State:
The system remains in this state until the call for heat is
cancelled or a fault occurs. The system returns to the Idle
state if the call for heat is cancelled, or satisfied and to the
appropriate fault state if a fault occur.
• Inter-Purge State:
The gas valve and the igniter are turned off. The blower is on.
After approximately 15 seconds the system goes back to the
Heating Igniter State.
• Post-Purge State
The gas valve and the igniter are turned off. If this stage
does not have a blower the system returns to the idle
state. If it does have a blower then the blower is left on.
After approximately 25 seconds, the system returns to
the Idle state.
• Error State
The gas valve and the igniter are turned off. The FCB micro
tells the MCB micro which error has occurred. The system waits
in this state until the MCB sends a command to clear the error,
and the system returns to the idle state.
• Cold Purge State
Cold purge occurs when the MCB micro commands the
blower to go on before lighting to clear the unit of residual
gases. This state normally lasts approximately 32 seconds
but on a special situation can last up to 5 minutes. If on
the first call for heat after power up, blower prover switch
is open, this state will take up to 5 minutes before declaring
an error.
The MCB will wait the normal cold purge time (normally 32
seconds) before checking for activation of all active blower
prover. If blower prover is active at this time the MCB cancels
the cold purge request. After the first cold purge has been
done a flag is set to prevent further activation of the shutter
adjust 5 minute delay. Any further requests for cold purge will
last the normal cold purge time. The FCB will go into fault mode