A.O. Smith Boiler VB/VW- 750 User Manual

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Theprocesswaitsinthisstateuntilthecallforheatissatised,the
thermostatinputisopened,orafaultoccurs.Whenheatissatised,
the sequence continues to Post-Purge and then back to Idle. If
three tries for ignition has been selected on the MCB dipswitch and
ame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ame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amesensorischecked.Ifameisdetected,theigniter
is turned off. The state machine then advances to the Heating
State.Ifafter5seconds,theame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satised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rstcoldpurgehasbeendone
aag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
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ltered.
• 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.
OPERATING SETPOINT ADJUSTMENT PROCEDURE
The system has a standard programmable Operating Setpoint
rangeof70°F(21°C)to190°F(88°C)foraVWandof70°F
(21°C)to220°F(104°C)foraVBforeitheraRemoteorinlet
probe depending on selection. The user can easily change
the Operating Setpoint at any time by using the following
procedure. When any configuration or setpoint is changed
(andtheSelectkeytouched),thenewvalueisIMMEDIATELY
saved to non-volatile memory. The factory default setting is
120°F(49°C).
1. Press the MENU key.
2. Scrollthe“>”withtheDOWNkeynexttotheUSERSETTING
SCREEN.
3. Press the SELECT key.
4. Scrollthe“>”withtheDOWNkeyuntilitispointingto
OPERATING SETPOINT.
5. Press the SELECT key.
6. Use the UP or DOWN key to select the value you wish to enter.
7. Press the SELECT key to accept and store the new value.
• HighLimit:
The outlet temperature probe contains both an ECO switch
and a thermistor for temperature measurement. The sensed
outlet temperature is used for the automatically resettable High
Limit setpoint. If the sensed outlet temperature exceeds the
HighLimitsetpoint,a“softlockout”conditionwilloccurand
the burner will be shut off. This fault condition is automatically
cleared when the temperature drops below the high limit
setpoint minus the high limit differential.
AUTO HIGH LIMIT SETPOINT ADJUSTMENT PROCEDURE/
OUTLET TEMPERATURE PROBE
TheHighLimitsetpointhasarangeof90°F(32°C)to210°F(99°C)
foraVWandarangeof90°(32°C)to235°F(113°C)foraVB.

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