Heat Cycle Defi Nitions - A.O. Smith POWER VENT GAS Installation And Operating Manual

HEAT CYCLE DEFINITIONS
STATE
Pre-purge
Inter-Purge
Post-Purge
Standby Mode
Lockout mode
Proven Ignition
If the heat engine initial call-for-heat is successful (all checks are passed), the combustion fan
is energized. The fan is started at a high speed for several seconds and the system verifi es the
fan speed. Input signals (from other parts of the water heater) are verifi ed before completion of
pre-purge including fan current verifi cation. If one of the checks fails then the unit moves into
the post-purge mode followed by the hard lockout mode and the appropriate error is displayed
(see Error Code List and Clear-out Procedure).
If the fl ame presence is detected the control delays a pre-determined amount of time keeping
the spark igniter energized. If fl ame presence is not detected within 5 seconds from the
moment the gas was allowed in the combustion chamber (three ignition trials) then the control
will initiate an ignition re-try for two more times. If fl ame presence cannot be verifi ed within
the given timing and after the ignitions' re-try (if applicable), the system enters the post-purge
mode and the gas fl ow is turned off.
The heat engine gas valve is now turned off and the fan speed is changed to high speed for
a predetermined time followed by a change of fan speed to a low value for the remaining
duration of the post purge mode. The control will also verify that the proper conditions are
maintained and there is no fl ame current sensed from the fl ame rod. If during the fi rst stage
of the post-purge mode (high speed fan operation) conditions for a new call for heat are met
then the system will allow the state change to "Initial call for heat" only after the fi rst stage of
the post-purge mode is complete.
After successful completion of post-purge, the fan is turned off and the control will wait until
the determined conditions for a call for heat are met. In standby mode the unit continually
monitors the input and output conditions and if abnormal conditions are detected the unit will
enter in a lockout mode. If a call for heat is required and all checks are passed, the control
will return to the initial call for heat state.
This is a non-operational mode following the detection of an error in the system. In this stage
the system displays an error message and continuously monitors input and output parameters.
The system can move from this state into the stand-by mode only if the error is cleared (typical
values for input and output parameters are recorded).
Once the power is turned on, the self-inspection is getting a normal status, water is turned
on and fan's current is normal then the ignition process will start for 5 seconds. If ignition is
successful, the heater will go to normal combustion state and the ignition sparking will stop
after 3 seconds. If ignition fails, the control will attempt to cycle through the heating stage
three times and then attempt to re-ignite the gas. If ignition cannot be achieved, the control
will end ignition attempts and prevent any more heating cycles, close the gas valve, turn the
blower on, and display the E28 error message to the user.
DESCRIPTION
TABLE 2
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