Theoretical and experimental investigations have elucidated how different changes in the design of the 12.5 m(2) HT flat-plate solar collector from the Danish company ARCON Solvarme A/S influence the solar collector efficiency and the yearly thermal performance. The collector is designed for medium and large solar heating systems. Based on the theoretical findings a prototype of an improved HT solar collector was built and tested side-by-side with the original HT solar collector. The improved HT collector makes use of a changed insulation material, an absorber with improved absorptance and emittance, and a changed antireflection treated glass cover. Calculations based on the measured efficiencies showed that the yearly thermal performance is increased by 23-37% at operation temperatures between 40degreesC and 80degreesC when using the improved HT collector. The cost of the collector was however only increased with about 5%. (C) 2004 Elsevier B.V. All rights reserved.