VCB-X Controller Field Technical Guide
SEQUENCE OF OPERATIONS
63
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 Heating and Vent Modes, the economizer will remain at
its minimum position. During Unoccupied Mode, the Economizer
can be used for night setback free cooling; otherwise it will remain
closed.
Dehumidifi cation Mode
On VAV, CAV, Single Zone VAV, and High Percentage Outdoor
Units with Space Temperature Control, the Dehumidi
fi
cation Mode
is initiated when the Indoor Humidity rises above the Low Indoor
Humidity Setpoint. The unit will leave the Dehumidi
fi
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 Dehumidi
fi
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 con
fi
guration options for Dehumidi
fi
cation operation.
In Occupied Vent Mode Only
—Dehumidi
fi
cation can only be
initiated in the Occupied Mode when there is no call for Heating or
Cooling. This creates a Vent Dehumidi
fi
cation Mode.
In Both Occupied and Unoccupied Vent Mode
—Dehumid-
i
fi
cation can be initiated in the Occupied and Unoccupied Modes
when there is no call for Heating or Cooling. This creates a Vent
Dehumidi
fi
cation Mode.
NOTE:
Do not use this option on a MUA unit that does not
have return air and which is not con
fi
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
—Dehumidi
fi
cation can be initiated
anytime in the Occupied Mode during Cooling, Heating, or Vent
Mode. This can create a Cooling Dehumidi
fi
cation Mode, a Heating
Dehumidi
fi
cation Mode, or a Vent Dehumidi
fi
cation Mode.
In All Modes while Occupied and Unoccupied
—Dehumid-
i
fi
cation can be initiated anytime in the Occupied or Unoccupied
Mode during Cooling, Heating, Vent, or Off Mode. This can create
a Cooling Dehumidi
fi
cation Mode, a Heating Dehumidi
fi
cation
Mode, or a Vent Dehumidi
fi
cation Mode. This con
fi
guration should
not be used for an MUA unit with no return air, since it could start
the unit in the Unoccupied Mode when the Outdoor Air damper is
closed. Any calls for Unoccupied Dehumidi
fi
cation use the same
Dehumidi
fi
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 con
fi
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 Dehumidi
fi
cation allowed during Heat Mode on a Heat
Pump unit.
NOTE:
Compressor Lockout Setpoints are ignored during
dehumidi
fi
cation since the compressors are controlled
by coil temperature.
Dehumidifi cation Operation
For DX Cooling Stages, the VCB-X activates the Cooling Stages
based on the actual Evaporator Coil Temperature compared to
the Evaporator Coil Temperature Setpoint. The Evaporator Coil
Temperature is calculated by using the Suction Pressure Sensor and
converting the pressure to temperature.
For Copeland Digital Scroll™ Compressor units, the VCB-X will
modulate the Copeland Digital Scroll™ Compressor to maintain the
Evaporator Coil Temperature Setpoint and activate Fixed Capacity
Compressors as necessary.
If the Fixed Capacity Compressor is activated, the Copeland Digital
Scroll™ Compressor will only be allowed to modulate within the
range of 70% - 100% in order to prevent the loss of reheat capacity
during low load conditions. If, with both compressors on, the digital
compressor has modulated down to its 70% minimum and the Coil
Suction Temperature falls below the Coil Temperature Setpoint minus
the Cooling Stage Control Window, then the second compressor will
stage off once its Compressor Minimum Run Time and the Stage
Down Delay Timers have been met. At that point, the Copeland
Digital Scroll™ Compressor can modulate down as needed to
maintain the Coil Temperature Setpoint.
SAFETY:
If the Coil Temperature drops below 32
˚
F, any
cooling remaining on will be forced to stage off.
For Modulating Chilled Water units, the VCB-X will modulate the
Chilled Water Valve to a
fi
xed 100% position during Dehumidi
fi
cation
to provide full moisture removal capabilities. With On/Off Chilled
Water units, the Cooling Relay will energize to open the valve.
During Dehumidification, the Economizer will be held to its
minimum position.If the unit will be using the CAV/MUA Dual
Mode (Hood On/Off) Operation, Dehumidi
fi
cation will require the
use of an Outdoor and Indoor Humidity Sensor.
Revised: 9/3/14
