Rational use of energy in buildings leads to a concept of active energy facades such as transparently insulated massive walls, solar thermal or PV facades, advanced glazings for daylighting purposes or double ventilated facades. The paper is concerned with the facade-integrated solar thermal collectors concept for water heating in the existing building stock in the Czech Republic (panel and brick blocks of flats), which are ready for major renovations. Thermal behavior of facade collectors compared with standard roof-located collectors in solar DHW systems was investigated. Facade solar collectors should have an area increased by approximately 30% to achieve the usual 60% solar fraction compared with conventional roof solar collectors with a 45° slope. Further increases in the solar fraction above 70% lead to a required area comparable with roof collectors but with less stagnation periods compared with roof collectors. Application of facade solar collectors affects the indoor comfort in buildings in a reasonable range. Indoor temperatures increase by no more than 1 K in all investigated configurations. Building behavior is not strongly affected by facade collectors when sufficient insulation layers are present.