VCB-X Controller Field Technical Guide
SEQUENCE OF OPERATIONS
67
Single Zone VAV
Single Zone VAV Operation is a hybrid CAV/VAV application where
the Supply Fan VFD is modulated to maintain the Space Temperature
Setpoint while heating or cooling is modulated to maintain the
Leaving Air Setpoint. This application can be con
fi
gured to use VAV
Cooling and either VAV Heating or CAV Heating.
VAV Cooling and VAV Heating require modulating cooling
and heating sources in order to maintain a constant Leaving Air
Temperature no matter what the fan speed is. CAV Heating must be
con
fi
gured if using a staged form of heat.
The Space Temperature Sensor determines the heating or cooling
mode of operation. Heating and cooling are enabled and disabled as
described previously in the Heating and Cooling sections.
In the Cooling Mode, the modulating cooling source will modulate
to maintain the Cooling Leaving Air Setpoint. The Supply Fan VFD
will begin operation at the Minimum VFD Cooling Speed (30%
default) and modulate between this setpoint and 100% as needed
to maintain the Space Temperature within the Space Cooling Reset
Window created by con
fi
guring a Space Cooling High and a Space
Cooling Low Reset Source Setpoint.
If the unit is con
fi
gured for VAV Heating, then in the Heating Mode
the modulating heating source will modulate to maintain the Heating
Leaving Air Setpoint. The Supply Fan VFD will begin operation
at the Minimum VFD Heating Speed (50% default) and modulate
between this setpoint and the Maximum VFD Heating Speed (100%
default) as needed to maintain the space temperature within the Space
Heating Reset Window created by con
fi
guring a Space Heating High
and a Space Heating Low Reset Source Setpoint.
If the unit needs to be con
fi
gured for CAV Heating, set the Minimum
VFD Heating Speed to be the same as the Maximum VFD Heating
Speed desired during heating. Once the unit enters the Heating Mode,
the Supply Fan will run at the set Maximum VFD Heating Speed
(100% default) and Heating will occur as described in the Heating
Section of this sequence.
In the Vent Mode of operation, the Supply Fan will operate at the
VFD Vent Speed (user-adjustable).
During Dehumidi
fi
cation, the fan will operate as described above,
depending on if the Space Temperature is calling for Cooling,
Heating, or Vent Mode of operation.
Whenever the unit is in CO
2
override operation of the Outdoor Air
Damper, the minimum VFD Fan Speed is forced to 75% and can
modulate up from there.
Supply Air Temperature Setpoint
Reset
Various sources can be configured to reset the Supply Air
Temperature (SAT) Setpoint. Since the Supply Air Temperature
Setpoints are not
fi
xed during reset, we refer to them as “Active
Supply Air Temperature Setpoints.” The following Reset Source
options are available in this release:
1. Space
Temperature
2. Outdoor
Air
Temperature
3. Return
Air
Temperature
4. Fan
VFD
Signal
5. Remote SAT Reset Signal
For whatever option is selected, a High and a Low Reset Source
Setpoint must be con
fi
gured that will correspond to con
fi
gured
Low and High SAT Setpoints. This must be done separately for the
Cooling Mode Setpoints and for the Heating Mode Setpoints.
When the Reset Source is at its highest con
fi
gured setpoint, the SAT
Setpoint will be reset to its lowest con
fi
gured setpoint. When the
Reset Source is at its lowest con
fi
gured setpoint, the SAT Setpoint
will be reset to its highest con
fi
gured setpoint.
In all cases, as the Reset Source value moves within its range
established by the con
fi
gured High and Low Reset Setpoints, the
Supply Air Setpoint will be proportionally reset within its range
established by the con
fi
gured Low and High SAT Setpoints.
If a Remote SAT Reset Signal is con
fi
gured as the Reset Source, a
con
fi
gurable voltage signal (between 0 and 10 VDC, direct or reverse
acting) can be used to reset the Supply Air Temperature Setpoint. You
can con
fi
gure what voltage will correspond to the Low SAT Setpoint
and what voltage will correspond to the High SAT Setpoint in both
the Heating and the Cooling Modes. The EM2 Expansion Module
is required for this option.
Morning Warm Up and Single Zone VAV
Revised: 4/21/14
Once the Cool-Down Mode has been terminated, it cannot resume
until the unit has been through a subsequent Unoccupied Mode. Only
one Cool-Down Mode is allowed per Occupied cycle.
If you have stand-alone VAV boxes that need to be forced wide
open during the Cool-Down Mode, you can con
fi
gure one of the
relay outputs to be used during this Mode. If the Cool-Down Mode
is active, the relay is activated. This relay then becomes the Force
Open Command for all VAV boxes to which it is wired.
