Emerson E2 User Manual download pdf (Page 170)

11-28 • E2 RX/BX/CX I&O Manual 026-1610 Rev 13 14-SEP-2011

11.7.8 The Effect of Enabling Econ-

omization

Both MultiFlex RTU and AHU applications support

the use of both two-position (digital) and variable-position

(analog) economizers. Digital economizers, when enabled,

behave like a first stage of cool. When analog economizers

are enabled, the MultiFlex RTU or AHU will modulate the

opening percentage of the dampers based on its own

mixed air temperature measurements.

11.7.9 Dehumidification Control

A Zone application is responsible for reading the rela-

tive humidity level within the Zone, comparing it to a

dehumidification setpoint, and sending a command to

dehumidify when the humidity is above the setpoint.

Once the Zone humidity level rises above the setpoint,

dehumidification will be active in all the Zone applica-

tion’s associated HVAC units until the indoor relative

humidity falls below the setpoint minus the dehumidifica-

tion hysteresis value. An example of this is shown in Fig-

ure 11-16.

The need for dehumidification may also be determined

by a digital humidistat. In this case, dehumidification is

active only when the humidistat input is ON.

11.7.10 The Zone Humidity Input

Note that unlike Zone Temperature, which has 16

inputs that combine into a single value, Zone humidity is

designed to be provided by a single input. However, there

may be some instances where one or more relative humid-

ity sensors exist within a Zone (such as an installation

where each MultiFlex RTU has its own humidity sensor

for use in Stand-Alone mode). If you have multiple

humidity sensors in a Zone and you wish to combine these

humidity sensor values to calculate Zone Humidity, use an

Analog Combiner application to make the combination,

and tie the Zone application’s Zone Humidity input to the

output of the combiner. Refer to Section 11.18, Analog

and Digital Combiners for Analog Combiner application

programming instructions.

11.7.11 The Effect of Enabling Dehu-

midification

When a Zone application determines that dehumidifi-

cation is needed, it sends an ON signal to all its associated

controllers, signaling them to begin dehumidification. It is

up to the individual MultiFlex RTU, RCB, or AHU to per-

form the dehumidification.

11.7.11.1 MultiFlex RTUs and RCBs

Once the MultiFlex RTU and RCB applications

acknowledge the signal to begin dehumidification, it will

search its outputs for a dehumidification device. If one is

configured, this device will be turned ON. Afterwards,

cool stages will be cycled ON (up to a user-defined maxi-

mum number of stages) until all stages are ON or until the

Zone application signals dehumidification is complete.

When dehumidification is complete, the stages will cycle

OFF followed by the dehumidification device.

11.7.11.2 AHUs

AHUs use a special dehumidification algorithm based

on the Proportional loop in PID control. When dehumidifi-

cation is enabled by the Zone application, the AHU appli-

cation will activate a percentage of its total cool stage

capacity, the amount of which depends on how close the

current relative humidity is to the setpoint. See Section

11.6, Air Handling Units (AHU), for more information.

11.7.12 Optimum Start/Stop (OSS)

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

alongside the Zone application’s occupied and unoccupied

temperature control modes. OSS takes control of heating

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-

Figure 11-16

- Dehumidification Setpoint Control

DEHUM SET POINT

DEHUM HYSTERESIS

Time (s)

Relative Humidity (%)

100%

26512040

DEHUM ON

NOTE: OSS applies only to Zone applications

that use a time schedule to change occupancy

states. Overrides initiated by the digital BY-

PASS TO OCC or BYPASS TO UNOCC inputs

will not initiate pre-starts or pre-stops.

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