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VCCX2 Controller Technical Guide
Dehumidication Mode
On VAV, CAV, Single Zone VAV, and High Percentage Outdoor
Units with Space Temperature Control, the Dehumidication
Mode is initiated when the Indoor Humidity rises above the
Indoor Humidity High Reset Source Setpoint. The unit will leave
the Dehumidication Mode when the humidity falls below the
Indoor Humidity Low Reset Source Setpoint.
On 100% Outdoor Air (MUA) units with Outdoor Air
Temperature Control, Dehumidication is initiated when the
Outdoor Air Dewpoint rises above the Outdoor Air Dewpoint
Setpoint by 2°F. The unit will leave the Dehumidication Mode
when the humidity falls 2°F below this setpoint. The Outdoor
Air Dewpoint is calculated using the Outdoor Air Temperature
and the Outdoor Air Humidity
There are four configuration options for Dehumidification
operation.
In Occupied Vent Mode Only—Dehumidication can only be
initiated in the Occupied Mode when there is no call for Heating
or Cooling. This creates a Vent Dehumidication Mode.
In Both Occupied and Unoccupied Vent Mode—
Dehumid-ification can be initiated in the Occupied and
Unoccupied Modes when there is no call for Heating or Cooling.
This creates a Vent Dehumidication Mode.
NOTE: Do not use this option on a MUA unit that does
not have return air and which is not congured for
space controlled Night Setback operation. Damage
to the unit could occur since the OA damper
remains closed in the Unoccupied Mode.
In All Modes while Occupied—Dehumidication can be
initiated anytime in the Occupied Mode during Cooling, Heating,
or Vent Mode. This can create a Cooling Dehumidication Mode,
a Heating Dehumidication Mode, or a Vent Dehumidication
Mode.
In All Modes while Occupied and Unoccupied—
Dehumid-ication can be initiated anytime in the Occupied
or Unoccupied Mode during Cooling, Heating, Vent, or O
Mode. This can create a Cooling Dehumidication Mode, a
Heating Dehumidication Mode, or a Vent Dehumidication
Mode. Any calls for Unoccupied Dehumidication use the same
Dehumidication Setpoint as during the Occupied Mode since
you never want humidity to get out of control.
Dehumidication Operation on DX
Compressor Units
Any DX unit doing dehumidication will utilize one or more
Refrigerant System Modules (RSMs). Each RSM will control the
compressors, condensers, and EXVs (on Bitzer VFD compressor
units) for one or two refrigeration circuits. Up to four RSMs may
be used in controlling up to eight circuits.
The RSMD is used for digital compressor units, including heat
pumps. The RSMV is used for Bitzer VFD compressor units (non
heat pumps). The RSMV-HP is used for Bitzer VFD compressor
units that are heat pumps. The RSMVC is used for Copeland
VFD compressor units.
In the Dehumidication Mode, the compressors are controlled
to maintain the Suction (Saturation) Temperature Setpoint. Each
RSM will independently control its compressors to achieve the
most ecient dehumidication control.
See the appropriate RSM Technical Guide for a more detailed
sequence of operation.
CAUTION: If the Coil Saturated Temperature drops below
32˚F, any cooling remaining on will be forced
to stage o.
During Dehumidication, the Economizer will be held to its
minimum position. If the unit will be using the CAV/MUA Dual
Mode (Hood On/O) Operation, Dehumidication will require
the use of an Outdoor and Indoor Humidity Sensor.
Dehumidication Operation on Chilled Water
Units
For Chilled Water units, the VCCX2 Controller will open the
Chilled Water valve to a xed 100% position to provide full
moisture removal capability.
NOTE: Do not use this option on a MUA unit that does
not have return air and which is not congured for
space controlled Night Setback operation. Damage
to the unit could occur since the OA damper
remains closed in the Unoccupied Mode.
NOTE: Compressor operation is subject to the Outdoor
Air Temperature Cooling Lockout during
Dehumidication.
SEQUENCE OF OPERATIONS
Dehumidication Mode
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