Emerson E2 Manuel D’Utilisation

air. 

The dehumidification setpoint is placed at the 0% end 

of the dehumidification PID throttling range. In other 
words, the dehumidification output will start at 0% when 
the humidity is equal to the setpoint and increase to 100% 
when the humidity is equal to or above the setpoint plus 
the throttling range. 

The dehumidification output percentage is used much 

like a heating or cooling output percentage is used in Tem-
perature Control. The percentage represents the percent-
age of total dehumidification capacity available to the 
AHU (including cool stages and other dehumidification 
devices). 

Some power companies offer curtailment programs 

that allow participating stores to disable user-defined loads 
during peak power times in return for discounts on utility 
rates.

If you are participating in a curtailment program, the 

power company will supply you with a digital curtailment 
device that must be wired to an input on the RS485 I/O 
Network.

To set up curtailment in the system software, you must 

designate which specific heating and cooling stages will 
be subject to curtailment. 

 When the power company sends a curtail command 

(i.e., the value of the curtailment device switches to 
“CLOSED”), all stages that are set up to be curtailed will 
be shut off and locked out.

Fan control is not directly affected by a call for curtail-

ment. The AHU fan will still run at a speed based on the 
number of active, non-curtailed stages (or, if using modu-
lated outputs, the curtailed modulating percentage). If this 
causes the fan to slow down or shut off during curtailment, 
there will be energy savings from the fans.

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

alongside the AHU Control application’s occupied and 
unoccupied temperature control modes. OSS takes control 
of heating and cooling several minutes before the building 
is scheduled to change occupancy states, and prepares the 
building 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-
pied to occupied mode occurs, the temperature will 
already be at or near the occupied heating setpoint. During 
the pre-stop, which occurs before AHU Control goes from 
occupied to unoccupied mode, heating is suspended and 
the temperature is allowed to “coast” down to the unoccu-
pied setpoint.

OCCUPIED
SET POINT

UNOCCUPIED

SET POINT

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OSS is designed to handle pre-starts and pre-stops in 

the most energy-efficient manner possible. Every time a 
pre-start or pre-stop occurs, OSS measures the amount of 
time it takes to bring the temperature from the previous 
setpoint to within the “comfort zone” of the new setpoint 
(a user-defined range of values above and below the set-
point within which the temperature is considered accept-
able). This duration is used to determine the average rate 
of temperature change, called the K factor. 

The K factor is stored in the memory along with the 

average value of the outdoor air temperature during the 
pre-start or pre-stop. Over time, collected K factor data 
will be sorted and combined into a table. As a result, by 
constantly observing and recording the results of previous 
pre-starts and pre-stops, OSS will be able to intelligently 
guess how much time a pre-start or pre-stop mode should 
last based on the outside temperature.

AHU Control keeps track of three different kinds of K 

factors:

• Heat K factor - used to guess pre-start dura-

tions for AHUs operating in heating mode.

• Cool K factor - used to guess pre-start dura-

tions for AHUs operating in cooling mode.

• Coast K factor - a measurement of the 

change in temperature when no heating or 
cooling is active. This is used to determine 
pre-stop durations for both heating and cool-
ing AHUs.