A.O. Smith 405, 400, 402, 401, 403, 404 instruction manual Description of FCB control states

•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 (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 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:

When any error is declared during idle state-The system will remain in this state until the CCB request a heating cycle, or a cold purge.

•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 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 CCB micro which error has occurred. The system waits in this state until the CCB sends a command to clear the error, 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 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. This special mode is used by service technicians to adjust the blower shutter.

The CCB 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 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.

•User Settings Screen:

Each setpoint or user setting has either a limited selection of values, or a limited range of values. The Up/Down keys are used to change values. After changing an item, the Select key is pressed to accept the change, or the Menu key is pressed to reject the change and restore the item to its original value. The following setpoints can be changed:

•Operating Setpoint:

This setpoint sets the base temperature for the control algorithm. See Figures 20A & 20B.

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