User manual A.O. Smith 101

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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.  
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.  
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 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.  
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.  
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.  
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:  
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.  
One hour passes (automatic restart after one hour)  
If Communications error system will automatically  
restart if communications re-establishes.  
Heating State:  
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.  
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.  
(
outlet water temperature drops to safe level). The fault  
is logged in the error history when the fault state is  
exited.  
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.  
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.  
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.  
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.  
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  
Description of FCB control states:  
Idle State:  
When any error is declared during idle state-The system  
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