Emerson E2 operation manual Unoccupied Hysteresis, Setpoint Reset

11.17.2 Unoccupied Hysteresis

When the Heat/Cool receives an indication that the building is unoccupied, the control mode changes from the PID method described previously, and uses simple ON/ OFF control of the heating and cooling outputs.

The Heat/Cool application in unoccupied mode makes use of a hysteresis value to form a range of temperature values across which the heating and/or cooling will be applied. As shown in Figure 11-26, when the temperature drops to the Unoccupied Heating Setpoint or rises to the Unoccupied Cooling Setpoint, the heat or cool outputs go to 100% ON. They remain in this state until the tempera- ture rises above the heating setpoint plus the hysteresis, or below the cooling setpoint minus the hysteresis, at which time the outputs will go to 0% ON.

pied setpoint.

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

Figure 11-27- 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

HYSTERESIS

100%

OUTPUT

0%

HEATING

SET POINT

TEMPERATURE

COOLING

SET POINT

26512035

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

Figure 11-26- Unoccupied Hysteresis Mode

11.17.3 Optimum Start/Stop (OSS)

NOTE: OSS applies only to Heat/Cools that use a time schedule to change occupancy states. Overrides initiated by the digital OCC STATE input will not initiate pre-starts or pre-

stops.

Optimum Start/Stop (OSS) is a feature that works

alongside the Heat/Cool’s occupied and unoccupied tem- perature control modes. OSS takes control of heating and cooling several minutes before the building is scheduled to change occupancy states, and prepares the building for the upcoming change in setpoints. As a result, when the occu- pancy 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 Heat/Cool goes from occupied to unoccupied mode, heating is suspended and the temperature is allowed to “coast” down to the unoccu-

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.

Heat/Cools keep track of three different kinds of K fac- tors:

•Heat K factor - used to guess pre-start durations for Heat/Cools operating in heating mode.

•Cool K factor - used to guess pre-start durations for Heat/Cools 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 Heat/Cools.

11.17.4 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.

To set up a setpoint reset for heating or cooling, the user must specify the minimum and maximum range of reset sensor values, and the maximum range of setpoint

Heat/Cool Control Software Overview • 11-47