23
• Economizer is unavailable, or if the Economizer is active,
mechanical cooling is available if the economizer is open
> 90% for at least 7.5 minutes, the SAT > [Minimum Cool-
ing SAT + 5°F (2.7°C)] and SPT > [Effective Cooling Set-
point + 0.5°F (0.27°C)].
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.
Any time the compressors are running the RTU Open control-
ler 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.
• Cooling – Without humidity control [RH Control = Disable]
or an economizer [Economizer Exists = No], the operation of
the unit is as follows:
If OAT > Cooling Lockout Temperature (45°F def) and
SPT > cooling setpoint, then a PID will calculate the re-
quired number of compressor cooling stages required to
satisfy the present space load conditions. After the mini-
mum compressor off time has expired, then the first com-
pressor stage will be started. The actual number of active
compressor stages [Active Compressor Stages] is dis-
played in the maintenance section.
As the compressor stage(s) operate, the controller moni-
tors the supply air temperature and compares the measured
value to the configured supply air temperature setpoint
[Stage 1 SAT Stpt if Active Compressor Stages = 1, see the
following table].
The fan will begin to operate (or currently be operating) at
the configured minimum fan speed [Min VFD Output]. If
the measured SAT falls below this setpoint, a PID will cal-
culate the required VFD output to increase the fan speed to
obtain the configured Stage “X” SAT Stpt depending on
the actual number of the operating stage (1-3). The fan
speed can increase as required but not greater than the con-
figured maximum speed [Max VFD Output or IDF Max
Speed Voltage].
Supply Fan
The RTU Open controller supply fan may be configured for 1
of 3 Fan Control modes:
• Single – The fan operates at one speed only and provides
on/off operation
• 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 op-
erates at high speed. This control mode is used with
48/50TC(Q), 48/50HC(Q) and size 04-06 48/50LC units
when Staged Air Volume (SAV™) option is provided.
• 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 horse-
power consumption. Fan speed is NOT controlled by static
pressure. This setting is default for LC WeatherExpert size
07-26 units, FC/GC units, and when WeatherExpert is select-
ed under Unit Type configuration.
The RTU Open controller supply fan may be configured for 1
of 3 Fan Modes:
• Auto – The fan cycles on/off in conjunction with heating
or cooling.
• Continuous – The fan runs continuously during occupan-
cy and intermittently during unoccupied periods with heat-
ing and cooling.
• Always On – The fan runs continuously regardless of oc-
cupancy or calls for heating and cooling.
Occupancy can be determined by Linkage, 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 immediate-
ly, regardless of the occupancy state or demand:
• Fire Shutdown
• Safety chain
• SAT sensor alarm
• SPT sensor alarms
The RTU Open controller does not include smoke-control
functions such as a smoke-purge, zone-pressurization, or
smoke-ventilation. Each of these modes require a field-de-
signed circuit to operate the following by local fine codes:
• RTU supply fan
• RTU economizer
• RTU power exhaust
The RTU Open controller many be configured to accept a Sup-
ply Fan Status input to provide proof the supply fan is operat-
ing. 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 Quali-
ty, if optional CO
2
sensor is installed.
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
If any of the mentioned conditions are not true, the economizer
will be set to its configured minimum position.
If the RTU Open controller is configured for VFD or 2-speed
fan, and the fan is on or at the highest speed, or if the unit is
configured for single-speed fan, then the economizer will use
the Vent Dmpr Pos/DCV Min Pos setpoint as the minimum
economizer position when occupied. If the unit is configured
for VFD or 2-speed fan, and the fan is on or at the lowest
speed, the economizer will use the Low Fan Econ Min Pos.
NOTE: For LC WeatherExpert units or any unit with Variable
Speed fan control, the RTU Open controller 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.
Active Compressor Stages Controlling SAT Setpoint
=1 Stage 1 SAT Stpt
=2 Stage 2 SAT Stpt
=3 Stage 3 SAT Stpt
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