Optimal Start

Optimal Start is used to anticipate the heating or cooling needs of a space by starting equipment early enough to reach setpoint just at the beginning of scheduled occupancy. The Optimal Start function in CBAS does this by calculating the difference between the Actual Temperature and the Occupied Temperature Setpoint. Based on a heating or cooling slope determined by the user, the unit is started early enough to bring the space temperature to the desired level. By doing this, you can avoid putting a heavy load on the equipment that would normally occur by starting right at the occupied time and playing catch-up. Under extreme conditions, playing catch-up could mean running at full capacity for several hours before reaching setpoint.

Depending on what version of CBAS you have (see Chapter 1: Getting Started), you will see something different when you open the Program Optimal Start screen. As of version 3.1.6, Adaptive Optimal Start and Internal Setpoints were added, making for a totally different setup. Both will be covered next.

This feature is available in 3.1.6, and requires a firmware upgrade on the controller hosting the Optimal Start point. The firmware is Revision 10.6 or higher, which will only work with CBAS version 3.1.6 or higher.

Optimal Start is a feature that can be found under the Optimal Start button in the Program screen of any Binary Output. Go there and you’ll find a screen similar to the screen to the right:

New Features in 3.1.6

Optimal Start is programmed on a Binary Output that starts and stops an Air Handling Unit. There must be a weekly schedule programmed on the point to begin with, as well as a Low/Heating Setpoint and a High/Cooling Setpoint. These are software setpoints and can be programmed either individually for each AHU or globally for all AHUs in the database to use. The only other point needed is the space temperature.

If looking at an existing Optimal Start, the “Setpoint is a CBAS Point” box would be checked and your Setpoints listed. You can still do Optimal Start that way, but now you can enter your Heating and Cooling Setpoints right there on the Optimal Start screen. Just check the “Setpoint is internal number” box and enter your setpoint numbers. This makes setting up Optimal Starts on multiple controllers much easier than before.

Adaptive Optimal Start

When you check the box to “Allow OPTIMAL START to ADAPTIVELY adjust the Cooling and Heating Slope,” CBAS will save several parameters about the last 16 Heating and 16 Cooling Optimal Starts. The parameters saved are:

A)Slope

B)What the temperature difference was when the Optimal Start began.

C)How long before the Schedule Start time did the Optimal Start begin. (Actually not stored but can be calculated from A and B above)

D)Did the Optimal start reach it’s goal? If not, then what was the temperature difference at schedule start time?

E)The day the Optimal Start ran.

F)Was the unit off for more than 36 Hours when the Optimal Start first evaluated? (Tells you if it was after a Holiday or a Monday)

You can click on the “Show Adaptive History” button to see the data on the above parameters. Once you have enabled Adaptive Optimal Start, CBAS will determine the “slope”, or amount of time required to change the temperature by 1 degree, and adjust on a daily basis.

The following applies to versions prior to 3.1.6:

Once these points are programmed, go to the Point Program Screen of the point containing the AHU schedule. Click the Program Optimal Start button to get the setup screen. (Pre-3.1.6 screen shown)

Click on the buttons next to Analog Input, Low Setpoint, and High Setpoint, and then select the appropriate points from the lists.

•Analog Input: Choose the Space Temperature point that applies to the AHU you are programming

•Low Setpoint: Choose the Heating Setpoint you created for this AHU or a global Heating Setpoint for the database

•High Setpoint: Choose the Cooling Setpoint you created for this AHU or a global Cooling Setpoint for the database

Cooling and Heating parameters need to be determined unless using Adaptive Optimal Start. The slope parameter is the number of minutes it takes the AHU to change the temperature by 1 degree. By looking at histories, you should be able to determine this information.

•Do a History Graph on the AHU Binary Out and select the space temperature point to be graphed also.

•Look at the time when the unit first comes on for the day. In cooling mode, you will see the binary point come on and the graph of the space temp will begin to go down.

•By zooming in on that part of the graph, you should be able to tell exactly how long it takes to lower the temperature by 1 degree. But, you might want to see how long it takes to change by 3 or 4 degrees and take the average.

•Do the same thing on a day when heat is required. Note: These slopes might have to be adjusted in the future.

•Enter the slope times in the appropriate places on the Optimal Start setup screen.

Once you have saved the Optimal Start by exiting the setup screen, you now have to determine when to start the Optimal Start and add that to the schedule. If using Adaptive Optimal Start, the Slope will be determined for you, and your schedule can be adjusted accordingly.

•Do a history on the Space Temperature point for a week, during the hottest and coolest times of the year.

•You will be able to see how high or low the temperature got in relation to the setpoint. Subtract the lowest and highest numbers from the respective setpoints to get the maximum number of degrees change needed.

•If you are using a “night Setback” or Unoccupied Setpoint to keep from getting too far away from the Occupied Setpoint, you can just subtract setpoint from the Unoccupied setpoint to get the maximum number.

•Multiply this number by the larger of the two slopes determined earlier to get the amount of time the Optimal Start schedule should precede the regular schedule. (Maximum Slope, 30 Mins) X (Maximum Degrees, 5) = 2.5 Hours)

•Open the Schedule for the point in question and you will notice that the first column of the schedule is labeled Optimal Start. You can edit this column or use the Graphical Editor by clicking the button.

•In the Graphical Editor, click and hold the pointer at 2.5 hours before the beginning of the regular schedule, and then drag down and to the right to fill up the space (see figure below).

•Let up the mouse button and the line around the time period remains.

•Click the Optimal Start button below and the time period will change to the color of the button. (Start, Optimal Start, and Stop sections are different shades, as are the corresponding buttons)

•Exit the Schedule Editor and the Schedule will be saved.

You will need to monitor the operation by graphing some histories of the space temperature and Start/Stop points for the unit to see if any parameters need to be adjusted. There should be no need to shorten time periods during mild weather periods, because the schedule will not turn the unit on until it needs to.

*Very Important Note! If using CBAS points for setpoints, you have to “command” your setpoints or they will not work correctly, even if you have set the limits to a range of 1 degree on the point program screen. When you create a setpoint in CBAS and set the range on the setpoint (70-74), the value of the setpoint is 0 until you command it the first time. The value of the setpoint is 0 even though it will be displayed as the lower limit (70). This causes PIDs and Optimal Starts to not work. For example, a user created a setpoint and set it’s range from 74 to 74 because he never wants the setpoint to change. He then used this setpoint in an Optimal Start. The Optimal Start would come on at the beginning of the scheduled time period every day. The setpoint was showing a value of 74 but really it was 0. As soon as he commanded the setpoint to 74 it started to work.