27
the supply airflow changes, the economizer minimum position is
adjusted to provide a constant amount of outdoor air.
When the fan operates at maximum speed, the economizer mini
-
mum position will be set to the Vent Dmpr Pos / DCV Min Pos
setpoint. For any fan speed between the minimum and maximum
speeds, the VAV-RTU Open control calculates a minimum econo
-
mizer position by a linear interpolation between these two values.
When cooling is required, if all preceding conditions are true and
the outdoor Enthalpy Status is Low, the economizer PID loop
modulates the damper between the minimum position and 100%
open to maintain the Supply Air Setpoint.
During economizer operation, the economizer position is reduced
as the SAT falls below the Supply Air setpoint – 3
Δ
°F (1.7
Δ
°C),
but never closes below the applicable minimum ventilation
position.
The VAV-RTU Open provides FDD (Fault Detection and Diag
-
nostics) for economizer operation in compliance with California
Title 24. The FDD logic will detect an economizer that fails to
close, fails to open, is stuck fully open, and fails to fully open.
Each condition will cause an Economizer Operation alarm to
occur and the specific fault condition will be displayed.
ENTHALPY CONTROL
An enthalpy switch can be used 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 out
-
door and return air streams. A differential enthalpy switch indi
-
cates when outside air is more suitable to be used than the return
air and is available for economizer cooling.
If no physical enthalpy switch is configured, a network point
(Object Name: oae) is available. This point is displayed in the i-Vu
application and an Equipment Touch as Enthalpy (BACnet). If no
enthalpy input is used, then only a dry bulb OAT comparison can
be used to determine the suitability of the outdoor air.
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 maintains only a position
that provides minimum ventilation during occupied periods. An
Enthalpy Status that is Low enables the economizer, if a need 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
2
) sensor to monitor conditions within the occupied space. A
CO
2
sensor may be terminated at the VAV-RTU Open, or a subor
-
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
2
level.
The following 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
2
value
increases), 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
2
minimum damper position is then com
-
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
The VAV-RTU Open may enable and disable an exhaust fan,
based on either the controller’s occupancy or a calculated econo
-
mizer damper position. The Power Exhaust Setpoint is automati
-
cally adjusted based on the fan’s air delivery in order to maintain a
constant building pressure. The Calculated PE Setpoint used for
control is displayed in MAINTENANCE→USER.
If Continuous Occupied Exhaust is Yes, the Power Exhaust binary
output (BO-8) is energized while the VAV-RTU Open is occupied
and de-energized when unoccupied.
If Continuous Occupied Exhaust is No, the Power Exhaust binary
output (BO-8) is energized when the economizer damper output
exceeds the Calculated Power Exhaust (PE) Setpoint value. The
output remains energized until the economizer output falls below
the Power Exhaust Setpoint value by a fixed hysteresis of 10%.
Heating (standalone mode)
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
• Heating is required (Space temperature is < occupied heat
-
ing setpoint – 1
Δ
°F (.5
Δ
°C) The unit has a valid Supply
Air Temperature input
• The unit has a valid Space Temperature input
• The unit has a valid Return Air Temperature (RAT) sensor
input
• Neither Cool nor Economizer modes have been active
previously.
The heating relays are controlled by the Heating Control PID
Loop and Heating Stages Capacity algorithm, which calculate the
required Supply Air Temperature to satisfy the space by compar
-
ing the RAT to the:
• Effective Occupied Heating Setpoint when occupied
When the heating algorithm preconditions have been met, the
Heating Stages Capacity algorithm calculates the Heating Control
Setpoint during any heating mode. Anti-recycle timers are em
-
ployed to protect the equipment from short-cycling. There are
fixed application specific minimum on and off times for each heat
-
ing output (gas = 120 seconds on and 60 seconds off / electric =
15 seconds on and 10 seconds off).
During heating operation, the VAV-RTU Open may reduce the
number of active stages as the rooftop’s Supply Air Temperature
approaches the Maximum Heating SAT setpoint. A heat stage
turned off in this fashion may be started again after the normal
time-guard period has expired, if the Supply Air Temperature has
decreased below the Maximum Heating SAT setpoint.
Heating Service Alarm Timer functions are available (1 for each
stage of heat). This function tracks the number of heat run hours
and generates an alarm when the accumulated runtime exceeds the
threshold set by the adjustable heat service alarm timers.
Содержание 48LCB07
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Страница 9: ...9 Fig 6 VAV RTU Open Control Wiring Diagram 48LC B08 12 ...
Страница 10: ...10 Fig 7 VAV RTU Open Control Wiring Diagram 48LC B14 26 ...
Страница 11: ...11 Fig 8 VAV RTU Open Control Wiring Diagram 50LC B07 ...
Страница 12: ...12 Fig 9 VAV RTU Open Control Wiring Diagram 50LC B08 12 ...
Страница 13: ...13 Fig 10 VAV RTU Open Control Wiring Diagram 50LC B14 26 ...
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