Emerson Guide to Bypassing Comfort Alert: FSD 11.3.6 vs 11.4.4 and FSD 11.4.8 Insights

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

comfortably within the range of the new setpoint.

Figure 11-15shows 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	UNOCCUPIED	PRE-START	OCCUPIED	PRE-STOP(COAST)	UNOCCUPIED
SET POINT
UNOCCUPIED
SET POINT
					26512036

Figure 11-15- Diagram of Pre-Start and Pre-Stop Operation

Intelligent Pre-Starts and Pre-Stops

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.

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