Zone Control
Software Overview
•
11
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29
and cooling several minutes before the Zone application is
scheduled to change occupancy states, and prepares the
area for the upcoming change in setpoints. As a result,
when the occupancy state changes, the temperature will be
comfortably within the range of the new setpoint.
Figure 11-15
shows an example of how pre-starts and
pre-stops work in a heating application. From unoccupied
mode, the pre-start period ramps the temperature up
slowly so that when the scheduled change from unoccu-
pied to occupied mode occurs, the temperature will
already be at or near the occupied heating setpoint. During
the pre-stop, which occurs before the Zone application
goes from occupied to unoccupied mode, heating is sus-
pended and the temperature is allowed to “coast” down to
the unoccupied setpoint.
Figure 11-17
- Diagram of Pre-Start and Pre-Stop Operation
OCCUPIED
SET POINT
UNOCCUPIED
SET POINT
UNOC
C
UPIED
UNOC
C
UPIED
PRE-ST
ART
OCC
U
PIED
PRE-ST
OP
(COAST)
26512036
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 set-
point within which the temperature is considered accept-
able). 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
will be sorted and combined into a table. As a result, by
constantly observing and recording the results of previous
pre-starts and pre-stops, OSS will be 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 dura-
tions for AHUs operating in heating mode.
•
Cool K-factor -
used to guess pre-start dura-
tions 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 cool-
ing AHUs.
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