Amortization: The system utilization rate of a solar domestic hot water system is undoubtedly higher than that of a heating-supported system. Heating-supported systems are designed for the transition period. In summer, these systems inevitably go into stagnation (there are also numerous publications on this, so if you need a source, you can find it on the pages of the relevant institutes – especially from ISE, ISFH or SPF). From experience, I would now say that one can expect a specific collector yield of just under 350 kWh m[SUP]2[/SUP] a[SUP]-1[/SUP] for a well-designed heating-supported system and 450 kWh m[SUP]2[/SUP] a[SUP]-1[/SUP] for a well-designed domestic hot water system. Of course, anyone interested in solar thermal energy should recalculate their specific case (or have it recalculated). However, the fact that a “smaller system” makes more monetary sense is – in my opinion – not the point of your question. Return temperatures: There are again numerous studies on the influence of return temperatures on the solar coverage ratio or system utilization rate (especially in the area of solar-supported local heat networks) (Szablinski, 2004). However, there are certainly a few tricks to reduce return temperatures in conventional radiators, but miracles cannot be expected. As a general rule, high return temperatures reduce the efficiency of solar systems. To really tackle this precisely, all details need to be on the table. I think that is the intention of €uro. He is right. I would still recommend a domestic hot water system.