Ah, I see... Well, I must admit I am indeed interested in more eco-friendly options, but it has to be financially viable.
At the moment, I see additional costs of about €15,000 for geothermal compared to a gas boiler... I rule out air heat because, in my opinion, it is not a sensible technical solution. I am starting to doubt recovering the €15,000 over a reasonable period, even if the gas price increases (which I do expect).