A.O. Smith 500 User's Information Manual page 12

except the fault states. If enabled the powered vent and the
IRI gas valve are turned on. A command is sent to all active
stages to cold purge the system. 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 Stages 1-4:
The system will command the FCB micros to start their heat
sequence starting with stage 1. Stages 2 through 4 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 CCB turns off the Powered Vent and the IRI Gas. The
pump remains on for the selected post-circulate time to cycle
the hot water out of the boiler. The FCB's are commanded to
shut down and the Alarm output is turned on. The green LED
turns off and the red "Service" turns on. The CCB remains in
this state until one of the following occurs:
• One hour passes (Soft fault automatic restart after one hour)
• If Communications error system will automatically restart
when communications are reestablished.
• 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 CCB turns off the Powered
Vent and the IRI Gas. The pump remains on for the selected
post-circulate time to cycle the hot water out of the boiler. The
FCB's are 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 CCB is in the Heating mode the activated FCB
stage 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 CCB 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:
If a Silicon Carbide Igniter is being used then its condition is
determined and any error is declared. The system will remain
in this state until the CCB requests a heating cycle or a cold
purge. The blowers, igniter, and gas relay are turned off.
• Pre-Purge State:
If the stage does not have a blower, this state is skipped and
the system advances to the Heat Igniter State. Otherwise 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 FCB advances to Flame Check status. If
flame is proven, the unit continues in run mode until set point
is reached.
• 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 boiler 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 boiler 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 and to the appropriate
fault state if a fault occur.
• Inter-Purge State:
The gas valve and the igniter are turned off. 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.
• Error State
The gas valve and the igniter are turned off. The FCB micro
tells the CCB micro which error has occurred. The system
waits in this state until the CCB sends a command to clear
the error. Then the Silicon Carbide Circuit is reset and the
system returns to the idle state.
• Cold Purge State
Cold purge occurs when the CCB micro commands all
blowers to go on before lighting the first stage to purge the
unit of 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, any blower prover
switch is open, this state will take up to 5 minutes before
declaring an error. This special mode is used by service
technicians to adjust the blower shutters.
The CCB will wait the normal cold purge time (normally 32
seconds) before checking for activation of all active blower
provers. If all are active at this time the CCB 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 min.
delay. Any further requests for cold purge will last the normal
cold purge time. The FCB will go into fault mode if the blower
prover does not activate after 15 seconds. This time allows
the blower relay to activate, the blower to come up to speed,
and the response from the blower prover to be filtered.
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