18
S
Heat mode is not active and the time guard between
modes equals zero.
S
If occupied and the SPT > (occupied cool setpoint plus
the T56 slider offset).
S
Space Temperature and supply air temperature values
are available.
S
If it is unoccupied and the SPT > (unoccupied cool
setpoint plus the T56 slider offset). The indoor fan will
be turned on by the staging algorithm.
S
If economizer is available and active and economizer
open > 85% and SAT > (SAT low limit + 5
_
F) and SPT
> effective set point + 0.5
_
F.
OR
Economizer is available, but not active
OR
Economizer is not available
S
OAT > DX Lockout temperature
If all of the above conditions are met, the controller uses a
PID logic loop to energize or de- energize compressors to
satisfy the cooling demand.
There is a fixed 3- minute minimum on time and a
5- minute off time for each compressor output and a
3- minute minimum time delay between staging up or
down.
Any time the compressors are running the RTU Open will
stage down the compressors if the SAT becomes less than
the cooling low supply air setpoint. After a compressor is
staged off, it may be started again after a normal
time- guard period and the supply air temperature has
increased above the low supply air setpoint.
Supply Fan
The RTU Open supply fan may be configured for 1 of 3
Fan Control modes:
S
Single -
The fan operates at one speed only and
provides on/off operation
S
Two Speed -
The fan operates at 1 of 2 speeds depending
on the mode of operation and load conditions. During fan
only or single stage cooling, the fan operates at low speed.
During heating, dehumidification, second stage cooling, or
if maximum economizer operation is required, the fan
operates at high speed.
S
Variable Speed -
The fan operates at a variable speed
to meet the load conditions and SAT safety
requirements to provide maximum energy savings by
minimizing fan horsepower consumption. Fan speed is
NOT controlled by static pressure.
The RTU Open supply fan may be configured for 1 of 3
Fan Modes:
S
Auto -
The fan cycles on/off in conjunction with
heating or cooling.
S
Continuous -
The fan runs continuously during
occupancy and intermittently during unoccupied
periods with heating and cooling.
S
Always On -
The fan runs continuously regardless of
occupancy or calls for heating and cooling.
Occupancy can be determined by BACnet schedules, BAS
schedules, or in response to a remote occupancy switch.
A Fan Off Delay allows the supply fan to continue
operating after heating or cooling stops.
If the following alarms are active, the fan turns off
immediately, regardless of the occupancy state or demand:
S
Fire Shutdown
S
Safety chain
S
SAT alarm
S
SPT alarms
The 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
fine codes:
S
RTU supply fan
S
RTU economizer
S
RTU power exhaust
The 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
and Indoor Air Quality, if optional CO
2
sensor is installed,
when the outside conditions are suitable.
The following conditions must be true for economizer
operation:
S
Indoor Fan has been on for at least 30 seconds.
S
Enthalpy is Low if the Enthalpy input is enabled.
S
SAT reading is available.
S
OAT reading is available.
S
SPT reading is available.
S
OAT <= High OAT economizer lockout configuration
(default = 75).
S
OAT <= SPT
If the RTU Open is configured for VFD or 2- speed fan,
and the fan is on high speed or is configured for
single- speed fan, and any of the preceding conditions are
not true, the economizer will be set to the Vent Dmpr
Pos/DCV Min Pos setpoint. If it is configured for VFD or
2- speed fan, and the fan is on low speed, and any of the
preceding conditions are not true, the economizer will be
set to the Low Fan Econ Min Pos.
If any of the mentioned conditions are not true, the
economizer will be set to its configured minimum position.
The minimum damper position can be overridden by the
IAQ routine described later in this section.
If the above conditions are true, the Economizer control
will calculate a damper position starting with a minimum
vent position and based on current space temperature and
TS
-
5580
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01