VAV Airflow Sensor Response in Relation to Poor Upstream Duct Geometry

Variable air volume (VAV) terminal placement has typically been considered good enough in the HVAC industry, regardless of upstream duct geometry that clearly affects the response of the flow sensor. This paper describes the loss of VAV flow sensor amplification, uniformity, and precision
following common upstream duct geometries from a recommissioning standpoint. Two brands of VAV terminals were tested in three different sizes: 250, 200, and 150 mm diameters (referred to as 10, 8, and 6 in. throughout this paper). Upstream geometry was modeled after transitions that were deemed poor during a recommissioning procedure that was completed at three facilities during ASHRAE Research Project 1137. The poor upstream conditions included combinations of concentric reducers, expanders, 90 elbows, and S-shape geometries with varying straight duct lengths prior to the sensor. This paper outlines some basic recommendations that HVAC designers, terminal manufacturers and commissioning agents can use to improve the accuracy of VAV airflow sensors both before installation and as part of a recommissioning procedure.