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RT-PRC007-EN28
Controls
VAV Units Only
Sequence of Operation
1
Supply Air Pressure Control
Inlet Guide Vane Control
Inlet guide vanes are driven by a
modulating 2-10 vdc signal from the VAV
Module. A pressure transducer
measures duct static pressure, and the
inlet guide vanes are modulated to
maintain the supply air static pressure
within an adjustable user-defined range.
The range is determined by the supply
air pressure setpoint and supply air
pressure deadband, which are set
through a unit mounted potentiometer.
Inlet guide vane assemblies installed on
the supply fan inlets regulate fan
capacity and limit horsepower at lower
system air requirements. When in any
position other than full open, the vanes
pre-spin intake air in the same direction
as supply fan rotation. As the vanes
approach the full-closed position, the
amount of “spin” induced by the vanes
increases at the same time that intake
airflow and fan horsepower diminish.
The inlet guide vanes will close when the
supply fan is shut down.
Supply Air Static Pressure Limit
The opening of the inlet guide vanes and
VAV boxes are coordinated, with respect
to time, during unit start up and
transition to/from Occupied/Unoccupied
modes to prevent overpressurization of
the supply air ductwork. However, if for
any reason the supply air pressure
exceeds the fixed supply air static
pressure limit of 3.5” W.C., the supply
fan is shut down and the inlet guide
vanes are closed. The unit is then
allowed to restart four times. If the
overpressurization condition occurs on
the fifth time, the unit is shut down and a
manual reset diagnostic is set and
displayed at any of the remote panels
with LED status lights or communicated
to the Integrated Comfort system.
Variable Frequency Drives (VFD) Control
Variable frequency drives are driven by a
modulating 0-10 vdc signal from the VAV
module. A pressure transducer
measures duct static pressure, and the
VFD is modulated to maintain the supply
air static pressure within an adjustable
user-defined range. The range is
determined by the supply air pressure
setpoint and supply air pressure
deadband, which are set through a unit
mounted potentiometer. Variable
frequency drives provide supply fan
motor speed modulation. The drive will
accelerate or decelerate as required to
maintain the supply static pressure
setpoint. When subjected to high
ambient return conditions the VFD shall
reduce its output frequency to maintain
operation. Bypass control is offered to
provide full nominal airflow in the event
of drive failure.
2
Supply Air Temperature Controls
Cooling/Economizer
During occupied cooling mode of
operation, the economizer (if available)
and primary cooling are used to control
the supply air temperature. The supply
air temperature setpoint is user-defined
at the unit mounted VAV Setpoint Panel
or at the remote panel. If the enthalpy of
the outside air is appropriate to use “free
cooling,” the economizer will be used
first to attempt to satisfy the supply
setpoint.
On units with economizer, a call for
cooling will modulate the fresh air
dampers open. The rate of economizer
modulation is based on deviation of the
discharge temperature from setpoint,
i.e., the further away from setpoint, the
faster the fresh air damper will open.
Note that the economizer is only allowed
to function freely if ambient conditions
are below the enthalpy control setting or
below the return air enthalpy if unit has
comparative enthalpy installed. If outside
air is not suitable for “economizing,” the
fresh air dampers drive to the minimum
open position. A field adjustable
potentiometer on the Unitary
Economizer Module, Tracer®, or a
remote potentiometer can provide the
input to establish the minimum damper
position.
At outdoor air conditions above the
enthalpy control setting, primary cooling
only is used and the fresh air dampers
remain at minimum position.
If the unit does not include an
economizer, primary cooling only is used
to satisfy cooling requirements.
Supply Air Setpoint Reset
Supply air reset can be used to adjust
the supply air temperature setpoint on
the basis of a zone temperature, return
air temperature, or on outdoor air
temperature. Supply air reset adjustment
is available on the unit mounted VAV
Setpoint Panel for supply air cooling
control.
a
Reset Based on Outdoor Air Temperature
Outdoor air cooling reset is sometimes
used in applications where the outdoor
temperature has a large effect on
building load. When the outside air
temperature is low and the building
cooling load is low, the supply air
setpoint can be raised, thereby
preventing subcooling of critical zones.
This reset can lower usage of primary
cooling and result in a reduction in
primary cooling energy usage.
There are two user-defined parameters
that are adjustable through the VAV
Setpoint Panel: reset temperature
setpoint and reset amount. The amount
of reset applied is dependent upon how
far the outdoor air temperature is below
the supply air reset setpoint. The amount
is zero where they are equal and
increases linearly toward the value set at
the reset amount input. The maximum
value is 20 F. If the outdoor air
temperature is more than 20 F below the