Diffuse ceiling ventilation system is an air distribution system based on the low-impulse supply of air through the perforated panels installed as the suspended ceiling. A ceiling with diffuse supply typically consists of active and passive panels, where active panels allow airflow. Diffuse ceiling ventilation system has been proven to have a higher cooling capacity compared to conventional air distribution systems, e.g. mixing or displacement ventilation systems. Several parameters have an impact on the cooling capacity of the system though. The present research study evaluates the cooling capacity of the diffuse ceiling ventilation system in connection to two essential parameters, i.e. the distribution of heat load in the room and the distribution of active diffuse panels in the ceiling. The evaluation is based on full-scale experiments performed in a laboratory-controlled environment as well as numerical studies with Computational Fluid Dynamics (CFD) simulations. The cooling capacity of the system in several scenarios with different heat load and active diffuse panel distribution are compared under a certain thermal comfort condition. The results indicate the highest cooling capacity in the scenario with evenly distributed heat load in the room and compact distribution of active diffuse panels in the ceiling, while the active panels cover almost one-third of the suspended ceiling. The system is at its minimum cooling capacity in the scenario with compact heat load distribution in the room and disperse distribution of active diffuse panels in the ceiling.