Advantages/Disadvantages: Gas + solar + water-heated fireplace for domestic hot water?

ok, understood, you have to have everything calculated by someone who understands it. Can any architect or energy consultant do that or are there "trademarks" that show me I have an "expert" in front of me?

It should be a sales-independent building services engineer familiar with small units, who is also an energy consultant. Architects usually do not meet these requirements due to lack of training in building services engineering! The same applies to run-of-the-mill energy consultants, as this is not a legally protected professional title.

best regards

Great, many thanks for the information. I will report back.