Increasing the share of solar energy convertors installed on buildings asks for using all the suitably positioned places, including facades. However, facades integration asks for collectors with various colours and/or shapes to increase the architectural acceptance. Following these constraints, a new solar thermal flat plate collector was designed and developed, having a rather small area (0.67 m(2)) and trapeze shape; this shape allows a much higher coverage degree of the buildings' facades, in lego-type arrays; additionally, variously coloured absorber plates (red, green) support the architectural acceptance. A trapeze demonstrator collector was developed and proved an optical efficiency of 62.38% during indoor testing using a solar simulator. Further on, this demonstrator was outdoor installed on the vertical facade of a laboratory building, in the R&D Institute of the Transilvania University of Brasov, Romania (mountain temperate climate, 45.65 degrees N, 25.59 degrees E, at 600 m above the sea level). The paper presents the results obtained during two years of monitoring for the outdoor installed trapeze collector, in terms of thermal power output and efficiency. Data are compared with those obtained using a commercial flat plate collector (with 2.1 m(2) active area and optical efficiency of 85.1%) vertically installed on the same facade as the trapeze collector. These data are extensively discussed in the paper, considering the monthly cumulative output, the efficiency and the peak values (highest and lowest), in direct correlation with the irradiance values and outdoor temperature. It is found that the thermal energy output of vertically installed solar thermal collectors is larger during the autumn months as compared to the summer thus protecting the solar thermal system against overheating. The results show that during predominant cloudy days the infield conversion efficiency is recommended to be reported not based on instantaneous values but over longer time frames (e.g. of hours) due to an inherent time delay between the higher irradiance values and the collectors' response. (C) 2018 Elsevier Ltd. All rights reserved.