Stability and accuracy of variable air volume box control at low flows. Part 2: Controller test, system test, and field test

Abstract

This article with its companion paper (Liu et al. 2013), summarizes the findings of ASHRAE Research Project 1353 (Stability and Accuracy of VAV Box Control at Low Flows). This project aims to identify the major factors that cause the airflow measurement in a variable air volume system to be inaccurate and unstable, especially at low airflow conditions. Both a laboratory test (including variable air volume sensor test, controller test, and system test) and field test were conducted; the companion work discussed the variable air volume sensor test. In this article, findings from the controller test, system test, and field test are summarized. The controller tests involved testing of four controllers from four different manufacturers. Testing was performed for accuracy, stability, resolution, and ambient temperature effect. For the system test, the variable air volume box and the controller were operated together and tested as terminal unit systems. Two terminal units were tested, and it was found that the performance of a variable air volume terminal unit is highly dependent upon on controller performance. Zeroing and balancing at a low airflow rate 560 fpm (2.84 m/s) or 200 cfm (0.09 m³/s) for an 8-in. (0.2-m) box were effective for achieving high system accuracy at low airflow ranges. For the field tests, five variable air volume terminal units were tested in real commercial buildings. It was found that system balancing was not always an effective way to reduce the variable air volume airflow sensor error in the field due to the uncertainty of reference airflow measurement methods commonly adopted in the field testing and balancing process.