Every planner and user is interested in an efficient all-season application of the solar collector system. The performance of low temperature collectors drops considerably with falling outside temperatures. In winter or with not optimal weather they can never achieve much more than a preheating.

If heat is needed at different levels at the same time, e. g. for heating and hot water supply, the simplest solution is to provide and storage it at the highest level i. e. with the highest thermodynamic value. This is impossible with low temperature collectors, what leads to higher planning efforts and much more complicated hydraulic systems.

The missing of the predicted energy harvest with most of the large-scale projects with low temperature collectors was always explained by higher return-flow temperatures than planned. In fact, the return-flow temperatures are never the problem but the collectors itself because they lose substantial performance under real working conditions.