Technologies for measuring flow rates of outdoor air into HVACsystems: Some causes and suggested cures for measurement errors

Although the rate of outdoor air (OA) ventilation has a substantial influence on building energy consumption and occupant health, the available data indicate the outdoor air ventilation rates are poorly controlled in many buildings. Technologies being marketed for real time measurement of the flow rates of outdoor air into HVAC systems should enable better control of OA ventilation. In laboratory research they have studied the performance of these technologies. Sources of measurement errors identified during conduct of this research include: low air speeds; high spatial variability in air speed and direction; large eddies downstream of outdoor air intake louvers; and backwards airflow through a portion of outdoor air dampers. Several suggestions for overcoming these sources of errors were developed including: design and control of the outdoor air intake system to avoid low, hard-to measure, air speeds; use of highly sensitive pressure and velocity sensors; measuring air speeds between blades of louvers, rather than downstream of louvers; smoothing out the airflow between the outdoor air louver and damper through proper louver selection and insertion of components to straighten air flow; and maintaining a pressure drop across the outdoor air damper that exceeds approximately 0.04 IWG (10 Pa).