80
VCCX2 Controller Technical Guide
In the Cooling Mode, the modulating cooling source will
modulate to maintain the Cooling Supply Air Setpoint. The
Supply Fan VFD will begin operation at a user-adjustable
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 configuring a Space Cooling High and a Space Cooling Low
Reset Source Setpoint.
If the unit is configured for VAV Heating, then in the Heating
Mode the modulating heating source will modulate to maintain
the Heating Supply Air Setpoint. The Supply Fan VFD will
begin operation at a user-adjustable 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 configuring a Space Heating High and a
Space Heating Low Reset Source Setpoint.
If the unit needs to be configured for CAV Heating, set the
Minimum VFD Heating Speed to be the same as the Maximum
VFD Heating Speed desired during heating (
page 76
). Once the
unit enters the Heating Mode, the Supply Fan will run at the set
Maximum VFD Heating Speed (100% default).
In the Vent Mode of operation, the Supply Fan will operate at
the VFD Vent Speed (user-adjustable).
During Dehumidification, the fan will operate as described above,
depending on if the Space Temperature is calling for Cooling
Dehumidification or Heating Dehumidification.
When the controller is in the Vent-Dehumidification Mode,
the Supply Fan VFD will still modulate based on the Space
Temperature. It looks at a temperature half way between the
Cooling and Heating setpoints and modulates between the
Cooling minimum and 100% as the Space Temperature goes from
the midway value to 0.5°F above that value. For example, if the
Cooling Enable Setpoint is 74°F, the Heating Enable Setpoint
is 70°F, and the Space Temperature is 72°F or less, the Supply
Fan will be running at the cooling minimum. As the Space
Temperature rises to 72.5°F, the fan will go to 100%.
Night Setback operation uses the same SZ VAV fan control logic;
however, since the setbacks would normally be larger than the
space reset range for the fan speed, the fan will typically always
be at 100%.
If the Hood On/Off operation is used on a SZ VAV unit, then
during Hood On, the mode enable will switch to the Outdoor Air
Temperature Sensor using Hood On Mode Enable Setpoints and
the Outdoor Air Damper will modulate to 100%. The Supply Fan
VFD will modulate to 100%.
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 fixed 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 configured that will correspond to configured
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 configured setpoint,
the SAT Setpoint will be reset to its lowest configured setpoint.
When the Reset Source is at its lowest configured setpoint, the
SAT Setpoint will be reset to its highest configured setpoint.
In all cases, as the Reset Source value moves within its range
established by the configured High and Low Reset Setpoints, the
Supply Air Setpoint will be proportionally reset within its range
established by the configured Low and High SAT Setpoints.
If a Remote SAT Reset Signal is configured as the Reset Source,
a configurable voltage signal (between 0 and 10 VDC, direct or
reverse acting) can be used to reset the Supply Air Temperature
Setpoint. You can configure 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.
NOTE:
This Supply Air Temperature reset cannot be used
on a Single Zone VAV Unit.
SEQUENCE OF OPERATIONS
IAQ Control, Morning Warm-Up and Cool-Down, and Single Zone VAV
