Some control issues of VAV systems

To obtain stable control of VAV systems, many considerations must be accommodated properly. Local and central control loops affect each other in ways that in many cases can lead to problems of poor control and even instability. In a VAV system (utilizing demand controlled ventilation), the stability of the supply air temperature must be very carefully commissioned. While stable control can be obtained more easily at full flow rates, instability can often become a problem during reduced flow rates. Disturbances such as outdoor air temperature swings and returned temperature swings from the local zones via heat exchangers also affect stability and may be critical at low flow rates. The strategy and controls chosen for sequencing of heating and cooling coils, and heat recovery units, may also lead to problems of instability. This paper summarizes experiences obtained from several detailed simulation case studies addressing demand controlled ventilation and VAV systems. The results show that the above emphasized problems often are present and must be taken seriously. The work also suggests practical solutions to reduce the risk of instability.