In this investigation, thermo-hydraulic performance of rectangular ducts roughened with a new configuration of 'V-down rib having gap' on one wide wall is determined. Small symmetrical gap equal to rib height is created at the centre of both legs of V of continuous V-down rib. The duct has aspect ratio (AR) of 12 and the Reynolds number (Re) ranged from 3000 to 15,000. To simulate the indoor testing of solar air heater, the roughened side of rectangular duct is heated with constant heat flux electric heater while the other sides are insulated. The roughness has relative roughness height of 0.043 and relative roughness pitch of 8. Five rib roughened plates having flow-attack-angle (alpha) from 30 degrees to 75 degrees have been tested. The thermo-hydraulic performance parameter based on equal pumping power (eta), friction factor (f) and Nusselt number (Nu), were found to be highest for alpha = 60 degrees. The results obtained at alpha = 60 degrees were compared with those of continuous V-down rib for same rib-roughness parameters. (C) 2011 Elsevier Ltd. All rights reserved.