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OCCUPIED HEATING
Occupied Heating operates exactly the same as Morning Warm-up
except that heating can occur any time during the occupied period.
All the same conditions that apply to Morning warm-up apply to
occupied heating except: the restriction - Neither Cool nor Econo-
mizer modes have been active. In its place is:
•
Neither Cool or Economizer modes are currently active
and the 5 minute time guard between modes has expired.
UNOCCUPIED HEATING
The following conditions must be true for the unoccupied heating
algorithm to operate:
•
The Outdoor Air Temperature is less than the Heating
Lockout Temperature setpoint
•
Heating is required RAT (space temperature if configured)
is < unoccupied heating setpoint – 1
•
The unit has a valid Supply Air Temperature input
•
The unit has a valid RAT (space temperature if configured)
input
•
Neither Cool nor Economizer modes are active and the
5 minute time guard between modes has expired
When the heating algorithm preconditions have been met, the
Heating Stages Capacity algorithm calculates the Heating Control
Setpoint during the heating mode and the heat operates the same
as Morning Warm-up.
SUPPLY AIR TEMPERING
The VAV-RTU Open provides the option to operate the heat, if so
equipped, to maintain a minimum supply air temperature during
conditions where very cold outdoor air causes the supply air tem-
perature to fall below the configured SA Tempering Setpoint. This
occurs during periods where DCV is active and increasing the
amount of outdoor air or in cases where the system is operating at
very low airflow and the calculated economizer position has in-
creased to maintain a constant ventilation rate.
The following conditions must be true for the supply air tempering
algorithm to operate:
•
The SA Tempering is set to Enable
•
The indoor fan is on
•
The System Mode is Fan Only or IAQ Override
•
The Outdoor Air Temperature < Minimum Cooling SAT
•
Heat type is gas or electric and Number Of Heat Stages > 0
If the above are true, the VAV-RTU Open will monitor the SAT
sensor value and operate the first stage of heat to temper the sup-
ply air as required in order to maintain the configured SA Temper-
ing Setpoint. Heat operation is subject to anti-recycle timers to
protect the equipment from short-cycling. There are fixed applica-
tion specific minimum on and off times for each heating output
(gas = 120 seconds on and 60 seconds off / electric = 15 seconds
on and 10 seconds off).
Demand Controlled Ventilation
(standalone mode)
The VAV-RTU Open provides the option to control the economiz-
er's minimum ventilation position based on a Demand Controlled
Ventilation (DCV) algorithm. The algorithm monitors the CO
sensor value and compares that value to the user defined setpoint.
A PID control calculates the required minimum economizer posi-
tion required to satisfy the ventilation requirements of the space. A
user adjustable DCV Max Vent Damper Pos is provided to limit
the maximum amount of outdoor air that can be brought into the
unit due to the DCV algorithm.
The following conditions must be true for DCV operation:
•
The DCV Control is set to Enable
•
The unit has a valid IAQ sensor input
•
The unit is in an occupied mode
Δ
Δ
°F (.5
°C)
2
28
If the above are true, the VAV-RTU Open will monitor the IAQ
sensor value and the OAQ sensor value if the option is provided. It
will compare those values to the IAQ setpoint and then calculate
the required minimum economizer position as required. If that cal-
culated value is greater than the configured DCV Max Vent
Damper Pos, the value is clamped to that maximum limit. If the
DCV damper position is less than the calculated economizer mini-
mum position for ventilation, the economizer will maintain that
calculated economizer minimum position.
Demand Limit (standalone mode)
The VAV-RTU Open may employ a demand limit strategy. De-
mand limiting in the VAV-RTU Open works by resetting the Sup-
ply Air Static Pressure Setpoint. The demand level at which static
pressure reset begins is defined by the parameter SP Reset De-
mand Level which is selected by the user. The selected SP Reset
Demand Level determines at which demand level the SP Reset be-
comes active. The amount of reset is determined by Maximum
Reset.
Unoccupied Free Cooling (standalone mode)
The control can run a night time free cooling (NTFC) mode called
Unocc Free Cooling. In this mode the damper is utilized to bring
in outdoor air for free cooling during unoccupied periods. The
following conditions must be true for unoccupied free cooling to
operate:
•
Unocc Free Cool Enable set to Enable
•
The system is unoccupied
•
The outside air temperature is below the Economizer High
OAT Lockout Temp
•
The outside air temperature is less than the space tempera-
ture or RAT
•
Enthalpy (if enabled) is Low
When the VAV RTU Open is unoccupied and the RAT (space tem-
perature if configured) rises at least 2
cupied Cooling Setpoint, the supply fan starts. The economizer
damper opens as necessary to maintain the Supply Air Setpoint
and cool the space. The VAV RTU Open continues to operate in
this mode until the RAT drops to 1
pied Cooling Setpoint or the outside air conditions are no longer
suitable for free cooling.
Unoccupied Fan Cycling (standalone mode)
Unoccupied Fan Cycling Enable - enables the indoor fan to run
and permits RAT sampling during unoccupied modes. The VAV-
RTU provides fan cycling during the unoccupied periods to deter-
mine if there is a valid demand for heating or cooling before initi-
ating an unoccupied heating or cooling mode. This will allow the
RAT to be used to sample the actual space temperature conditions
more accurately during unoccupied periods when the fan is off
and without have to remotely mount the RAT sensor inside the re-
turn air duct inside the building. If enabled, the feature will start
the supply fan once every hour (default), allow it to operate for 5
minutes (default), and then use the measured RAT sensor value to
determine the unit's required operating mode for the last 1 minute
(default). The feature is disabled if a network RAT value is used, a
valid space temperature sensor is installed and functional, or if
Open Linkage is active.
The following conditions must be true for Unoccupied Fan Cy-
cling to operate:
•
Unoccupied Fan Cycling set to Enable
•
The system is unoccupied
•
No space temperature sensor or network RAT temperature
is configured
Fire Shutdown (standalone mode)
Fire Shutdown may be configured on Binary Input 5. A typical
application involves a smoke detector or fire shutdown contact,
which, when active, immediately shuts down equipment operation.
Δ
Δ
°F (1.1
°C) above the Oc-
Δ
Δ
°F (.5
°C) below the Occu-
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