An investigative analysis of an advanced trim and respond control strategy for variable air volume HVAC systems

Abstract

<p>Energy efficiency, particularly HVAC energy use in buildings, offers a large potential of reducing overall energy consumption. By optimizing fan controls in variable air volume (VAV) HVAC systems, up to 30 to 50% of fan energy can be saved using trim and respond (TR) strategies compared to constant pressure strategies. The tiered trim and respond (TTR) strategy has shown promise in realizing significant fan energy savings in real buildings without issues of static pressure oscillation.</p> <p>This study proposed the demonstration of the TTR strategy at five Iowa Army National Guard facilities comparing against fixed static pressure (FSP) and traditional TR strategies over a ten-month period. The DDC and HVAC systems at each site were equipped with the necessary hardware and software needed to properly test and record all data needed. Functional mode and power verification testing were conducted to confirm the correct implementation of the TTR method and fan power readings from the DDC system and installed data logging systems.</p> <p>After a ten-month period, the fan energy savings results for the TTR and FSP comparison were as follows: Boone RC AHU-1 20.86% and AHU-2 10.69%, Des Moines MEPS AHU-1 27.30%, JFHQ AHU-1 4.73%, AHU-4 12.83%, AHU-9 59.92% and AHU-12 15.83%, Muscatine AFRC RTU-1 18.91%, RTU-3 29.46%, RTU-4 36.54% and Waterloo RC RTU-1 33.80%. The fan energy savings results of the TTR and TR comparisons are as follows: JFHQ AHU-2 -25.90% and AHU-3 -47.27%. The temperature control comparisons results on the TTR and TR comparison air handling units (AHUs) are as follows: JFHQ AHU-2 4.35% and AHU-3 7.76%.</p> <p>While the original proposal of 30 to 50% of fan energy savings is possible, a value closer to 20 to 30% fan energy savings is more realistic. Numerous instances of mechanical failure, setpoint alterations, scheduling errors and other issues that while hindered the capabilities of the TTR strategy, reflected the true nature of a real building. The study showed that the TTR strategy is most successful with less number of VAV zones and proper control of AHU supply air temperature. One recommendation is to recommission heating or cooling airflow setpoints so the VAV zone damper is not left wide open when under the control of the TTR strategy.</p> <p>Static pressure control was as expected on 3 of the 5 sites studied. The TTR strategy was able to respond to building loads while minimizing or eliminating issues with static pressure oscillation. However, the TTR strategy displayed numerous instances of frequent static pressure oscillation, especially at sites that had difficulty in controlling zone temperatures from inactive boiler or chiller service. The results from the TTR and TR comparison showed that temperature control with a radiant in-floor heating system was difficult for both static pressure reset strategies studied.</p> <p>In future studies, boiler and chiller data including: operation, temperature, setpoints, etc. should be trended. A comparison of the TTR strategy with fixed supply air temperature and an outside air based supply air temperature reset strategy would be insightful. Parameters such as the TM and RP rates, Step Timer and damper position thresholds could be refined to maintain a quick response to changing building loads. Lastly, as industries and professional standards progress with improved building standards, the focus of future studies should shift from comparing against fixed static pressure strategies to existing static pressure reset strategies in not only fan energy savings, but whole building energy savings as well.</p>