On the other hand, in a thermosiphon installation, the hot water rises then it goes down again to be cooled.
In reality, hot water tends to rise because its density is lower and after cooled in heat exchangers (radiators), its density increases, which tends to bring it down.
Further:
I do not understand. How to keep the temperature high enough?
Due to the use of thermostatic valves, it is possible to dissipate only the necessary heat in the living rooms despite a high water temperature in the circuit, temperature maintained by a sufficiently bright fire (well, it's of course easier with an automatic boiler controlled by a probe ...), this to avoid the dew point.