The ventilation rate has equal importance as pollutant concentration when determining air pollutant emissions from barns at concentrated animal feeding operations (CAFOs), as the emission rate is the product of ventilation rate and pollutant concentration. This paper evaluated fan monitoring and performance verification technologies, and fan model development methodology for a large commercial layer farm emission measurement site, where there were two manure belt barns, each equipped with 88 exhaust fans that housed 250,000 layers. All the fans were individually and continuously monitored with fan rotational speed (FRS) sensors or vibration sensors. Differential static pressures (ΔAPs) across the walls of each barn were also continuously measured with pressure transducers. The Fan Assessment Numeration System (FANS) and fan airflow traverse method were applied periodically to assess the actual fan performance at barn conditions. Two groups of fan ventilation models, each with three speed regimes of high, medium and low FRS, were developed based on off-site laboratory fan chamber tests, on-site fan performance checks, and the first and second fan laws. The corresponding degradation factors of the fan models were also estimated. The mean ventilation rates of the two barns from Jan. 1 to Dec. 31, 2009 were calculated. Barn ventilations was influenced by indoor temperature variations, FRS, ΔPs conditions and fan performance degradation situations.