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first. The Off and Running status of each manifold and
burner are provided on the user interface.
When the gas heat is in Warmup or Running state, the
Symbio 800 controller will report the status as
Running. Burners will not turn on if the heater
minimum off timer is active, the heater inter-stage
timer (3-minutes) is active, or modulating heater is not
at maximum rate. Burners will not turn off if the heater
is in process of starting, heater is undergoing a retry or
modulating heat is not at a minimum rate.
Modulating and Ultra Modulating gas heat types
modulate the first burner on manifold A. The minimum
and maximum modulation rates are defined by each
stage of heating. The modulating stage controls to
maximum modulation rate before another burner stage
is added. Likewise, the modulating burner must reach
minimum rate before a stage is subtracted.
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Call for heat initiated
1. A call for heat is initiated by the digital Modbus
signal.
2. Draft Inducer is energized, an air pressure switch
closes initiating pre-purge for 30 seconds.
3. At end of pre-purge period, Spark and Gas Valve
are energized for an ignition trial period for 4
seconds.
4. Burners ignite.
5. Flame detected by flame sensor and control
operates in steady state heating condition.
6. Unit continues in operation until the call for heat
terminates.
7. The Gas valves are de-energized and unit enters its
post-purge for 30 seconds.
If ignition is not achieved within the trial period, the gas
valve is shut off; the inducer continues to run for a 30
second post-purge period. Additional ignition trials
follow the specified sequence. If 3 attempts for ignition
have occurred without flame detection, the control will
lock out.
The control board will reset after one hour. Control
lockout can be cleared by cycling the power off for a
minimum of 5 seconds.
If flame is lost once it has been established, the control
will shut off the gas supply within 2.0 seconds and
enter the post-purge period. Control will allow a
predetermined number of flame losses before lockout.
If flame sensor indicates presence of flame during
purge period when no flame should be present, the
inducer will remain energized but the gas valve will
remain off until the cause of the “false flame” is
removed.
If the air pressure switch is closed when the inducer is
de-energized, or does not close after the inducer is
energized, the control will wait a predetermined time
for the air switch to correct itself, if not then lock out.
Control will go into lockout with all outputs off.
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Call for Heat Initiated
1. A call for heat is initiated by the digital Modbus
signal.
2. The gas heat control board will then go through a
system check to ensure that the high temperature
limit and rollout switches are closed, the air
pressure switch is open, and the modulating valve
is positioned correctly.
3. The control will then enter the pre-purge cycle
where the inducer will run at the programmed
purge pressure. During this cycle, the control will
look for the air pressure switch to close and open at
the correct settings..
4. Once the system check and pre-purge cycles are
complete, the control will enter the ignition cycle.
a. The modulating valve and inducer will go to
their “ignition” settings.
b. The DSI ignition module will be energized and
the spark igniter will activate.
c. The redundant safety valve will open, allowing
gas flow.
d. The burners will ignite and the control will
receive a signal from the flame sensor.
e. The spark igniter will remain active for the
duration of the ignition cycle regardless of flame
status.
5. If flame is not established during the ignition cycle,
the control will perform a post-purge and repeat the
pre-purge and ignitions cycles up to three times.
After three failed ignition attempts, the board will
enter a 1 hour lockout.
6. Once flame has been established, the control will
enter a warmup period to ensure flame stabilization
and reduce condensation in the heat exchanger.
7. After the warmup period, the gas heat control will
enter the run cycle. During the run cycle, the burner
firing rate and draft inducer pressure are
determined based on the heat demand received by
the control via a Modbus signal.
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e:: If the control is paired with a split manifold, steps
1 through 6 pertain to the primary burners. Once
the control exits the warmup period and the
firing rate is dictated by the rooftop control, the
control will ignite the secondary burners and
modulate the primary burners based on the
demand for heat.
The run cycle will continue until any of the following
conditions are met.
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1. Any of the safety devices (high limit, air pressure,
rollout, etc.) are triggered.
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