Emerson E2 operation manual Diagram of Pre-Start and Pre-Stop Operation

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 11-15shows 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	UNOCCUPIED	PRE-START	OCCUPIED	PRE-STOP(COAST)	UNOCCUPIED
SET POINT
UNOCCUPIED
SET POINT
					26512036

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

cooling is active. This is used to determine pre-stop durations for both heating and cool- ing AHUs.

Zone Control Software Overview • 11-29