Buildings' facades usually have small sized and variously shaped opaque surfaces to integrate traditional (2 m(2), rectangular shaped) solar thermal collectors, thus resulting a reduced coverage factor (and thermal output) with rather low architectural acceptance. To tackle these issues, a novel type of flat plate, small sized (0.083 m(2)) solar thermal collector, with triangle shape was developed. Due to its rather low dimensions, the collector has no internal pipes and the water (thermal fluid) forced flow runs through a central body composed of an absorber plate and a cavity below it. This design rises specific issues to minimize the central body's deformation and optimize the flow distribution targeting a good thermal efficiency. The central body of the triangle collector was virtually prototyped using SolidWorks and transferred to ANSYS to identify the optimal solutions that mitigate the deformation and allow to evaluate the stagnation zones. Further on, the optimal thickness of the thermal insulation and of the air gap between the absorber plate and the glazing were evaluated using a radiative mathematical model. Based on the simulation results, three collectors (with black, green and orange absorber plates) were manufactured; efficiencies of 55%, 42% and 35% were obtained on the indoor testing rig. (C) 2019 Elsevier Ltd. All rights reserved.