Storage of wood pellets has resulted in deadly accidents in connection with off-gassing and self-heating. A forced ventilation system should be in place to sweep the off-gases and control the thermal conditions. In this study, multiple purging tests were conducted in a pilot scale silo to evaluate the effectiveness of a purging system and quantify the time and volume of the gas needed to sweep the off-gases. To identify the degree of mixing, residence time distribution of the tracer gas was also studied experimentally. Large deviations from plug flow suggested strong gas mixing for all superficial velocities. As the velocity increased, the system dispersion number became smaller, which indicated less degree of mixing with increased volume of the purging gas. One-dimensional modelling and numerical simulation of the off-gas concentration profile gave the best agreement with the measured gas concentration at the bottom and middle of the silo.