Automated Terminal Unit Performance Monitoring
Keeping continuous tabs on your VAV units via your BAS may be worth the effort.
In May’s column, I detailed the benefits to the owner of defining and enforcing a standard VAV terminal unit control sequence. These benefits included the ability to automate performance monitoring through the central BAS. The May column also included an example list of potential operating conditions that would result in alarms.
Programming the BAS to generate maintenance alarms when certain conditions exist at any VAV box may relieve the owner of the burden of periodically recommissioning hundreds of terminal units on a regular basis. More likely, the owner does not have the labor resources or budget to implement a regular terminal unit recommissioning program, and recommissioning only occurs when trouble calls are received from occupants. In worst-case scenarios, recommissioning does not even happen then because the maintenance responders are too busy to do anything but apply band aid solutions. Such quick and dirty solutions can mask the root cause, waste energy, and potentially upset the system as a whole.
Therefore, programming the BAS to continuously collect and analyze data and alarm energy- or comfort control-related problems saves substantial labor otherwise spent manually measuring and recording data. It is also far more effective. Just about zero building owners perform 100% sampling of terminal units during recommissioning efforts. As such, they are missing opportunities to improve performance and save energy. Continuous BAS monitoring catches problems as soon as they occur in hopes of correcting them as soon as possible and not waiting until the next scheduled recommissioning event.
Of course, no benefit is gained from automated BAS monitoring and alarming unless the root cause of the alarm is diagnosed and repaired. Continuous monitoring will generate more work for a building’s operations staff than the standard wait-for-complaints approach. Historically, maintenance departments have lived in the “ignorance is bliss” world. With the BAS analyzing terminal unit performance for them, it can turn into a “beware of what you wish for” world.
One client of ours with thousands of terminal units has committed to continuous BAS monitoring and alarming. They started with a prototype project of 16 VAV boxes and are methodically working their way through all of their buildings. When they were at 1,500 monitored terminal units, the alarms generated an average of about one hour of work orders every day. Assuming about 500 sq ft per VAV zone and a 24/7/365 operation, that equates to an additional 365 hrs/year for 750,000 sq ft, or 1 hr/year for every 2,000 sq ft. That doesn’t sound so bad, but for a large campus setting, this would add up to one additional full-time equivalent maintenance person for every 3.5 million sq ft.
Unfortunately, automated BAS monitoring does not mean you never need to physically visit a terminal unit again. It will still be necessary to validate the calibration of the BAS sensors on which the monitoring program relies, but there may be opportunities for automated calibration checks. For example, some terminal unit controllers schedule a zeroing calibration of their airflow measuring stations during typical unoccupied hours. Or the root cause of alarms generated by the BAS may be uncalibrated sensors. For example, if the discharge air temperature sensor is reading too high when the reheat valve is commanded closed, the problem may be a leaking-past reheat valve or a faulty discharge air temperature sensor.
The benefits our client has realized so far can be summarized in terms of the following problem types identified and corrected because of the BAS monitoring.
Failed or out-of-adjustment devices (dampers, valves, actuators, sensors)
Poor thermostat locations
Poor space reconfigurations (blocked airflow, desks below diffusers)
Original design issues
Both occupant satisfaction (fewer trouble calls) and energy conservation have improved.
In summary, for a building operator who really cares about (1) proactively providing the best temperature and ventilation control to building occupants and (2) doing so as energy efficiently as possible, continuously monitoring terminal unit performance through the BAS offers affordable opportunities to succeed.