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Airflow measurement is essential to efficiently operate air handling units (AHUs) in commercial buildings. Due to the space and cost limitations for physical meters, many virtual airflow meters have been developed for induction motor driven fan systems. On the other hand, electronically commutated motor (ECM) fans have been widely applied in AHUs. However, the applicability of virtual airflow meters to ECM fan systems remains unclear. The purpose of this paper is to develop and validate a virtual airflow meter for ECM fan systems using measured fan head and motor input power and speed. First, a virtual airflow meter model is developed by referring to an ECM fan energy model and the corresponding calibration procedure is established, then the developed model is calibrated using the manufacturer data of a 7,865 m3/h ECM fan, including fan airflow rate, head and motor input power at different motor speeds, and validated in a test AHU with an array of four calibrated ECM fans. The validation result shows that the developed meter can accurately calculate the total AHU airflow rate with the root mean squared error of 660 m3/h and the normalized root mean squared error of 4%. [ABSTRACT FROM AUTHOR]