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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.
Note: 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.
Call for Heat Terminated
1. Any of the safety devices (high limit, air pressure,
rollout, etc.) are triggered.
2. Once the run cycle has terminated, the redundant
safety valve will close, the modulating valve will return
to its set position, and the draft inducer will ramp up to
its "ignition" setting for a 30 second post - purge cycle
3. After the conclusion of the post-purge, the gas heat
control will enter the "OFF" state. All system outputs are
de-energized, but all safety devices are still monitored.
Flame Failure
Once locked out on flame failure, the IC board will not
reactivate the ignition/combustion control circuit until it is
reset manually. To do this, press the reset button on the
front of the (IC) board case.
RT-SVX063G-EN
Note: The modulating gas heaters are factory adjusted for
safe operation and to reach the nameplate rated
firing MBH for most areas of the country. The proper
air/gas ratio must be reached by the service tech
during start-up.
Electric Heat - Staged
When staged electric heat is configured in the unit, the
controller manages four binary outputs to provide up to 6-
stages of heat control; depending on size of the electric
heat installed. If four or less electric heat stages are
installed, the binary outputs are energized in sequence to
add additional heat capacity. If more than four electric heat
stages are configured, the binary outputs are energized in
combination to provide increasing heating capacity with
each stage.
Electric Heat - Modulating
Modulating Electric Heat provides improved control of
electric heat by modulating the first stage of heat. When the
first stage of heat reaches 100% capacity, the next binary
stage of electric heat is energized. The first modulating
stage of heat is reset to 0% capacity and continues to
modulate as heating capacity demand increases.
An airflow limit control is applied when modulating electric
heat is configured to reduce the risk of high temperature
limit switch trips. This feature quickly reduces modulating
electric heat capacity when the supply airflow decreases
due to duct pressure control or external supply fan control.
Electric Heat Protection
There are three automatic high temperature limit switches
that will trip when exposed to a high temperature. They will
reset automatically once the temperature drops into an
acceptable range. If any one limit switch trips, all stages of
heat are turned off. The limits are mounted vertically on the
heater terminal plate and all three have the same
temperature trip point.
A manual reset temperature limit is also mounted near the
top of the terminal plate and serves as backup protection.
When exposed to high temperature, it will turn all stages of
heat off until the reset button is manually reset. The electric
heat protections operate independently of the controller.
External Heat
The Symbio 800 controller supports external field supplied
heat sources. There is no factory wiring or coordination
between the controller and external heat. When External
Heat is configured, the supply fan operates as required in
heat modes and provides ventilation defined by the mode.
Single Zone-VAV units have an External Heat Supply Fan
Speed Setpoint (adjustable) that defines the supply fan
speed in Occupied Heat modes of operation. The supply
fan operates at maximum speed for all other heating
modes. Continuous and cycling fan modes are both
supported.
Sequence of Operation
85
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