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Unit Start-Up
Sequence of Operation
See ACC-APG002*-EN, Application Guide, Symbio™ 700
Controller with Precedent™ Packaged Rooftop Air-
Conditioners.
Ignition Module – Two-stage
Two-stage (IGN) runs self-check (including verification that
the gas valve is de-energized). (IGN) checks the high limit
switches (TC01 and TC02) for normally closed contacts,
the pressure switch (PS) for normally open contacts, and
the flame rollout (FR) switch for continuity.
(IGN) energizes inducer blower on high speed to check
pressure switch closure. If the pressure switch is closed,
the inducer blower starts a 20 second pre-purge (15
seconds on high speed followed by 5 seconds on low
speed). If the pressure switch (PS) is still open, the inducer
blower will continue to be energized on high speed until
pressure switch closure.
After pre-purge completes, the IGN energizes the first
stage gas output for first stage heat demand and energizes
second stage gas output for second stage heat and detects
flame and de-energizes spark.
At this point indoor fan would start with its minimum speed
for the corresponding heat (first stage or second stage)
demand.
The (IGN) enters a normal operating loop where all inputs
are continuously monitored.
If a call for second stage heat is initiated after a call for first
stage heat is established, the (IGN) energizes the second
stage of the gas valve and the second stage of inducer
blower.
When the zone thermostat is satisfied, the (IGN) de-
energizes the gas valve. The (IGN) senses loss of flame.
The (IGN) initiates a 5 second inducer blower post purge.
The Symbio™ 700 initiates a 90 second indoor blower-off
delay at the current speed. The (IGN) de-energizes the
inducer blower at the end of the post purge. The Symbio
700 de-energizes the indoor blower at the end of the
selected indoor blower-off delay
Note: For complete Gas Heat User information, Operation,
Start-up, Shutdown and Maintenance refer to
Heat Operation and Maintenance," p.
Table 13. Ignition module diagnostics
Steady OFF
Steady ON
Slow Flash Rate
1 Flash
38
"Gas
53.
No Power/Failure/ Internal Failure
Normal – No Call for heat
Normal, call for heat (¾ second on, ¼
second off)
Loss of Communication
Table 13. Ignition module diagnostics (continued)
2 Flashes
3 Flashes
4 Flashes
5 Flashes
6 Flashes
7 Flashes
8 Flashes
9 Flashes
Ignition Module – Modulating
Gas
Call for Heat
1. Thermostat (or heat enable) contact closes providing
call for heat.
2.
The provided analog input is 2.0 Vdc (0.5% ModBus
firing rate) or higher, the modulating actuator is placed
in the park position and the draft inducer is energized,
the air pressure transducer is calibrated, and the draft
inducer is run to the pre-purge setting. A 15 to 30
second pre-purge is initiated. The display on the
VB1285 will show p p u u r r . See below.
3. The modulating actuator is moved to the ignition
position during this time.
4. At the end of the pre-purge period, the ignition source is
activated, and the main gas valve opens allowing gas
to flow to the burners for the trial for ignition period
[I I g g n n ].
5. Burners ignite and cross light. Flame is sensed and
control continues the heating cycle.
6. Control enters a Warm-up period [H H E E A A ] that allows for
burner stabilization prior to active modulation. Warm-up
duration is program controlled.
7. At the end of the warm-up period, the 1
the run state characterized by normal burner
modulation in response to the 2-6 Vdc analog input (1 -
56% ModBus firing rate) signal to the control [r r u u n n ].
8. The modulating gas valve is set as commanded by the
analog FIRING RATE input signal:
System Lockout: Failed to detect or
sustain flame. (failure to ignite, no
spark, low/no gas pressure, etc.)
Pressure switch problem detected. (no
vent air flow, bad CBM, closed at initial
call for heat). Auto reset.
High Limit switch protection device
open (excessive heat in combustion
chamber, low airflow). Auto reset.
Flame sensed and gas valve not
energized, or flame sensed and no call
for heat.
Flame Rollout Switch open. (CBM
failure, incorrect gas pressure, incorrect
primary air). Requires manual reset of
the switch.
Weak Flame
Internal Error
Hardware Mismatch
st
stage enters
RT-SVX077B-EN
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