the top) is important so that the combisystem can supply hot or cool water and space heating and cooling water at different temperatures. A solar combisystem may therefore be described as being of type B/DS, CS, etc. Within these types, systems may be configured in many different ways. For the individual house they may – or may not – have the storage tanks, controls and auxiliary heater and cooler integrated into a single prefabricated package. In contrast, there are also large centralised systems serving a number of properties. The simplest combisystems – the Type A – have no "controlled storage device". Instead they pump warm (or cool) water from the solar collectors through underfloor central heating pipes embedded in the concrete floor slab. The floor slab is thickened to provide thermal mass and so that the heat and cool from the pipes (at the bottom of the slab) is released during the evening. Combisystem design The size and complexity of combisystems, and the number of options available, mean that comparing design alternatives is not straightforward. Useful approximations of performance can be produced relatively easily, however accurate predictions remain difficult. Tools for designing solar combisystems are available, varying from manufacturer's guidelines to nomograms (such as the one developed for IEA SHC Task 26) to various computer simulation software of varying complexity and accuracy. Among the software and packages are CombiSun (released free by the Task 26 team, which can be used for basic system sizing) and the free SHWwin (Austria, in German ). Other commercial systems are available. Solar combisystems generally use underfloor heating and cooling . Concentrating solar thermal technology may be used to make the collectors as small as possible. Technologies Solar combisystems use similar technologies to those used for solar hot water and for regular central heating and underfloor heating, as well as those used in the auxiliary systems - microgeneration technologies or otherwise. The element unique to combisystems is the way that these technologies are combined, and the control systems used to integrate them, plus any stratifier technology that might be employed. Relationship to low energy building By the end of the 20th century solar hot water systems had been capable of meeting a significant portion of domestic hot water requirements in many climate zones. However it was only with the development of reliable low-energy building techniques in the last decades of the century that extending such