Most building control systems are designed to meet the expected maximum system demand. However, the maximum demand is usually required only a few times per year for short periods of time. As a result, the system is oversized for the normal load so that it can meet this exceptional demand.

The duty cycling (DC) function coordinates the operation of all available equipment to compensate for additional equipment capacity that is not normally needed. During normal system operation, some equipment is cycled off for short periods of time to distribute equipment wear and reduce operating costs.

Applications that can use ON/OFF switching in hourly patterns work best with a duty cycling program.

CAUTION
Be careful when designing your programs so that functions do not conflict.

If objects used for DC are also commanded by another application, such as Time-of-Day (TOD), one program may interfere with the operation of the other if both functions are trying to control a object during the same time period.

Example

The following example uses duty cycling in a school building to control a hall fan, called HFAN.

  • In order to conserve energy, the fan will run only during the times when students are in the hall.
  • In this school, classes start on the hour and last for 50 minutes.
  • Students are in the hall during a 10-minute period before the next class.
  • Since the hall that HFAN serves has good ventilation, the fan should run for the last 15 minutes of the hour and for the first five minutes of the next hour.
  • The rest of the time the fan is shut off.

The code for duty cycling HFAN is as follows:

1000

C  THIS DC COMMAND DUTY CYCLES HFAN

1010

C  ACCORDING TO THE FOLLOWING SCHEDULE:

1020

C

1030

C  FIRST 15 MINUTES – ON, OFF, OFF (1)

1040

C  SECOND 15 MINUTES – OFF, OFF, OFF (0)

1050

C  THIRD 15 MINUTES – OFF, OFF, OFF (0)

1060

C  FOURTH 15 MINUTES – ON, ON, ON (7)

1070

C

1100

DC(HFAN,7001)