Emerson E2 Installation And Operation Manual page 173

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.
OCCUPIED
SET POINT
UNOCCUPIED
SET POINT
Figure 11-17
- Diagram of Pre-Start and Pre-Stop Operation
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
Zone Control
cooling is active. This is used to determine
pre-stop durations for both heating and cool-
ing AHUs.
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