New construction! Help with heating system!

So, I have a gas condensing boiler + solar thermal system and underfloor heating throughout the house... if I were you, I wouldn’t listen to a heating engineer but rather consult an energy advisor...

As I already wrote, a water-based fireplace is very demanding in terms of control... so it’s not exactly cheap either... the few times a year it is fired up will never recover the additional investment costs. So you can safely do without it unless you get it for free...

The same applies to solar thermal... at least on the scale you describe... not in terms of control, but in terms of investment, but if you depend on [KfW] or something similar, then you have to have the 5 sqm installed on the roof.

And a mixed heat distribution system (underfloor heating and radiators) is probably the biggest money pit...

Imagine the following:

Your underfloor heating floats around at 35° supply temperature and heats the house continuously... now you come and turn on your radiators, but they need about twice the supply temperature to work effectively... meaning your heating system kicks in and runs the burner at full blast... how was it again with physics, how much energy is needed to heat water to twice the temperature?

So have everything explained by someone who knows how everything works and doesn’t just screw pipes together...

But my tip:

Radiators belong in old buildings... if you don’t overheat or undercool your house from the start, you hardly have to do anything and it regulates itself... then you don’t even need to quickly “turn up” a room — it’s warm or cold when necessary...

Hello,

..... und gesagt hat.....

The more components involved in heat generation, the lower the efficiency, the more prone to failures and complicated (competing demands)! Simply the best!

..... aber es muss ja auch dafür einen Weg/Möglichkeit geben!?...

Yes, first have a solid and exact basic assessment done => actual demand (power, energy) for heating and hot water.
Then the rest becomes clear!

..... Fußbodenheizung und restlichen Räume im OG (3 Schlafräume) Heizkörper damit wenn ich in den Schlafräumen mal kurzfristig es schnell warm haben möchte kurz Heizkörper aufgedreht und fertig!...

Underfloor heating is very slow, so heating ad libitum is usually ruled out. Here you heat continuously at a low level. At best, temporary lowering of supply temperature. Radiators, on the other hand, are ideal for occasional differentiated heating.

.....Ichbewusstsein nehmen soll ....

That will also not work like that.

....aber die Angaben im Internet bezüglich Verbrauch solch einer Anlage schwankt bei manchen angegebenen Werten von 2500 kWh verbrauch im Jahr und bei anderen um 6000 kWh im Jahr

There are also some with 1000 kWh/a. Such absolute comparisons are completely meaningless unless the actual energy demand is known!
You can cook with gas, with a heat pump you can cool, which can be especially interesting in lightweight construction.

best regards