The interaction between a fluidized bed and the air-feed system (pipes, valves, fan and air plenum) was studied in a 8.5 x 0.7 x 0.12 m cold circulating fluidized bed unit operated with Group B solids. The study comprises measurements of pressure fluctuations in the bed and in the air plenum as well as of the airflow into the bed, The configuration of the air-feed system was changed by means of valves at two locations in the system. The flow and pressure fluctuations in the system were modeled by solving the continuity and momentum equations of the gas flow. The measurements and the simulations show considerable fluctuations in the inflow to the bed. Furthermore, it was shown that the interaction between the fluidized bed and the air-feed system depends on the configuration, such as length of pipes, characteristics of the fluidizing fan and location of control valves. The modeled flow and pressure fluctuations in the air-feed system agree with the experiments. The results have important implications on the understanding of the dynamics of the bubble flow and on the choice of boundary inlet-conditions for numerical modeling of fluidized beds.