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HVAC Zone Software Overview
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5.22.10 Enabling Dehumidification
Effect
When an HVAC Zone application determines that
dehumidification is needed, it sends an ON signal to all its
associated controllers, signaling them to begin
dehumidification. It is up to the individual MultiFlex RTU,
RCB, or AHU to perform the dehumidification.
5.22.10.1 MultiFlex RTUs and RCBs
Once the MultiFlex RTU and RCB applications
acknowledge the signal to begin dehumidification, it will
search its outputs for a dehumidification device. If one is
configured, this device will be turned ON. Afterwards,
cool stages will be cycled ON (up to a user-defined maxi-
mum number of stages) until all stages are ON or until the
Zone application signals dehumidification is complete.
When dehumidification is complete, the stages will cycle
OFF followed by the dehumidification device.
5.22.11 Optimum Start/Stop (OSS)
Optimum Start/Stop (OSS) is a feature that works
along with the HVAC Zone application’s occupied and
unoccupied temperature control modes. OSS takes control
of heating and cooling several minutes before the Zone
application is scheduled to change occupancy states. It
prepares the area for the upcoming change in setpoints. As
a result, when the occupancy state changes, the
temperature is comfortably within the range of the new
setpoint.
Overrides initiated by the digital BYPASS TO OCC or
BYPASS TO UNOCC inputs will not initiate pre-starts or
pre-stops.
The figure below shows an example of how pre-starts
and pre-stops work in a heating application. From
unoccupied mode, the pre-start period slowly increase the
temperature up so that when the scheduled change from
unoccupied to occupied mode occurs, the temperature is
already at or near the occupied heating setpoint. During
the pre-stop, which occurs before the HVAC Zone
application goes from occupied to unoccupied mode,
heating is suspended and the temperature is allowed to
“coast” down to the unoccupied setpoint.
5.22.12 Intelligent Pre-Starts and
Pre-Stops
OSS is designed to handle pre-starts and pre-stops in
the most energy-efficient manner possible. Every time a
pre-start or pre-stop occurs, OSS measures the amount of
time it takes to bring the temperature from the previous
setpoint to within the “comfort zone” of the new setpoint
(a user-defined range of values above and below the
setpoint within which the temperature is considered
acceptable). This duration is used to determine the average
rate of temperature change, called the K-factor.
The K-factor is stored in the memory, along with the
average value of the outdoor air temperature during the
pre-start or pre-stop. Over time, collected K-factor data is
sorted and combined into a table. As a result, by
constantly observing and recording the results of previous
pre-starts and pre-stops, OSS is able to intelligently guess
how much time a pre-start or pre-stop mode should last
based on the outside temperature.
AHU Control keeps track of three different kinds of K-
factors:
• Heat K-factor - used to guess pre-start durations for
AHUs operating in heating mode.
• Cool K-factor - used to guess pre-start durations for
AHUs operating in cooling mode.
• Coast K-factor - a measurement of the change in
temperature when no heating or cooling is active.
This is used to determine pre-stop durations for
both heating and cooling AHUs.
5.22.13 Setpoint Reset
If desired, Heat/Cools may be configured with a Set-
point Reset that varies the value of the heating and/or
cooling setpoints based on an analog value from a reset
sensor. This is most often used to vary the value of a heat-
ing or cooling setpoint based on the outside air tempera-
ture.
NOTE: OSS applies only to Zone applications
that use a time schedule to change occupancy
states.
Figure 5-12
- Optimum Start/Stop (OSS)