A partly solar heated building area comprising 50 residential units has been built in Anneberg, Sweden. The system includes low-temperature space heating with seasonal ground storage of solar heat. Heating is supplied by 2400 m(2) solar collectors and individual electrical heaters for supplementary heating. The ground storage comprises about 60,000 m(3) of crystalline rock with 100 boreholes drilled to 65 m depth and fitted with double U-pipes. The collectors will have favourable working conditions but the store is rather small, the estimated heat loss from the beat store is about 40% of stored solar heat and the average solar fraction is estimated to 70% after 3-5 years of operation. An initial evaluation after 2 years of operation shows that, although problems have occurred and several parts seem to work less efficient than expected, the overall system idea works as intended. (C) 2007 Elsevier Ltd. All rights reserved.