In this study, flat plate, finned and v-corrugated air heaters were investigated both experimentally and theoretically in an effort to improve the performance of conventional air heaters. Collectors were also tested in double pass mode to investigate the extent of improvement in efficiency that could be achieved without increasing the collector size or cost. A series of experiments were conducted, based on the ASHRAE standard, under climatic conditions of Singapore. The performance of all three collectors was examined over a wide range of operating and design conditions. The v-corrugated collector was found to be the most efficient collector and the flat plate collector to be the least efficient. Results showed that the v-corrugated collector is 10-15 and 5-11% more efficient in single pass and double pass modes, respectively, compared to flat plate collectors. The double pass operation of the collector improved the efficiency of all three collectors. The improvement in efficiency for double pass mode was most significant in the flat plate collector and least in the v-groove collectors. (C) 2005 Elsevier Ltd. All rights reserved.