Buildings account for a large portion of the total energy use in the US; therefore, improving the operation of typical variable-air-volume (VAV) systems in buildings can provide a tremendous economic opportunity. ASHRAE Guideline 36 recommends a resetting strategy for supply air temperature (SAT) for VAV systems based on outside air temperature. However, this strategy may not produce optimal performance, particularly when simultaneous cooling and heating occurs in zones. In addition, there is no strategy recommended in the Guideline to reset the zone minimum airflow set point in a single-duct VAV terminal unit with reheat, although this setpoint has a great impact on zone reheat requirements and ventilation efficiency. Thus, this paper introduces new strategies to reset both the SAT and zone minimum airflow rate set points to improve the efficiency of typical VAV systems. The strategies were tested under various conditions through experiments performed in fully instrumented VAV systems located in the HVAC lab at the University of Cincinnati. The experiments were conducted on a chilled-water VAV system that serves three controlled zones with hot-water reheat VAV boxes controlled by a typical commercial BACnet web-based building automation system BAS. The simulation studies were performed using the building energy simulation software EnergyPlus to evaluate the strategies at a larger scale in various locations. The simulation results show that the proposed resetting strategies can provide fan energy savings between 1.6% and 5.7% and heating load savings between 7.7% to 33.7%, depending on the location. The laboratory testing shows that the proposed strategies can provide stable control performance in actual systems as well as achieving the anticipated reheat and fan energy savings. The result offers significant improvements that can be implemented in the Guideline for single-duct VAV system operation and control.

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.