Emerson E2 Manuel D’Utilisation

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.).

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.

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.

DEHUM SET POINT

DEHUM HYSTERESIS

Time (s)

Rela

ti

v

e

 Humidity 

(%)

100%

0%

26512040

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.

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.

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.

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.

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.

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