Compliance with the 2016 Energy Saving Ordinance with the following heating

I decided on gas because, as can be read in the forum, heat pumps have higher investment costs.

This is not generally true and can be the opposite in individual cases. A gas condensing boiler is usually cheaper to purchase than a heat pump, but that is not the whole story. It depends on the overall setup (considering the legal framework, according to which gas is rather bad and electricity is rather good), which includes the gas connection and compensatory measures according to the Energy Saving Ordinance and the Renewable Energies Heat Act (often solar thermal).

I pay for my heating for a 150 sqm house, including central controlled residential ventilation with heat recovery 8500€ and underfloor heating 4700€ and brine-water heat pump including 120m drilling 21,500€ for everything according to the offer. I can still choose the system installer either Vaillant, Viessmann or Wolf. Which company would you choose for both?

Originally planned: Gas heating Underfloor heating Solar thermal Controlled residential ventilation + heat recovery Could the solar system be omitted? Or would the composition of the heating then no longer comply with the Energy Saving Ordinance 2016?

Has the question already been answered elsewhere because I am currently seriously considering whether to choose gas or geothermal energy.

Originally planned: Gas heating Underfloor heating Solar thermal Controlled residential ventilation + heat recovery

Could one do without the solar system? Or would the composition of the heating then no longer comply with the Energy Saving Ordinance 2016?

We planned it exactly like that (gas without solar, underfloor heating, controlled residential ventilation + heat recovery). However, we submitted the building application shortly before the turn of the year, so the old Energy Saving Ordinance still applies to us. I believe that in the currently valid version, gas without solar is no longer allowed.

The Energy Saving Ordinance does not prohibit gas as an energy source for heating nor does it require a solar system if gas is used. It is up to the builder and their assistants with which concept the Energy Saving Ordinance is fulfilled.

For gas, the currently valid Energy Saving Ordinance ST seems to be the "lesser evil" in many cases, if gas is chosen at all...