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Zone Control Software Overview • 11-29
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
UNOCCUPIED
UNOCCUPIED
PRE-START
OCCUPIED
PRE-STOP (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 tempe
rature 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 th
e 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 dif
ferent 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.
• Coas
t 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.