Operating Setpoint Adjustment Procedure - A.O. Smith VB-500, VB-750 Instruction Manual

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 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 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
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.

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.
41