Solar heating systems which provide both domestic hot water and space heating, so called solar combisystems or SDHW&H systems, are increasing their market share in several countries such as Austria, Germany, Denmark, the Netherlands and Switzerland. In some countries, such as Sweden, they have been the dominant solar system type for a long time.

Much is already known about solar domestic hot water systems, but solar combisystems are more complex and have interactions with extra subsystems. These interactions profoundly affect the overall performance of the solar part of the system. The general complexity of solar combisystems has led to the development of a large number of widely differing system designs, many only very recently introduced onto the market. After the first period of combisystems (1975-1985), where design of nonstandard and complex systems by engineers was the rule, a new period has been opened since 1990. Now the design is done essentially by solar companies trying to sell simpler and cheaper systems. But current designs are based mainly on field experiences and they have not yet been carefully optimized. Experts believe that there is a great potential for cost reduction, performance improvement and increase in reliability, and that this needs to be scientifically addressed.

Through international cooperation experts can analyze and review more designs and ideas than one country alone could cover. Collaborative work in analyzing and optimizing combisystems is therefore a proactive action that can favour good systems on a more global market than a national market.

Task 26 is a major research projects of the IEA Solar Heating and Cooling Programme. The Task involves 25 experts from 9 IEA member countries and 11 solar industries.