This report describes performed investigations of four small low flow solar heating systems, with mantle tanks, for domestic hot water supply. The systems are of the following types:System: Type: 1. Solar collector: Batec A/S Storage tank: Nilan A/S (Installed by IBE)2. Solar collector: Batec A/S Storage tank: Nilan A/S (Installed by IBE)3. Solar collector: Batec A/S Storage tank: Nilan A/S (Installed by IBE)4. Nordsol ApS (Installed by the manufacturer)Further an independent investigation of the tanks in system 1, 2 and 3 has been made. The systems 1, 2 and 3 are identical with the exception of the tanks, which have the same volume, however. Tank 1 is high, slim (H/D=3.6) with a wide mantle. Tank 2 is high, slim (H/D=3.6) with a slim mantle, and tank 3 is somewhat lower and wider (H/D=2.0) with a slim mantle. The properties of the tanks were investigated at different steady and dynamic tests. The tests showed that the solar collector fluid is cooled best in a high slim tank with a wide mantle. Further, a theoretical model describing the convective heat transfer coefficient in the mantle has been developed. The model was validated with the measured results, and the calculated results agreed well with the measured results. The model was then implemented in an existing computer simulation program which is used for the calculation of annual thermal performance of low flow solar heating systems with mantle tanks. The solar heating systems are installed on the testing ground of Department of Buildings and Energy at a test facility especially intended for test of solar heating systems. The systems were installed in the summer of 1996 and they have been tested under similar test conditions so that an immediate comparison of the systems is possible. Measurements on the systems have been carried out for about 6 months. In addition to that calculations have been made of the annual thermal performance of the systems with the above-mentioned computer simulation program. The computer models of the systems have been verified from the measurements on the systems. For all the systems a good agreement was obtained between the measured and calculated results. The models have been used for calculating annual thermal performance for the systems with weather data from the Danish Test Reference Year TRY. The calculations imply a daily hot water consumption of 160 l heated from 10°C to 50°C. The calculations showed that system 1 has a thermal performance1 which is about 7% higher than the performance of system 2 and about 13% higher than that of system 3. These differences are mainly due to the differences of the tank design. Further, the calculations showed that the system from NordSol ApS has improved its thermal performance by more than 8% compared to its predecessor from Dansk Solvarme A/S. This improvement is due to the fact that the system is now a low flow system based on a mantle tank.