Techniques for airflow measurements to determine the real efficiency of heat recovery in ventilation systems

Heat recovery in ventilation is essential to reduce energy use and thus mitigate greenhouse gas emissions from the building sector. Heat recovery efficiency of at least 80 % in new buildings is required according to Norwegian standards. However, measurements show that the real heat recovery efficiency during operation is commonly 10-20 % lower. Measuring heat recovery efficiency in buildings is challenging, mainly due to difficulties measuring airflow rates close to the air handling unit (AHU). This study assesses the following duct airflow measurement techniques and equipment: pressure differential, velocity traversal technique, ultrasound and tracer gas. The pressure differential method can provide accurate flow rates and thus it is used as the reference measurement. However, it is not suitable for duct flow measurements due to its high pressure penalty and long straight duct requirement. Velocity traverse and tracer gas methods introduce less disturbance to the flow. Nevertheless, both methods require intensive labour work and cannot track quick changes of the airflow with time. The application of ultrasound to measure airflow is relatively novel and it can automatically measure constant and fluctuating airflows with low pressure drop and acceptable accuracy when the proper installation and minimum straight duct are provided.