This paper discusses the fluid dynamics of an underfloor air distribution (UFAD) ventilation system in which, in contrast to conventional air-conditioning systems, cool air is delivered from diffusers in the floor rather than from overhead vents. In order to produce more realistic models of UFAD systems, we extend previous work on a simplified system consisting of a single heat source at floor level and a single cooling diffuser, to the case of multiple cooling diffusers and a single heat source located at different heights above the floor. We carry out experiments in which the heat source is represented as a buoyant plume, and the cooling diffusers are modelled using negatively buoyant vertical jets. The experiments show that the properties of the system are determined by the entrainment into the plumes and the negatively buoyant jets. We characterize these entrainment processes by entrainment coefficients, and develop a theoretical model based on layered models of ventilation flows. The model predictions are compared with the laboratory experiments, and used to determine the dependence of these parameters on the cooling load, the ventilation rate and the properties of the cooling diffusers.