Emerson E2 Separate Setpoints, AHU Zone Control, Intelligent Pre-Starts and Pre-Stops

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 AHU Control goes from occupied to unoccupied mode, heating is suspended and the temperature is allowed to “coast” down to the unoccu- pied setpoint.

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

Figure 11-15- 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.

11.6.11 Separate Setpoints

The Separate Setpoints strategy for AHU allows a Cut In/Cut Out setpoint to be set up for each heat and cool stage instead of just one cool and one heat setpoint set up for each stage (Normal strategy). Dehumidification con- trol can be performed while controlling with separate set- points.

11.6.12 AHU Zone Control

Unlike MultiFlex RTU rooftop controller applications, AHU applications are not required to be grouped into Zone applications (AHUs are usually large enough to be “zones” in and of themselves).

However, if desired, you may associate an AHU Con- trol application with a Zone application. The AHU will then use the Zone’s Temperature Control setpoints, occu- pancy state, summer/winter state, and economization and dehumidification enable signals. More information on Zone control is available in Section 11.7, Zone Control.

11.6.13 Hardware Overview

To set up an AHU for control by an E2, numerous tem- perature and humidity sensors for several different appli- cations must be connected to the I/O Network, as well as fan and cool proof checking devices, economization checking devices, curtailment devices, and all of the heat- ing, cooling, and dehumidification outputs.

Listed below are wiring instructions for some of the inputs and outputs that are part of a typical AHU setup.

Inputs	Sensor Type	Wiring
		Instructions
Space Temperature	Temperature	See Table 9-1 on
		page 9-3
Space Humidity	Humidity	See Table 9-1 on
		page 9-3
Supply Air Temp	Temperature	See Table 9-1 on
		page 9-3
Return Air Temp	Temperature	See Table 9-1 on
		page 9-3
Outdoor Air Temp	Temperature	Set up as Out-
		door Air Provider
		in Global Data
		(see Section
		10.17).
Table 11-10- Suction Group Inputs

11-24 • E2 RX/BX/CX I&O Manual 026-1610 Rev 13 14-SEP-2011