Orion System VCC-X Controller - Manual (Page 57)

VCC-X Controller Technical Guide
SEQUENCE OF OPERATIONS
57
Economizer Operation & Dehumidifi cation
Economizer Operation
Economizer operation is enabled when the Outdoor Air (OA) drybulb,
wetbulb, or dewpoint temperature falls below the Economizer Enable
Setpoint by 1°. Economizer operation is disabled when the OA
temperature rises 1° above the Economizer Enable Setpoint.
The Economizer acts as the 1 st stage of cooling and controls to the
Active Supply Air Cooling Setpoint. An Economizer Minimum
Position can be programmed into the controller. During Economizer
Operation, the economizer will modulate between this minimum
position and 100%. If the economizer reaches 100% and the Supply
Air Temperature is still above setpoint, mechanical cooling is then
allowed to stage up while the economizer is held at the full open
position. Any time cooling stages are currently running, and the
economizer becomes enabled, it will immediately open to 100%.
During Heat and Vent Modes, the Economizer will remain at its
minimum position. The only exception to this can occur during “VAV
Operation with Outdoor Air Temperature Control (VAV Tempering)”.
See that section for more details. During Unoccupied Mode, the
Economizer can be used for night setback free cooling; otherwise
it will remain closed.
IAQ (CO
2
) Override of the Economizer simply resets the Economizer
Minimum Position higher. See IAQ (CO
2
) Control Operation section
for more details.
If utilizing the Title 24 Economizer option, an Economizer feedback
signal (0-10 VDC) can be wired into the VCCX-EM1 Expansion
Module for status monitoring. Several Title 24 alarm conditions
can also be annunciated and are listed in the Alarms section of this
manual.
Dehumidifi cation Mode
On VAV, CAV, Single Zone VAV, and High Percentage Outdoor
Units with Space Temperature Control, the Dehumidifi cation Mode
is initiated when the Indoor Humidity rises above the Low Indoor
Humidity Setpoint. The unit will leave the Dehumidifi cation Mode
when the humidity falls 1% below this setpoint.
On 100% Outdoor Air (MUA) units with Outdoor Air Temperature
Control, Dehumidification is initiated when the Outdoor Air
Dewpoint rises above the Outdoor Air Dewpoint Setpoint. The unit
will leave the Dehumidifi cation Mode when the humidity falls 1%
below this setpoint. The Outdoor Air Dewpoint is calculated using
the Outdoor Air Temperature and the Outdoor Air Humidity.
There are four confi guration options for Dehumidifi cation operation.
In Occupied Vent Mode Only —Dehumidifi cation can only be
initiated in the Occupied Mode when there is no call for Heating or
Cooling. This creates a Vent Dehumidifi cation Mode.
In Both Occupied and Unoccupied Vent Mode —Dehumid-
ifi cation can be initiated in the Occupied and Unoccupied Modes
when there is no call for Heating or Cooling. This creates a Vent
Dehumidifi cation Mode.
NOTE: Do not use this option on a MUA unit that does not
have return air and which is not confi gured 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 —Dehumidifi cation can be initiated
anytime in the Occupied Mode during Cooling, Heating, or Vent
Mode. This can create a Cooling Dehumidifi cation Mode, a Heating
Dehumidifi cation Mode, or a Vent Dehumidifi cation Mode.
In All Modes while Occupied and Unoccupied —Dehumid-
ifi cation can be initiated anytime in the Occupied or Unoccupied
Mode during Cooling, Heating, Vent, or Off Mode. This can create
a Cooling Dehumidifi cation Mode, a Heating Dehumidifi cation
Mode, or a Vent Dehumidifi cation Mode. Any calls for Unoccupied
Dehumidifi cation use the same Dehumidifi cation Setpoint as during
the Occupied Mode since you never want humidity to get out of
control.
NOTE: Do not use this option on a MUA unit that does not
have return air and which is not confi gured for space
controlled Night Setback operation. Damage to the
unit could occur since the OA damper remains closed
in the Unoccupied Mode.
There is no Dehumidifi cation allowed during Heat Mode on a Heat
Pump unit.
NOTE: Compressor operation is subject to the Outdoor Air
Temperature Cooling Lockout during Dehumidifi ca-
tion.
Dehumidifi cation Operation
Once in Dehumidifi cation Mode, units with fi xed compressors
will activate the compressors to maintain the Evaporator Coil
Suction (Saturated) Temperature Setpoint. An RSMV Module will
be required for each fi xed compressor or each fi xed compressor
tandem pair in order to monitor the suction (saturated) temperature
and control the compressor(s) accordingly. Each RSMV will
independently activate and stage its compressor(s) to maintain that
circuit’s Suction (Saturated) Temperature Setpoint. The staging of
compressors on an RSMV is subject to Stage Up and Stage Down
Delays as well as compressor Minimum Run Times and Minimum
Off Times.
A suction pressure transducer is used and the VCC-X Controller
converts that to a Suction (Saturated) Temperature value
On units with one or more sets of VFD/Fixed tandem compressors,
each set of tandem compressors (one circuit) will be controlled by
a separate RSMV Module with its own Suction Pressure Transducer
(converted to a Suction (Saturated) Temperature). Once the unit
enters the Dehumidifi cation Mode, each RSMV will initially activate
the VFD compressor as required to maintain the Suction (Saturated)
Temperature Setpoint. At that point each RSMV will modulate its
VFD compressor and enable the fi xed compressor, as necessary,
to maintain the Suction (Saturated) Temperature Setpoint for that
circuit.