RT-SVX073A-EN
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configured; otherwise, the supply fan operates at full
capacity in all other heat operating mode
All heat types, when the control terminates heating
capacity or exists a heating mode; a 5-minute post heat
timer is enforced. This keeps supply fan On to remove heat
from the unit before transitioning to a cooling mode or
cycling the supply fan off.
Gas Heat
There are two types of gas heat available: Staged and
Modulating. Burners will not turn on if the heater minimum
off timer is active, the heater inter-stage timer (three
minutes) is active, or modulating heater is not at maximum
rate.
Gas Heat Sequence
Call for heat initiated
1. A call for heat is initiated by a relay.
2. Inducer pressure switch is open prior to starting the
inducer fan.
3. The inducer fan is started, and pressure switch is
tested for closure. The supply fan proving switch and
inducer fan proving switch must be closed within 10
seconds of energizing the inducer fan to initiate pre-
purge for 60 seconds.
4. Ignition speed is reduced to ignition level and trial for
1.5 seconds.
5. Flame is detected and stabilized.
6. Heater is Stage 1 for staged or minimum fire for
modulating.
7. Stage 2 Enable, requests the second stage of heat
from the gas heat controller.
While heat operation is in progress, the gas heat controller
is monitoring the flame, inducer fan, and thermal limit
switch. If any of these items open, a lockout code is set.
Unexpected Shutdown
If an unexpected shutdown occurs when the gas heat
controller reflects standby without an error code, an
unexpected shutdown diagnostic is set. This can happen
when there is a component failure upstream of the gas heat
controller.
Lost Gas Heat Flame
If flame is lost, the gas heat controller shuts down and
requires a reset to clear the lockout. After clearing the
lockout, the unit controls request a retry for ignition. If after
three unsuccessful attempts to re-ignite the heater, a
manual lockout is set, and diagnostic communicated. This
diagnostic will auto reset after 90s, and heat will be allowed
to attempt to start again if requested due to temp control.
If gas/flame is not detected, such as air in lines, a failure to
start diagnostics is set.
Failure to Start
If pre-purge is not detected within 60 seconds, after
request for Stage 1 or minimum fire, it indicates a problem
where the gas heat controller is not receiving the heat
enable signal. A diagnostic message of “heater failed to
start” is set. No retries are attempted.
“Heater failed to start” diagnostic also sets if starting is not
proven within six minutes of closing the enable relay if no
other diagnostics occur. This allows for the retries due to
failure to prove flame at ignition and covers any unforeseen
sequence issues at startup.
If the gas heat controller does not sense a flame after an
ignition attempt, it will lockout (NO FLAME AT END OF
TRIAL FOR IGNITION) and a corresponding non-latching
diagnostic is called out. The diagnostic is reset after 90
seconds and then anther request for ignition is initiated. If
after three attempts, a successful ignition is not achieved,
heat is locked out and a latching diagnostic set.
• The 90 second delay to auto reset the diagnostic
begins when the Symbio controller receives
communication for the failed ignition lockout and sets
the diagnostic. 90 seconds allows for post purge and
other delays.
• The enable relay stays closed throughout the retries
and delays.
• At the end of the 90 second delay, a reset command is
sent to the gas heat controller through the reset relay
output which causes the lockout and diagnostic to
clear.
• Check for Failure to Start detection is performed only
on first attempt.
• If while in the 90 second delay the diagnostic is reset
through user interface or externally, the delay is
canceled and retry is attempted.
• The retry count is reset by either: a successful start,
removal of call for heat, occurrence of lockout
diagnostic, or power cycle of Symbio.
Call for Heat Terminated
When the unit has determined the request for heat is no
longer required, the enable signal is removed and the gas
heat controller will de-energize the valve and the inducer
fan starts its 60-second post purge.
Electric Heat - Staged
When staged electric heat is configured in the unit, the
controller manages four binary outputs to provide up to
eight-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.
Unit Startup
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