A recycle operation design using a flat-plate solar water heater with rectangle flow conduits was theoretically and experimentally investigated. Devices with differing flow-conduit geometries (i.e. aspect ratio) and external recycle were designed to create a solar heater with low heat-transfer resistance between the absorber and working fluid to increase the convective heat-transfer coefficient. Considerable solar water heater collector efficiency improvement has been obtained employing rectangle flow conduits and a recycle operation, instead of recycle solar collector constructed with circular pipes operated at the same total mass flow rate. Under a fixed absorber area and distance between the flow conduits, the collector efficiency increases with increasing flow conduit aspect ratio, total mass flow rate and recycle ratio but with decreasing inlet water temperature. The incident solar radiation, mass flow rate, recycle ratio and flow conduit aspect ratio influences on the collector efficiency and energy consumption are also discussed. (c) 2010 Elsevier Ltd. All rights reserved.