A study on improvement in device performance of multi-pass solar air heaters with external recycle has been carried out under countercurrent-flow operations. An absorbing plate and insulation sheet were inserted horizontally and vertically, respectively, into a parallel-plate channel to divide the open duct into four subchannels. The coupled nonlinear ordinary differential equations were derived and solved numerically. The theoretical results show that the new device of four-pass flat-plate solar air heater with external recycle can enhance the heat transfer rate compared with that in an open conduit (without the vertical insulation sheet inserted and the. absorbing plate glued on the bottom insulation plate, the so-called a downward-type in gle pass solar air heater) under the same flow rate and working dimensions. The effects of inserting the absorbing plate and insulation sheet on the transfer efficiency enhancement as well as on the hydraulic dissipated power increment have been also discussed. (c) 2004 Elsevier Ltd. All rights reserved.