As a large part of the education year occurs during the cold weather season, the proportionally largeglass areas permit excessive heat loss in the classrooms. The problem with large glass areas arisesfrom the lack of control of mechanically generated heating systems. A solution for the improvement of these classrooms, is to use solar energy in the design, which is economical and practical. In thisstudy, a school in Istanbul, which has existing large classroom windows and has a solar energycollector behind the south classroom window is introduced and measurements are evaluated.