Zone
Zone
SEQUENCE OF OPERATIONS
VCB-X Controller Field Technical Guide
70
CAV/MUA Dual Mode ( Hood On/Off
Operation)
The VCB-X Controller can be con
fi
gured as a CAV controller but
switch to MUA operation when an exhaust hood is energized. This
MUA force mode occurs when a 24 VAC wet contact closure is
received on the Hood On binary input on the VCB-X EM1 Expansion
Module. Under normal operation (CAV), the unit will operate as a
recirculating Space Temperature (and Space Humidity) controlled
unit.
When the Hood On contact is made, the unit will open the Outdoor
Air Damper to its full open position. The Heating and Cooling Modes
will then be determined by the Outdoor Air Temperature Sensor using
the Hood On Outdoor Air Heating and Cooling Setpoints which are
used only in Hood On operation. Dehumidi
fi
cation would then be
initiated by an Outdoor Dewpoint Setpoint.
When the Hood On Force Mode is removed, the unit will revert to
CAV operation with the Outdoor Damper returning to its minimum
position (unless economizer operation is enabled) and with mode
control initiated by the Space Temperature and Humidity Sensors.
Space Temperature Control of High
Percentage Outside Air Units
This option allows for Space Temperature control of 100% Outside
Air MUA Units or units with a high percentage of Outdoor Air
(normally 50% or greater). For this application you would con
fi
gure
“Space Temperature w/High OA %” for the Controlling Sensor
option. The intent of this sequence is to allow Space Temperature
control of the unit while preventing the dumping of hot or cold
outside air into the space during the Space Vent Mode of operation.
Once the Space Temperature is satis
fi
ed, before switching to Vent
Mode, the controller compares the Outside Air Temperature to the
Hood On HVAC Setpoints (Hood On MUA Setpoints) to determine
if a continued demand for heating or cooling is required to prevent
dumping. If there is no demand, the VCB-X Controller switches to
Vent Mode. If the Outside Air Temperature is greater than the Hood
On HVAC Cooling Setpoint or less than the Hood On HVAC Heating
Setpoint, plus the Occupied Deadband, the VCB-X Controller will
continue mechanical cooling or heating operation and stage it as
necessary to maintain the Vent Mode Supply Air Setpoint (Preset
to halfway between the Mode Enable Setpoints).
CAV/MUA and Condenser Fan/Water Valve Operation
During this Vent Mode Tempering operation, indoor humidity
(space or return air) will continue to control the dehumidi
fi
cation
operation. So, if dehumidi
fi
cation is con
fi
gured and the humidity
is above setpoint, the unit will be in Vent Mode Dehumidi
fi
cation
(see the Dehumidi
fi
cation Mode section for more details). Reheat
will be controlled to the calculated Vent Mode Supply Air Setpoint
described earlier.
A call for Heating or Cooling from the Space Sensor will cancel the
Outdoor Air Tempering operation.
NOTE:
All Minimum Run times must be satis
fi
ed before
mechanical cooling or heating is de-energized.
VAV Operation with Outdoor Air
Temperature Control
(Alternative Operation to VAV with
Supply Air Tempering)
On a VAV unit that may need daytime heating to be able to maintain
the Cooling Supply Air Setpoint, previous controllers used a Supply
Air Tempering sequence with the Supply Air Temperature Sensor
configured as the Controlling Sensor. The VCB-X Controller
accomplishes the same result using the Outdoor Air (OA)
Temperature Sensor as the Controlling Sensor.
First con
fi
gure for OA Temperature Control. Then con
fi
gure OA
Cooling and OA Heating Mode Enable Setpoints as well as Cooling
and Heating Supply Air Setpoints. Con
fi
gured this way, whenever
the OA temperature rises above the OA Cooling Setpoint, the unit
will be in Cooling Mode, controlling to the Cooling Supply Air
Setpoint. Whenever the OA Temperature falls below the OA Heating
Setpoint, the unit would be in the Heating Mode, controlling to the
Heating Supply Air Setpoint. The Cooling and Heating Supply Air
Setpoints can be set to the same values (e.g. 55
̊
F). The OA Heating
Setpoint should be calculated as the OA temperature that, when
mixed with the Return Air with the OA Damper (Economizer) at
its Minimum Position, will require Heating in order to achieve the
Supply Air Setpoint.
Finally, con
fi
gure the VCB-X for Duct Static Pressure Control and for
Morning Warm-Up control (Morning Warm-Up is now an available
option for any application except 100% Outdoor Air units).
This con
fi
guration will still allow the Economizer to be used for free
cooling when the unit is in the Cooling Mode.
Revised 2/14/14
