Orion System VCC-X Controller - Manual (Page 58)

Zone
Zone
SEQUENCE OF OPERATIONS
VCC-X Controller Technical Guide 58
Reheat and Return Air Bypass Damper Control
If the VFD compressor reaches 100% for the Stage Up Delay and
the Suction (Saturated) Temperature is still above setpoint, then the
fi xed compressor will stage on while the VFD compressor modulates
as needed. If the VFD compressor has modulated down to 0% and
the Suction (Saturated) Temperature is 5 degrees below the Suction
(Saturated) Temperature for the Stage Down Delay period, the fi xed
compressor will stage off and the VFD compressor will continue to
modulate. During this operation, compressor Minimum Run Times
and Minimum Off Times must be met.
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 VCC-X will modulate the
Chilled Water Valve to a fi xed 100% position during Dehumidifi 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, Dehumidifi cation will require the
use of an Outdoor and Indoor Humidity Sensor.
Reheat
During the Dehumidifi cation Mode, the VCC-X activates Cooling to
extract moisture from the Supply Air and utilizes either Modulating
Hot Gas Reheat, On/Off Hot Gas Reheat, or Heating to reheat the
Supply Air. Hot Gas Reheat is the standard form of Reheat.
Reheat is always controlled to the Active Supply Air Setpoint (see
the Supply Air Temperature Setpoint Reset section for further
explanation) which will be different depending on whether
the unit is in Cooling Mode Dehumidifi cation, Heating Mode
Dehumidifi cation, or Vent Mode Dehumidifi cation.
During Cooling Dehumidifi cation, Reheat is controlled to the Active
Cooling Supply Air Setpoint. During Heating Dehumidifi cation,
Reheat is controlled to the Active Heating Supply Air Setpoint.
During Vent Dehumidifi cation, Reheat is controlled to a calculated
setpoint that is halfway between the Heating and Cooling Mode
Enable Setpoints.
If the unit is equipped with a Modulating Hot Gas Reheat Controller
(MHGRV-X), then during Dehumidifi cation, it will modulate the
reheat valve to maintain the Supply Air Temperature at the Active
Supply Air Temperature Setpoint.
If the unit is equipped with an On/Off Hot Gas Valve, then one of
the relays will be confi gured for Reheat. The Reheat Relay will be
activated if the SAT is less than the SAT Setpoint. The Hot Gas
Reheat Relay will remain on during the Dehumidifi cation Mode
regardless of the Supply Air Temperature. This is to ensure a steady
Supply Air Temperature.
The HVAC unit’s Heat Source or a Heat Source located in the
Supply Air Duct can be used for Reheat if the unit is not equipped
with Hot Gas Reheat or to supplement Hot Gas Reheat. Please read
the warning that follows regarding applications that operate Heating
and Cooling simultaneously.
WARNING: Simultaneous Heating and Cooling cannot be
approved unless the HVAC unit has been specifi cally designed
for this purpose. A Special Price Authorization (SPA) must be
obtained from the AAON ® factory for these applications to
avoid warranty and/or rating problems. WattMaster Controls
Inc. assumes no liability for any Simultaneous Heating and
Cooling application if a SPA is not obtained from the AAON ®
Factory at the time the HVAC unit is ordered.
When Heating is used for Reheat instead of Hot Gas Reheat, the
VCC-X can activate the Heat Source(s) discussed in the Heating
Mode section. When Heating is used to supplement Hot Gas Reheat,
the VCC-X restricts the Heating to one form of Modulating Heat or
one stage of Gas or Electric Heat.
Coil (Saturated) Temperature Reset
The Indoor Humidity can be used to reset the Coil (Saturated) Tem-
perature Setpoint. A user adjustable range of Indoor Humidity values
can be used to reset the Coil
(Saturated) Temperature Setpoint between
a user adjustable range of values. As the Indoor Humidity rises within
its range the Coil
(Saturated) Temperature Setpoint will be lowered
within its range.
Return Air Bypass Damper Control
The Return Air Bypass (RAB) Damper is only used on constant
air volume units with space temperature configured as the
HVAC Mode Enable sensor. The RAB Damper is only active
during the Dehumidifi cation Mode and is used as the fi rst form
of Reheat. If the HVAC unit is equipped with modulating Hot
Gas Reheat, the RAB Damper needs to be at 100% before the
modulating Hot Gas Reheat can be used. The RAB Damper
modulates from 0-100% as the Space Temperature falls below
the Cooling Setpoint. When the Space Temperature is equal to
the Cooling Setpoint, the RAB Damper will be at 0%. When the
Space Temperature falls to halfway between the Cooling and Heating
Setpoints, the RAB Damper will be at 100%.
If the HVAC unit is equipped with separate actuators for the
Outdoor Air and Return Air Dampers, the Return Air Damper will
proportionally close more as the RAB Damper opens. The rate at
which the Return Air Damper closes is user-adjustable. The purpose
of closing the Return Air Damper more as the RAB Damper opens
is to allow more air to bypass the evaporator coil through the RAB
Damper. If you want more air to pass through the RAB Damper,
enter a larger number in the Return Air Damper Factor setpoint. If
you want less air to pass through the RAB Damper, enter a smaller
number in the Return Air Damper Factor setpoint.