11.7.8The 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.

100%

 

(%)

DEHUM SET POINT

Humidity

DEHUM ON

DEHUM HYSTERESIS

Relative

 

0%

 

 

Time (s)

 

26512040

Figure 11-16- Dehumidification Setpoint Control

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.

11.7.11The 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)

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.

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 11-15shows 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-

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Emerson E2 operation manual Effect of Enabling Econ- omization, Zone Humidity Input, Effect of Enabling Dehu- midification