19
If all of the above conditions are met, the controller uses a
PID logic loop to energize or de--energize heat outputs to
satisfy the heat demand. If the SAT begins to exceed the
high supply air setpoint, a ramping function will cause the
Heat Stages Capacity algorithm to decrease the number of
stages until the SAT has dropped below the setpoint.
There is a fixed one minute minimum on time and a one
minute off time for each heat output. Heat staging has a 2
minute stage up and 30 second stage down delay.
Heat pump operation is the same as above except for what
is explained below. There is a fixed 3 minute on and 5
minute off time for the first heat stage output, and a one
minute on and one minute off time for the second heat
stage output. There is a 10 minute minimum stage up
delay if the heat demand is <= 3
_
F, and a 2 minute
minimum stage up delay if heat demand is > 3
_
F. The
stage down delay is still 30 seconds. If the Compressor
Safety Alarm is active, the second heat stage will come on
with the first stage with no delay.
Indoor Air Quality
If the optional indoor air quality sensor is installed, the
RTU--OPEN will maintain indoor air quality within the
space at the user configured differential set point. The set
point is the difference between the indoor air quality and
an optional outdoor air quality sensor. If the outdoor air
quality is not present then a fixed value of 400ppm is
used. The following conditions must be true in order for
this algorithm to run:
S
The mode is occupied.
S
Indoor Fan has been ON for at least 30 seconds.
S
Indoor Air Quality sensor has a valid reading.
As air quality within the space changes, the minimum
position of the economizer damper will be changed thus
allowing more or less outdoor air into the space depending
on the relationship of the indoor air quality to the
differential setpoint. If all the above conditions are true,
the IAQ algorithm will run and calculates an IAQ
minimum position value using a PID loop. The IAQ
minimum damper position is then compared against the
user configured economizer minimum position and the
greatest value becomes the final minimum damper
position of the economizer output.
If the calculated IAQ minimum position is greater than
the DCV Max Vent Damper Pos configuration, then it will
be clamped to the configured value.
Dehumidification
The RTU--OPEN will provide occupied and unoccupied
dehumidification only on units that are equipped with the
Humidi--MiZer
t
option from the factory. This function
requires a space relative humidity sensor or a humidistat
for control. The space relative humidity senor can be
installed and configured as one of the two analog input
channels (inputs 1 or 2 on J4), or a humidistat can be
installed and configured as switch input 9 on J5. When
using
a
relative
humidity
sensor
to
control
dehumidification,
occupied
or
unoccupied
dehumidification setpoints are use accordingly. When
using a humidistat, setpoints are not used and the
dehumidification call comes when the humidistat
indicates high humidity.
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, 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.
