Fans are key components of mechanically ventilated poultry housing. When installed, the fan is often fitted with 1 or more accessories, including safety guards, shutters, and discharge cones. These first 2 accessories usually reduce the airflow and fan efficiency, whereas discharge cones improve airflow. Field performance of a fan is further impacted by accumulated dirt on the blades and shutters, mechanical wear, and degree of maintenance. Performance of all ventilation fans on 2 commercial broiler farms was determined during an air emission monitoring project. Each fan was tested using the Fan Assessment Numeration System at a range of static pressures typical of its regular operating range. The performance of otherwise identical fans was shown to vary by up to 24%. This variation in performance is attributed to accumulated dirt and corrosion, differences in the resistance to flow imposed by the shutters, and differences in motor and bearing wear due to run time and aging. A small reduction in fan speed from slipping or worn belts had a large effect on airflow generated by the 1,220-mm (48 in.) diameter fans. The power consumption of each fan was also measured as part of the evaluation process and revealed considerable variation among these fans.