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RT-SVX073A-EN
Functional Description
EXV Calibration
An EXV Calibration (also known as overdrive closed)
procedure will be initiated in the following circumstances:
• Whenever the unit controller is power cycled. This is
under the control of the unit controller software, as part
of the software startup procedure.
Note: Under the most common scenario when both the
EXV electronic controller and unit controller are
power cycled, EXV Calibration may occur twice.
• When the circuit cycles off and the circuit has run for at
least EXV Recalibration Time since the previous
calibration.
Note: EXV calibration time is used for both HGBP and
EXV valve calibration.
• Whenever the EXV electronic controller is power
cycled. The EXV electronic controller does this
automatically.
EXV Manual Override Control
See Programming Guide
EXV Control Sequence of Operation
The EXV control algorithm can be in one of the following
states:
Table 36. EXV Control States
Control State Description
Stopped EXV closed. Overdrive closed (calibration) performed if criteria are met.
Starting EXV pre-position is calculated, and the valve arrives at the requested opening
before any compressors on the circuit start.
Stopping EXV commanded to close. Remain in state until EXVs report that they are
closed.
Running EXV operates in pre-start control, or in automatic control after the pre-position
hold time expires.
Running Mode Sub-States Description
Running – Wait for Circuit Start Time after valve arrives at pre-position value, but before a compressor on the
circuit starts.
Running – Pre-Position Hold Time After a compressor on the circuit starts, but before automatic control begins.
Running – Suction Superheat Control Normal automatic superheat control
Running – Suction Pressure Control Controlling suction pressure instead of suction superheat, to maintain
adequate compressor pressure ratio or to limit high suction saturated
temperature.
Running – Evaporator Dryout Control Special version of suction pressure control, to dry out a flooded evaporator and
prepare to control suction superheat or suction pressure again.
Operating Modes
Calibrating_EXV
Circuit submode displayed when EXV is going through the
calibration process. Most often active when circuit top-level
mode is Stopped, Run Inhibit, or Auto.
Prepositioning_EXV
Circuit submode displayed when EXV(s) are moving
toward their commanded pre-position opening prior to
starting the first compressor on circuit startup.
Manual_Override_Evaporator
Displayed when performing manual control override on the
EXV in the Evaporator position.
Economizing
Symbio 800 supports a a single outdoor air / economizer
damper. Economizer high-limit decision methods are
defined as follows.
Unit Startup
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