Why does the amount of energy required for DHW depend on the supply temperature defined with HVAC_HOTWATER_ASX?

[gguidatiethz]

I ran a simple experiment with a single house that uses HVAC_HOTWATER_AS1 (High temperature water (60/10)) for delivering domestic hot water. I made two variants with HVAC_HOTWATER_AS2 (Medium temperature water (45/10)) and HVAC_HOTWATER_AS3 (Low temperature water + sanitation (35/10)). When I ran the building energy demand, I noticed that the energy demand for DHW decreased with the temperature of delivery. In fact, it was almost exactly proportional to the difference between suply and return temperature.

I could understand this if the equal amount of cold water was heated up to 60, 45 or 35 °C. But according to my modest knowledge of the practicalities of heating systems, that's not how this is done, isn't it? If my boiler delivers 60 °C and I want to wash my hands at 30 °C, a small amount of 60°C water is mixed with cold water. If my supply temperature is only 35 °C I take a larger stream of the hot water and mix it with less cold water. But the overall energy that is required is always the same.

Since I am new to CEA this may simply be a misunderstanding on my side. Any help is very welcome.

Thanks in advance and best regards, Gianfranco

PS: I use version v3.38.0