Danke :o)
Thank your for the answer
Actually, I think it is corrosion that is the issue – hence Atmos also recommends the kettle return temperature to be above 65 Celsius.
That is why I have selected the Laddomat pump should start at 75 (S37) to ensure the return and kettle temperature reaches the recommended values as fast as possible.
My pipes are well insulated.
I have also a question about flue gas temperature – but I will post that in a new thread – and attempt to do it in German … be aware …:o) I don’t quite trust Google translate … 
