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•
Safety chain
•
SAT sensor alarm
•
SPT sensor alarms
The VAV-RTU Open does not include smoke-control functions
such as a smoke-purge, zone-pressurization, or smoke-ventilation.
Each of these modes require a field-designed circuit to operate the
following by local fire codes:
•
RTU supply fan
•
RTU economizer
•
RTU power exhaust
The VAV-RTU Open many be configured to accept a Supply Fan
Status input to provide proof the supply fan is operating. When
enabled, a loss or lack of fan status will stop heating and cooling
operation.
A supply Fan Alarm Service Timer function is available to track
the number of supply fan run hours and generate an alarm when
the accumulated runtime exceeds the set threshold.
Economizer
The Economizer dampers are used to provide free cooling when
the outside conditions are suitable and Indoor Air Quality, if op-
tional CO
sensor is installed.
2
The following conditions must be true for economizer operation:
•
Indoor Fan is on.
•
Enthalpy is Low if the Enthalpy input is enabled.
•
SAT reading is available.
•
OAT reading is available.
•
SPT reading is available.
•
OAT <= High OAT economizer lockout configuration
(default = 75).
•
OAT <= SPT
For LC VAV rooftop the VAV-RTU Open will determine the re-
quired minimum economizer position (Calculated Min Econ Pos)
based on the fan's actual operation speed. The minimum damper
position can be overridden by the IAQ routine described later in
this section.
Since the VAV-RTU Open provides variable supply airflow to the
VAV system, as the supply airflow changes, the economizer mini-
mum position is adjusted to provide a constant amount of outdoor
air. If the fan operates at maximum speed, the economizer mini-
mum position will be set to the Vent Dmpr Pos / DCV Min Pos
setpoint. When the fan operates at its lowest speed, the economiz-
er minimum position will be set to the Low Fan Econ Min Pos.
For any fan speed between the minimum and maximum speeds,
the VAV-RTU control calculates a minimum economizer position
by a linear interpolation between these two values. When cooling
is required, if all preceding conditions are true and the outdoor En-
thalpy Status is Low, the economizer PID loop modulates the
damper between the minimum position and 100% open to main-
tain the Supply Air Setpoint.
During economizer operation, the economizer position is reduced
as the SAT falls below the Supply Air setpoint – 3
but never closes below the applicable minimum ventilation posi-
tion. The Calculated Min Econ Pos used for control is displayed in
the Maintenance section.
VAV-RTU Open provides Fault Diagnostic and Detection (FDD)
for economizer operation in compliance with California Title 24.
The FDD logic will detect an economizer that fails to close, fails
to open, stuck fully open, and fails to fully open. Each condition
will cause an Economizer Operation alarm to occur and the specif-
ic condition will be displayed.
Enthalpy Control
You may use an enthalpy switch to indicate the suitability of out-
door air for economizer cooling. You can use either an outdoor air
or differential enthalpy switch. A differential enthalpy switch has a
sensing device in both the outdoor and return air streams. A differ-
ential enthalpy switch indicates when outside air is more suitable
to be used than the return air and is available for economizer cool-
ing. If no enthalpy switch is configured, a network point (Object
Name: oae) is available. This point is displayed in the i-Vu appli-
cation and Equipment Touch as Enthalpy (BACnet).
The sequence of operation for economizer cooling is the same
with or without an enthalpy switch, except that an enthalpy switch
imposes one more validation on the suitability of outside air for
economizer cooling. An Enthalpy Status that is High disables the
economizer and the outside air damper goes to its minimum posi-
tion. An Enthalpy Status that is Low enables the economizer if a
call for cooling exists and the remaining preconditions are met.
Space Air Quality
Space Air Quality (AQ) is controlled on rooftop equipment using
the economizer. The Space AQ sequence utilizes an air quality
(CO
) sensor to monitor conditions within the occupied space. A
2
CO
sensor may be terminated at the VAV-RTU Open, or a subor-
2
dinate zone controller, when part of a zoned system.
When an outdoor air quality sensor is not installed, the algorithm
uses 400ppm as the fixed outdoor air CO
conditions must be true for the Space AQ algorithm to operate:
•
The system is occupied
•
The supply fan has been operating for at least 30 seconds
•
The Space AQ sensor has a valid reading
As the air quality in the space decreases (Space AQ CO
creases), the minimum position of the economizer increases, al-
lowing more outdoor air to enter the space. The amount of in-
crease depends on the relationship between the Space AQ level
and the DCV Max Ctrl Setpoint.
The Space AQ algorithm calculates a minimum position value us-
ing a PID loop. The CO
pared against the Vent Dmpr Pos / DCV Min Pos setpoint and the
greatest value becomes the minimum damper position utilized for
the economizer. When the minimum economizer position is being
reset by the Space AQ algorithm, the System Mode displays IAQ
Override. The maximum amount the economizer may be opened
to outdoor air by the Space AQ algorithm is limited by the DCV
Max Vent Damper Pos, which is adjustable between 10 and 75%.
Power Exhaust
VAV-RTU Open may enable and disable an exhaust fan based on
either the controller's occupancy or its economizer damper posi-
tion. If configured for continuous occupied operation, it will be
energized whenever the controller is in the occupied mode and
disabled when in the unoccupied mode. If configured for damper
position control, it will be energized whenever the economizer ex-
ceeds the power exhaust setpoint and disabled when the econo-
mizer drops below the setpoint by a fixed hysteresis of 10%. The
Power Exhaust Setpoint is automatically adjusted based on the
fan's air delivery. The Calculated PE Setpoint used for control is
displayed in MAINTENANCE→USER.
Heating
Δ
Δ
°F (1.7
°C),
The heat outputs are controlled by the Heating Control PID Loop
and Heating Stages Capacity algorithm.
MORNING WARM-UP
The specific heating sequence is determined by the controller's
application and configuration. The VAV-RTU Open controls up to
two stages of gas or electric heating. Morning Warm-Up occurs
after the first transition from unoccupied operation to occupied op-
eration if heat is required.
The following conditions must be true for the morning warm-up
algorithm to operate:
•
The Outdoor Air Temperature is less than the Heating
Lockout Temperature setpoint
23
level. The following
2
minimum damper position is then com-
2
value in-
2
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