Emerson E2 Digital Economizer Control, Analog Economizer Control, Dehumidification Control

economization is enabled.

6.In vs. Out Enthalpy - This strategy requires indoor and outdoor humidity sensors and also indoor and outdoor temperature sensors. The 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 economization checking method for use in summer and winter months.

11.6.5.2Economization Lockout Features

In addition to the methods listed above, there are two economization lockout features that apply to all AHUs using economization.

Max Outside Air Humidity

The Max Outside Air Humidity is the highest allow- able humidity level for the outside air. If the outside rela- tive humidity is higher than this setpoint, economization will not be allowed to occur.

Minimum Supply Temp

The Minimum Supply Temp is a user-defined setpoint that locks out economization if the supply air temperature falls below a minimum supply temperature setpoint. This feature ensures that the air coming from outside is not too cold.

11.6.6 Digital Economizer Control

Control of digital or two-position economizers is rela- tively simple. When conditions are favorable for econo- mization, the dampers will be opened when cooling is needed. If not favorable, the dampers will be closed.

11.6.7 Analog Economizer Control

For variable-position dampers, economization is gen- erally used by the AHU just as a cool stage would be in Temperature Control. If cooling is needed, and conditions are favorable for economization, the dampers will open and economization will begin. If more cooling is needed, the cooling stages would then cycle on as normal.

The position of the analog economizer damper is important only for the purposes of keeping the mixed air temperature (a combination of the outdoor air coming through the dampers and the return air temperature) near the user-specified setpoint. The damper is controlled using PID control.

11.6.8 Dehumidification Control

Dehumidification control uses the AHU’s existing cool stages (and a separate dehumidification device such as a desiccant wheel, if available) to remove moisture from the

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).

11.6.9 Curtailment

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.

11.6.10 Optimum Start/Stop (OSS)

NOTE: OSS applies only to AHU Control ap- plications that use a time schedule to change occupancy states. Overrides initiated by the digital BYPASS TO OCC or BYPASS TO UN-

OCC inputs will not initiate pre-starts or pre-stops.

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

Air Handling Units (AHU) Software Overview • 11-23