Operators also can set trending for points incorrectly. Overtrending can deplete system memory, while undertrending will not show enough information, since the issue might have occurred in the gaps between trends and, thus, not be logged into the BAS.
Operators should not alarm binary points by time but by change of value (COV). If trended by COV, the moment a 0 becomes a 1 or vice versa, the system will time-stamp and log the event. This also goes for setpoints. Room setpoints, and static-pressure setpoints, for example, do not change until commanded by an operator or adjusted in the field.
Operators also can trend setpoints by COV. This way, there will be a record of the exact time the change is logged. For example, a room occupant changed the space setpoint once during a five-day workweek by 2 degrees. If trended by COV every 1 degree, only two trend points would be logged. But if this was trended every 15 minutes, the system would log and record 480 trend points.
Operators should trend analog output points that modulate to maintain a desired result at an analog input point by time. For major systems, every five minutes is a good starting point. This time schedule will record 12 trend points an hour, as opposed to four an hour if trended every 15 minutes. Many things can happen to mechanical equipment every 15 minutes, and information can be lost if they are not trended enough.
With that said, operators can relax the trending by time if the point is also trended by COV. Consider a valve command for a preheat coil maintaining a supply-water temperature. If the mixed air temperature remains constant, the valve might not modulate much, and a trend every 15 minutes might be fine.
What happens if the proportional integral derivative (PID) loop overreacts and causes freeze stat trips? A five-minute trend might not gather the information at the time of trip, but adding a trend by COV every 5 percent of valve command can fill in the blanks between those 15-minute periods if the PIDs are overreacting. If not, and the system remains stable, the system will not record new trends.
For some instances, such as clean rooms and lab rooms, trending must be aggressive. Yet some control systems can only trend at a minimum of every minute. Where tight temperature or humidity control is critical, COV trending might be the best answer. Showing a room temperature every 0.25 degrees of change can demonstrate system performance better than logging a trend every minute. This approach uses more memory, but for critical processes, memory is an easy obstacle to overcome.
Scott Lance, EBCP, LEED AP O+M, is a senior project engineer with Horizon Engineering Associates, www.horizon-engineering.com. He has more than 10 years of hands-on experience in project management and systems engineering. Lance is experienced with all aspects of building automation and controls systems, including conceptual design, engineering, installation, start-up, commissioning, operations and maintenance, and customer training.
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