Sequence of Operation
RT-SVX21AC-EN 55
The supply fan will accelerate or decelerate as required to
maintain the supply static pressure setpoint.
Supply Air Static Pressure Limit
The control of the supply fan 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 VAV boxes are closed. The
unit is then allowed to restart three times. If the
overpressurization condition occurs on the fourth 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.
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 Potentiometer 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 Economizer Actuator, 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 potentiometer for supply air
cooling control.
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 Potentiometer: 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 reset temperature setpoint the
amount of reset is equal to the reset amount setpoint.
Reset Based on Zone or Return Temperature
Zone or return reset is applied to the zone(s) in a building
that tends 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 potentiometer. The maximum value is
3°F. If the return or zone temperature is more than 3°F
below the reset temperature setpoint the amount of reset
is equal to the reset amount setpoint.
Zone Temperature Control
Unoccupied Zone Cooling
During unoccupied mode, the unit is operated as a CV unit.
VAV boxes are driven full open and the supply fan is
commanded to full speed. The unit controls zone
temperature to the Unoccupied zone cooling 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
potentiometer or at a remote panel.
Morning Warm-up (MWU)
Morning warm-up control (MWU) is activated whenever
the unit switches from unoccupied to occupied and the
zone temperature is at least 1.5°F below the MWU
setpoint. When MWU is activated the VAV box output will
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