ᐅ Control the climate using underfloor heating or through the ventilation system?

This summer, it was quite warm inside my house despite having external shading on the windows, but it was still manageable for me. However, I can imagine that in 20 years, when I am older, it might be different. Otherwise, split units, like those used in split air-source heat pumps, consist of a heat exchanger with a fan indoors, and a box containing the compressor and a fan outdoors, connected by cables and pipes carrying the heat transfer fluid.

Don’t forget the condensate drain connection – this is unfortunately the biggest challenge when retrofitting.

Is it possible to direct this outside? And then let it drip with a few centimeters (inches) of clearance, similar to decentralized ventilation systems (if the position allows).

You can do that – if you don't care about airtightness and want to create a nice thermal bridge.

Unfortunately, this is often how retrofitting is done.

With a small trap, it becomes airtight, and in winter you can put a plug on it. Airtight and insulated with plastic/air is reasonably effective; you will have a significant thermal bridge anyway where it passes through, especially when retrofitting. In my opinion, this might be better than spending several thousand euros on installing pipes inside the house (which is a hassle once the house is finished). Thanks for the information about this solution.

I don’t have a thermal bridge because it is constructed from the cold attic space.

Where do you want to install the siphon? Outside on the wall? And no one will put a plug on it during winter, probably.

But of course, it definitely won’t be a major issue – it just needs to be considered.