ACC-APG002A-EN
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Heat Types
Two heating types are supported, staged electric and staged gas heat. Space temperature control
variable speed and multi-speed fan units will increase supply fan speed with increased heating capacity.
Multi-Zone VAV units with staged heat will always operate the supply fan at full airflow, see Maximum
Heat, in Heat Cool Modes section for details.
Staged Gas Heat
Symbio™ 700 supports 2 stages of gas heat for primary heating capacity. Gas heat is composed of a
single manifold with a two-stage, single gas valve. The burner is the inshot type with induced draft. As
demand for heat increases, incremental stages of heat are added. Stage 1 provides approximately 70%
of available heating capacity.
Staged Gas Heat Sequence
1. A call for heat is initiated by the Symbio 700 via a digital Modbus™ communication signal to the
White Rogers ignition module.
2. Pre-purge for 30 seconds. The draft inducer is energized at high speed for 25 seconds, the air
pressure switch closes, the inducer fan reduces to low speed for 5 seconds.
3. At end of pre-purge period, spark and gas valve are energized.
4. The burner ignites Stage 1 with the gas valve at low fire and low inducer fan speed. Trial for ignition
is seven seconds, in which time flame must be sensed.
5. The unit will add Stage 2 with the gas valve at high fire and high inducer fan speed, as required.
6. Unit continues in operation until the call for heat terminates.
7. The gas valve is de-energized and unit enters its post-purge at low inducer fan speed for 30
seconds.
8. Draft inducer is shut off.
9. When the control terminates gas heating capacity or exits a heating mode, a post heat fixed timer is
enforced. The supply fan is kept on to remove heat from the unit before transitioning to a cooling
mode or cycling the supply fan off.
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 gas heat ignition controller will lock out. Note, see
Appendix for a full list of gas heat diagnostics.
The ignition control board will reset lockout diagnostic after one hour. Control lockout can be cleared by
cycling the power off for a minimum of ten seconds. An ignition module heat lockout can also be cleared
by removing the call for Heat at the Thermostat, Zone Sensor or BAS System for 5 seconds and back to
heat.
Electric Heat — Staged
When staged electric heat is configured, the controller manages two outputs to provide two stages of
heat control.
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