Demonstration of Energy Saving and Control Performance of Tiered Trim and Respond Method in AHU Static Pressure Reset (ST-16-C017)

Both the ASHRAE Standard 90.1 and California Title 24 Building Energy Efficiency Standards require AHU supply duct pressure setpoints on variable-air-volume (VAV) systems with direct digital controls (DDC) be reset at the zone level. While many different implementation methods have been proposed, the Trim and Respond (TR) method is one of the more popular strategies. Although the TR methods are popular many are difficult to successfully implement due to issues maintaining stable control, complexity of tuning parameters, or sacrificed zone level comfort. A newer “Tiered Trim and Respond” (TTR) strategy has improved control stability, increased response time and eased implementation in the field compared to the traditional TR method, while achieving similar fan energy savings. The TTR method recognizes that the instability of the Trim and Respond (TR) methods is often caused by targeting the system’s maximum damper position at nearly 100% open. At these extreme positions the system is most sensitive to disturbances or load changes and therefore stable control is difficult. The TTR method compares the maximum VAV damper command or position value to three different tiers of high/low thresholds, and responds by varying the trim and respond rates to adjust the static pressure set points. The targeted average maximum VAV damper value is lowered from the traditionally recommended 95% or 98% open position to a lessor range of 80% to 90% open. The TTR method pushes the setpoints slightly off the “ideal static pressure curve”, but provides more stable system control while maintaining a quick response to load changes. The TTR method is being implemented in a year-long demonstration at five different building sites in various building types and usage on four different building automation systems. Preliminary demonstration results show that among seven AHUs fan energy savings vary from 6% ~ 47% with the TTR compared their normal or existing fixed static pressure control setup.  In four of the RTUs the fan energy savings are 33% to 36%. Additionally, two of the demonstration AHUs utilize two implementations of Trim and Respond strategies. The TTR method is more responsive to load changes and maintains better indoor air quality while using a similar amount of fan energy. This paper describes the TTR methodology, the five demonstration sites and their direct digital systems, the preliminary energy savings compared to fixed static pressure control, and control performance characteristics compared to two different TR methods.