This paper reports experimental data for forced convection heat transfer in a heat exchanger, which is a key part of the underfloor distribution system. The experiments were performed to study heat transfer and pressure drop characteristics of tested units with different geometry of flow ducts. The heat exchanger was made of Plexiglas and PVC. The top side of plastic exchanger was covered by a layer of a concrete screed with thickness of 0.035 m. The bottom side was insulated with 0.1 m of polystyrene. The length of the tested units was I m and the width 0.5 m. The fluid in the heat exchanger was air. The results of experimental study were presented in a dimensionless form. The average Nusselt number and Fanning friction factor were expressed in terms of the Reynolds number. The experimental results presented in this paper provide useful information for designing of an underfloor air distribution system in the residential housing sector. (c) 2005 Elsevier B.V. All rights reserved.