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When the indoor relative humidity becomes greater then
the dehumidification setpoint (or switches from low to
high), a dehumidification demand will acknowledged.
Compressor state is monitored and time guards are
honored. If a compressor was just turned off prior to the
dehum call the dehumidification output will be delayed
the 5 minute minimum off time of the compressor. When
ok to dehumidify, the dehumidification output (J11- 7, 8)
will be energized. This will bring on the supply fan (at
high fan speed if Fan Control is set to "Two Speed"), all
compressors, and the dehumidification relay placing the
unit in Hot Gas Reheat dehumidification mode. If
dehumidification is called for during cooling or cooling is
called for during dehumidification, the unit will run in
Subcooling dehumidification mode. Individual unit
circuits can be in different dehumidification modes based
on the demand. Refer to the base units operation for
additional information.
NOTE: There is a fixed 5% hysteresis that the indoor
relative humidity must drop below the active setpoint to
end the dehumidification mode and de- energize the
dehumidification output. The output will also de- energize
if the fan relay is de- energized.
Demand Limit
If the RTU Open receives a level 1 (one degree offset), 2
(two degree offset), or a 3 (4 degree offset) to the BACnet
demand limit variable, the controller will outwardly
expand the heating and cooling setpoints by the
configured demand limit setpoint value and remain in
effect until the BACnet demand limit variable receives a 0
value.
Unoccupied Free Cooling
When the unit is equipped with an economizer, the control
can run a night time free cooling (NTFC) mode called
Unocc Free Cooling. In this mode the damper is utilized
to bring in outdoor air for free cooling during unoccupied
periods. The following conditions must be true for
unoccupied free cooling to operate:
S Unocc Free Cool Enable set to Enable
S The system is unoccupied
S The outside air temperature is below the Economizer
High OAT Lockout Temp
S The outside air temperature is less than the space
temperature
S Enthalpy (if enabled) is Low
When the RTU Open schedule is unoccupied and the
space temperature rises at least 1 degree above the
Occupied Cooling Setpoint, the supply fan starts. The
economizer damper opens as necessary to cool the space.
The RTU Open continues to operate in this mode until the
space is satisfied or the outside air conditions are no
longer suitable for free cooling.
Optimal Start
The RTU Open may utilize Optimal Start, which adjusts
the effective setpoints to achieve the occupied setpoints
by the time scheduled occupancy begins. The Optimal
Start recovery period may begin as early as 4 hours prior
to occupancy. The algorithm works by moving the
unoccupied setpoints toward the occupied setpoints. The
rate at which the setpoints move is based on the outside
air temperature, design temperatures, and capacities. The
following conditions must be true for unoccupied free
cooling to operate:
S Under SETPOINT, Optimal Start Value must be set
greater than zero and less than or equal to four (0
disables Optimal Start.)
S The system is unoccupied
S The RTU Open has a valid outside air temperature
S The RTU Open is running occupancy based on a
schedule, were next time occupied is known.
Fire Shutdown
Fire Shutdown may be configured on Binary Input 5. A
typical application involves a smoke detector or fire
shutdown contact, which, when active, immediately shuts
down equipment operation.
Compressor Safety
Compressor Safety may be configured on Binary Input 3.
A compressor safety tripped indicator circuit is available
on most Bryant rooftop equipment. A Compressor Safety
Alarm indicates that the equipment requires attention.
Cooling, heating, and supply fan outputs are not
interrupted except where the RTU Open is configured for
Heat Pump operation. When configured for Heat Pump,
and in the heating mode, a compressor safety fault will
cause the available stages of electric heating to be enabled
in place of mechanical heating. Normal operation resumes
when the compressor safety circuit is de- energized.
Fan Status
Fan Status may be configured on any unused binary input
channel. A typical application would be an airflow switch,
current sensing relay, or other device that provides a
supply fan running verification. Enabling this function
displays the supply fan's status on the equipment graphic.
If the controller loses fan status during operation, heating
and cooling are disabled, the economizer damper (if
available) is closed, and an alarm for loss of status is
indicated. If the fan status is on when the controller is
commanding the fan off, the unit remains in the off state.
An alarm is generated indicating that the fan is running
when it should be off.
Filter Status
Filter status may be configured on any unused binary input
channel. A typical application is a differential pressure
switch that senses the pressure drop across a filter bank.
When the pressure across the filter bank exceeds the setpoint
of the differential pressure switch, the Filter status is
displayed as Dirty on the controller graphic. An alarm
indicates a dirty filter.
Door Switch
A Door Contact may be configured on any unused binary
input. A typical application is an occupancy sensor mounted
within the space served by a single zone rooftop. Door
Contact disables mechanical cooling and electric or gas
heating, when active. Economizer cooling, if available,
continues to operate.
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