Artificial roughness has been found to enhance the heat transfer from the collector plate to the air in a solar air heater. However, it would result in increase in frictional losses and hence, power required by fan or blower. This paper presents the results of an experimental investigation of thermohydraulic performance of roughened solar air heaters with metal rib grits. The range of variation of system and operating parameters is investigated within the limits of, e/D(h): 0.035-0.044, p/e: 15-17.5 and 1/s as 1.72, against variation of Reynolds number, Re: 3600-17000. The study shows substantial enhancement in thermal efficiency (10-35%), over solar air heater with smooth collector plate. The thermal efficiency enhancement is also accompanied by a considerable increase in the pumping power requirement due to the increase in the friction factor (80-250%). The optimum design and operating conditions have been determined on the basis of thermohydraulic considerations. It has been found that, the systems operating in a specified range of Reynolds number show better thermohydraulic performance depending upon the insolation. A relationship between the system and operating parameters that combine to yield optimum performance has been developed. Crown copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.