A.O. Smith VF VB 500, User Manual

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waits 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 occurs.
• 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. The blower
is then left on for 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 if the