Trane Symbio 210 Installation, Operation And Maintenance Manual page 49

The airflow control algorithm compares the active airflow setpoint with the measured airflow and
calculates the necessary air valve movement to minimize error. The airflow setpoint is limited by
applicable minimum and maximum flow setpoints.
Reheat Control
There are two (2) types of reheat control:
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Reheat is allowed to turn ON if the zone temperature is below the heating setpoint. Space temperature
control can use reheat if the following conditions are all TRUE:
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Potential Equipment Shut Down and/or Damage Due to Overheating!
Failure to follow instructions below could result in equipment damage due to overheating.
There are potential maintenance issues, such as overheating, that can occur if the electric
reheat is allowed to energize when the airflow passing over the element is too low or is below
the Airflow Setpoint Minimum Local Heat. Refer to the equipment catalog for Trane F-Style VAV
boxes for minimum airflow settings based on box inlet size and heating element kW.
Electric Heat
Electric Heat Pulse-width Modulation (PWM)
Single-duct units can have from one to three stages of electric heat pulse-width modulation. Fan
powered units can have up to three stages of electric heat pulse-width modulation. Energizing for a
portion of a three-minute time period modulates the electric heater. This allows exact load matching for
energy efficient operation, and optimum zone temperature control. The Airflow Setpoint Minimum Local
Heat is enforced during reheat. The amount of reheat supplied is dependent on both the degrees that
the space temperature is below the active heating setpoint and the time that the space temperature has
been below the active heating setpoint. If not already OFF, reheat de-energizes when the zone
temperature rises more than 0.5°F (0.28°C) above the heating setpoint. When reheat is de-energized,
the cooling Airflow Setpoint Minimum is enforced.
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Electric Heat SCR
SCR is a heat controller that controls a single stage electric heater proportional to an analog signal. The
analog output signal is proportional to the amount of reheat required. Typically with SCR heat, the
heater is turned on and off on a very short cycle time to provide proportional control of heat output. This
allows for closer matching of heating capacity to the heating load, resulting in more stable temperature
control.
When the space requires cooling, the air valve modulates between Airflow Setpoint Maximum and
Airflow Setpoint Minimum, to maintain space temperature at the active cooling setpoint; and the electric
heater is off. When the cooling load has decreased to the point where the air valve has reached Airflow
BAS-SVX091A-EN
Hydronic heat
Electric heat
The unit is not calibrating.
Heat Cool Mode Status is heat.
For local reheat only, the fan, if present, is not being overridden to Off (communicated fan override).
The supply air temperature is less than the configured reheat enable setpoint.
For electric reheat, the unit is in Pressure Independent mode or the Pressure Dependent Mode
Reheat Enable point is enabled.
Stage 1; the ON time is proportional to the amount of reheat required. For example, when 50% of
stage 1 capacity is required, reheat is ON for 90 seconds and OFF for 90 seconds. When 75% of
stage 1 capacity is required, reheat is ON for 135 seconds and OFF for 45 seconds. When 100% of
stage 1 capacity is required, reheat is ON continuously.
Stages 2 and 3; the same ON time logic as stage 1, except stage 1 is always energized. For
example, when 75% of unit capacity is required, stage 1 is energized continuously, and stage 2 is
ON for 90 seconds and OFF for 90 seconds.
Calibration, Operation Modes, and Control
NOTICE
49