Optimal Control Strategy for Parallel Fan-Powered VAV Systems

ASHRAE Guideline 36 recommends resetting the Supply Air Temperature (SAT) for Variable Air Volume (VAV) systems to balance fan power, heating and cooling loads and zone reheat requirements. This is achieved by employing a trim and response algorithm in conjunction with using the outside air temperature and readings from zone cooling loops. However, resetting the SAT for the VAV systems with parallel fan-powered terminal units according to Guideline 36 recommendations may not produce the best performance. Reducing the reheat requirement in parallel fan-powered terminal units can be done by increasing the air recirculating at the zone level rather than at the system level. This will allow keeping the system level SAT as cold as possible to reduce fan airflow for the zones in cooling with no or little effect on heating requirement for the zones in heating. Therefore, this study evaluates SAT control strategies and fan-powered terminal airflow rates to maximize total energy efficiency. Multiple airflow rate designs and operational variables such as the size of the fan-powered terminal unit and minimum airflow rate set point are included in this study. The simulation results for a typical small commercial building in various locations show that the new resetting method with local zone air recirculation enhancement can significantly reduce fan energy use with little effect on the heating requirement. The result may significantly improve the guideline related to the sequence of operation in a parallel fan-powered terminal unit.