Emerson E2 operation manual Effect of Enabling Economization, MultiFlex RTUs and RCBs

enthalpy of the outdoor air is calculated and com- pared to the enthalpy of the indoor air. If the out- door air enthalpy is less than the indoor air enthalpy, economization is enabled. Otherwise, economization is disabled.

You may choose a different method for use in summer and winter months. Also, an alternate method may be specified that will be used as a fail-safe when the primary method is not available (due to sensor failure, etc.).

11.7.8The Effect of Enabling Economization

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 Dehumidification

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

11-28 • E2 RX/BX/CX I&O Manual 026-1614 Rev 4 5-JAN-2013