Figure 11-14- System Log Report Example

11.6Air Handling Units

(AHU)

11.6.1 Overview

The E2 BX supports the simultaneous control of up to six air handling units (AHU). AHU Control applications govern all aspects of an AHU’s operations, including heat- ing and cooling, dehumidification, fans, and economizer control.

Unlike rooftop control units (RTUs), AHUs are con- trolled directly by the E2 itself and not by a separate unit control board.

An AHU Control application actually consists of three separate control algorithms. The main control algorithm monitors the indoor temperature and activates heating and cooling stages when necessary to maintain the user- defined temperature setpoint. Another algorithm monitors the indoor humidity and uses a desiccant wheel or addi- tional AHU cool stages to reduce the humidity level. The third loop controls either a two-position (digital) or vari- able-position (analog) economizer damper based on the outside air conditions.

11.6.2 Temperature Control

In its most basic form, Temperature Control simply reads a control input value, compares it to the active tem- perature setpoint, and activates or deactivates heating or cooling stages in an effort to satisfy the setpoint. The majority of user setup that must be done in Temperature Control involves specifying which input is to be used as

the control source, defining different setpoints for use in occupied, unoccupied, summer, and winter modes, and setting up the operating characteristics of the heating and cooling stages.

11.6.3 Alternate Setpoints

For both the heating and cooling setpoints, you may choose to use different setpoints during occupied or unoc- cupied building times, and different setpoints for summer and winter seasons. In other words, AHU Control may have four different pairs of heating and cooling setpoints, as shown in Table 11-9.

Cooling

Heating

 

 

SUMMER COOL OCC

SUMMER HEAT OCC

 

 

SUMMER COOL UOC

SUMMER HEAT UOC

 

 

WINTER COOL OCC

WINTER HEAT OCC

 

 

WINTER COOL UOC

WINTER HEAT UOC

 

 

Table 11-9- Possible Heating/Cooling Setpoints

AHU Control looks at the E2’s Global Data parameters to determine whether it should run in summer or winter mode. Refer to Section 10.17, Set Up Global Data, for information on how to set up summer/winter control parameters.

The AHU chooses occupied or unoccupied mode by reading an occupied state input, which is most commonly tied to the output of a Time Schedule application.

11.6.4 Fan Control

The E2 BX can control three different types of AHU fans. However, only one fan can be controlled per AHU. The fan types include: single-speed, two-speed, and vari- able-speed. Controls for all fans are similar in that they operate in either of three modes:

Continuous - The fan is always on, even when the AHU is not in heating, cooling, or dehumidification mode.

Auto - The fan is only on when the AHU is in heating, cooling, or dehumidification mode.

Summer ON, Winter Auto - This mode allows the AHU fan to operate in Continuous mode during the summer months and in Auto mode during the winter months.

The operation of the fans during Continuous and Auto modes is largely dependent on the fan type.

11.6.4.1Single-Speed Fans

Single-speed fans require no advanced control parame-

Air Handling Units (AHU)

Software Overview 11-21

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Image 163
Emerson E2 operation manual Air Handling Units, Temperature Control, Alternate Setpoints, Single-Speed Fans