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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%
0%
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.