32
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 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 (89 mm) 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.
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 (-6.7°C). If the outdoor air
temperature is more than 20°F (-6.7°C)
below the reset temperature setpoint
the amount of rest is equal to the reset
amount setpoint.
b
reset based on zone or return
temperature
Zone or return reset is applied to the
zone(s) in a building that tend to
overcool or overheat. The supply air
temperature setpoint is adjusted based
on the temperature of the critical
zone(s) or the return air temperature.
This can have the effect of improving
comfort and/or lowering energy usage.
The user-defined parameters are the
same as for outdoor air reset.
Logic for zone or return reset control is
the same except that the origins of the
temperature inputs are the zone sensor
or return sensor respectively. The
amount of reset applied is dependent
upon how far the zone or return 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 potentiometer on the
VAV Setpoint panel. The maximum
value is 3°F (-16.1°C). If the return or
zone temperature is more than
3°F (-16.1°C) below the reset
temperature setpoint the amount of
reset is equal to the reset amount
setpoint.
3
Zone Temperature Control
Unoccupied Zone Heating and Cooling
During Unoccupied mode, the unit is
operated as a CV unit. Inlet guide vanes
and VAV boxes are driven full open.
The unit controls zone temperature to
the Unoccupied zone cooling and
heating (heating units only) setpoints.
Daytime Warm-Up
During occupied mode, if the zone
temperature falls to a temperature
three degrees below the Morning
Warm-up setpoint, Daytime Warm-up
is initiated. The system changes to CV
heating (full unit airflow), the VAV
boxes are fully opened and the CV
heating algorithm is in control until the
Morning Warm-up setpoint is reached.
The unit is then returned to VAV
cooling mode. The Morning Warm-up
setpoint is set at the unit mounted VAV
Setpoint panel or at a remote panel.
