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VCCX2 Controller Technical Guide
In the Defrost Cycle, the Reversing Valve Signal is switched
to the opposite operation, and the compressors are brought to
maximum capacity. Auxiliary Heat will be used to attempt to
maintain the Heating SAT Setpoint. Emergency Heat cannot be
used because it is locked out because the compressors are on.
The unit will leave the Defrost Mode after 10 minutes have
elapsed or the Defrost Coil Temperature Switch opens.
If the unit leaves the Compressor Heating Mode, the Defrost
Interval will restart once the unit re-enters the Compressor
Heating Mode.
Heat Pump Adaptive Defrost Operation
The Adaptive Defrost operation adjusts the time interval
(Adaptive Defrost Timer) in between Defrost Mode cycles.
As stated above, the unit will leave the Defrost Mode after 10
minutes have elapsed or the Defrost Coil Temperature Switch
opens. If the Defrost Cycle is terminated because the 10 minute
timer runs out, this could be an indicator that the unit may need
more defrost time. To address this issue, the Adaptive Defrost
Timer value will be subtracted from the original Defrost Interval.
If the Defrost Cycle is terminated between the 8th and 9th minute,
the Defrost Interval will not be changed.
If the Defrost Cycle is terminated before the 8th minute, this
could be an indicator that the unit may need less defrost time.
To address this issue, the Adaptive Defrost Timer value will be
inversely proportionally added to the original Defrost Interval as
the termination time moves from 8 minutes to 0 minutes.
Adaptive Defrost can be disabled by setting the Adaptive Defrost
Timer Setpoint to 0.
Water Source Heat Pump Operation
A reversing valve relay output can be configured to activate with
the first compressor stage in the Heating Mode or the Cooling
Mode of operation.
In Heating Mode, Auxiliary Heat can be used with Compressor
Heat to achieve the Active Supply Air Heating Setpoint. Auxiliary
Heat can be:
•
Modulating Heat
•
Staged forms of Heat,
•
A modulating form of heat followed by staged heat.
When Auxiliary Heat comes on in conjunction with a digital
compressor heat stage, the digital compressor will be locked at
100% until the Supply Air Temperature (SAT) rises above the
SAT Heating Setpoint plus the Heat Staging Window.
At that point, the Auxiliary Heat will stage off (after a stage-
down delay) and the Digital Compressor Heat will be allowed
to modulate.
There is no Defrost Mode on a Water Source Heat Pump Unit.
The unit can be configured for a percentage of glycol. The options
are 0%-40% in 5% increments.
Electronic Expansion Valve (EXV)
Operation
If using EXVs with Bitzer VFD Compressors and the RSMV
Module, then a Coil (Suction Line) Temperature Sensor
will measure the Coil (Suction Line) Temperature after each
Evaporator Coil line for each compressor, and this sensor will
be connected to an RSMV Module. This temperature will be
used in conjunction with the calculated saturated refrigerant
temperature to calculate the Superheat of each evaporator coil.
The EXV for each coil will then be controlled to maintain the
Superheat Setpoint.
Head Pressure Control
The Refrigerant System Module (RSM) can monitor a Head
Pressure Transducer and control a Condenser Fan to maintain a
Head Pressure Setpoint.
In Cooling Mode, the Condenser Signal will modulate to maintain
the Cooling Head Pressure Setpoint.
In Dehumidification Mode, the Condenser Output Signal controls
to the Reheat Head Pressure Setpoint. High Head Pressure
conditions produce the same effects as in the Cooling Mode.
See the appropriate RSM Technical Guide for a more detailed
sequence of operation.
SEQUENCE OF OPERATIONS
Heat Pump Adaptive Defrost, WSHP, EXV and Head Pressure
