Zone
Zone
SEQUENCE OF OPERATIONS
VCB-X Controller Field Technical Guide
64
Dehumidifi cation and Economizer Operation
Reheat
During the Dehumidi
fi
cation Mode, the VCB-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 Dehumidi
fi
cation, Heating Mode
Dehumidi
fi
cation, or Vent Mode Dehumidi
fi
cation.
During Cooling Dehumidi
fi
cation, Reheat is controlled to the Active
Cooling Supply Air Setpoint. During Heating Dehumidi
fi
cation,
Reheat is controlled to the Active Heating Supply Air Setpoint.
During Vent Dehumidi
fi
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 Dehumidi
fi
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 con
fi
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 Dehumidi
fi
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 speci
fi
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
VCB-X can activate the Heat Source(s) discussed in the Heating
Mode section. When Heating is used to supplement Hot Gas Reheat,
the VCB-X restricts the Heating to one form of Modulating Heat or
one stage of Gas or Electric Heat.
NOTE:
MODGAS cannot be used in conjunction with
Modulating Hot Gas Reheat to add additional reheat.
Coil Temperature Reset
The Indoor Humidity can be used to reset the Coil Temperature Setpoint.
A user adjustable range of Indoor Humidity values can be used to reset
the Coil Temperature Setpoint between a user adjustable range of values.
As the Indoor Humidity rises within its range the Coil 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 Dehumidi
fi
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.
NOTE:
See
page 6
for speci
fi
c AAON
®
DPAC and PAC
operation.
Revised: 8/14/13
