Gas boiler without solar - from when is that allowed?

One should understand the principle; it doesn’t hurt to google and read the [Energieeinsparverordnung] once. It mandates the use of renewable energy in new buildings. It can be, but doesn’t have to be solar. Heat recovery via heat exchangers would also be a possibility. But the more renewable energy is used, the sloppier I can insulate. Gas as a fossil energy source that produces CO2 without absorbing any is, after all, penalized under the [Energieeinsparverordnung]. Pellets, for example, also emit CO2 into the air, but since it is wood, it absorbed CO2 while growing. Pellets really fare better. Whether that makes sense is another question. That’s just how it is. I still wanted gas, for various reasons. So I had to compensate with solar and good windows and a good wall structure. By the way, this solar system was the sole hot water provider until early November and can be again from mid-March. It’s not that bad. What would be possible with gas instead of solar? As I said, controlled residential ventilation plus super insulation might also work. Gas plus PI system. More expensive and nonsense. You can’t get around having an engineer calculate it. But only once your building concept is finalized. Then you’ll see if it’s feasible or not. An experienced engineer can tell at a glance whether it might work or is hopeless. Tips to enable it during planning. Don’t glaze too generously, especially not on the north side. Karsten