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a setpoint that is halfway between the effective cool and
heat setpoints. If the SAT drops below the cooling low
supply air setpoint (+ 5_F), the economizer will ramp
down to minimum position.
Power Exhaust
RTU Open may enable and disable an exhaust fan based
on either the controller's occupancy or its economizer
damper position. If configured for continuous occupied
operation, it will be energized whenever the controller is
in the occupied mode and disabled when in the
unoccupied mode. If configured for damper position
control, it will be energized whenever the economizer
exceeds the power exhaust setpoint and disabled when the
economizer drops below the setpoint by a fixed hysteresis
of 10%. If the Fan Control is set to Two Speed or Variable
Speed, the Power Exhaust Setpoint is automatically
adjusted based on the fan's air delivery. The Calculated
PE Setpoint used for control is displayed in the
Maintenance section.
Pre- Occupancy Purge
Pre- Occupancy Purge allow the rooftop equipment with
an economizer damper to utilize outdoor air to purge the
space of contaminates just prior to the beginning of the of
the occupied period.
The following conditions must be true for pre- occupancy
purge to operate:
S Pre- Occupancy Purge set to Enable
S Economizer Exists set to Yes
S A local time schedule is configured
S The local time schedule is currently unoccupied and the
remaining them is less than the configured Purge Time
When the RTU Open schedule is unoccupied and the
remaining unoccupied time is less than the purge time, the
supply fan starts. The economizer damper opens to the
configured Economizer Purge Min Pos. The RTU Open
continues to operate in this mode until the occupied start
time is reached. The Pre- Occ Purge state is displayed in
the Maintenance section.
Heating
The heat outputs are controlled by the Heating Control
PID Loop and Heating Stages Capacity algorithm. They
will be used to calculate the desired number of stages
needed to satisfy the space by comparing the SPT to the
Occupied Heat Setpoint the T56 slider offset when
occupied and the Unoccupied Heat Setpoint plus the T56
slider offset if unoccupied. The following conditions must
be true in order for this algorithm to run:
S Indoor Fan has been ON for at least 30 seconds.
S Cool mode is not active and the time guard between
modes equals zero.
S If occupied and SPT <(occupied heat setpoint plus T56
slider offset)
S SPT and supply air temperature values are available
S If it is unoccupied and the SPT < (unoccupied heat
setpoint plus T56 slider offset). The indoor fan will be
turned on by the staging algorithm.
S OAT < High OAT lockout temperature.
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
Perfect Humidityt 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.
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