A solar combisystem provides both solar space heating and cooling as well as hot water from a common array of solar thermal collectors, usually backed up by an auxiliary non-solar heat source. Solar combisystems may range in size from those installed in individual properties to those serving several in a block heating scheme. Those serving larger groups of properties district heating tend to be called central solar heating schemes. Many types of solar combisystems are produced - over 20 were identified in the first international survey, conducted as part of IEA SHC Task 14 in 1997. The systems on the market in a particular country may be more restricted, however, as different systems have tended to evolve in different countries. Prior to the 1990s such systems tended to be custom-built for each property. Since then commercialised packages have developed and are now generally used. Depending on the size of the combisystem installed, the annual space heating contribution can range from 10% to 60% or more in ultra-low energy Passivhaus-type buildings; even up to 100% where a large interseasonal thermal store or concentrating solar thermal heat is used. The remaining heat requirement is supplied by one or more auxiliary sources in order to maintain the heat supply once the solar heated water is exhausted. Such auxiliary heat sources may also use other renewable energy sources (when a geothermal heat pump is used, the combisystem is called geosolar) and, sometimes, rechargeable batteries. During 2001, around 50% of all the domestic solar collectors installed in Austria, Switzerland, Denmark, and Norway were to supply combisystems, while in Sweden it was greater. In Germany, where the total collector area installed (900,000 m2) was much larger than in the other countries, 25% was for combisystem installations. Combisystems have also been installed in Canada since the mid-1980s. Some combisystems can incorporate solar thermal cooling in summer. Classification Following the work of IEA SHC Task 26 (1998 to 2002), solar combisystems can be classified according to two main aspects; firstly by the heat (or cool) storage category (the way in which water is added to and drawn from the storage tank and its effect on stratification); secondly by the auxiliary heat (or cool) management category (the way in which non-solar-thermal auxiliary heaters or coolers can be integrated into the system). Maintaining stratification (the variation in water temperature from cooler at the foot of a tank to warmer at