Underfloor heating in the children's room? Some rooms planned without underfloor heating? Air-to-water heat pump removed?

But here I also want to bring science/medicine on board, which as far as I know says that it is significantly more advisable to have below 20°C in the bedroom. But to each their own

Because you mentioned ventilation, you have a ventilation system, right? Can't it also cool in summer when it gets warmer than XX °C in room XY? Of course, it then depends on the heating system, with an air-to-water heat pump you cool using electricity. But with a trench collector brine heat pump, cooling in summer is easier (in the ground significantly below the 30°C air temperature during the day), right?

hmm sure – and what about all the millions of people living in southern countries? Are they all living unhealthily because they have to sleep too warm?
Well, let's leave it at that.

Yes, we have a ventilation system, but without air conditioning attached. Maybe that will be retrofitted someday if our summers get even longer and hotter.
And as long as you don't have air conditioning, you can only cool a warm house with a heat exchanger if you can bring cool air in from outside. And even if it is cooler outside, the capacity isn’t enough – the system isn’t designed for that.

Whether it works with a geothermal heat pump, probably, but I haven’t dealt with it further.

With early planning, an air well can be installed.

That is why households in southern countries very often have air conditioners - in contrast to here in Germany, where the air conditioner still is rather rare. Depending on where and how long climate change steers, it may be that in 50-100 years - similar to the southern countries - we will also all have air conditioners in Germany. Yes, the trench collector brine heat pump should also be able to cool in summer with appropriate technology.

An air well can be installed with early planning.

Never heard of it before. Very good ecological and economical valuable tip in combination with a ventilation system

Brine heat pumps can cool; personally, I think it's a great feature. Of course, you shouldn't expect the effect of an air conditioner, but you can lower the house temperature by 2 to 3 degrees, which is very pleasant, especially with almost no ongoing costs.

The cooling effect through controlled residential ventilation (e.g., via [Lufbrunnen]) is significantly smaller to almost imperceptible. Physically, this makes sense because air is an incredibly poor medium for transferring heat/cold.

so the children's rooms have to be especially warm for us right now, definitely not below 22 degrees, otherwise they freeze. We have underfloor heating in the living room currently at 23 degrees, in the bedroom at 21 degrees. However, it is also possible to have only 19 degrees in the bedroom, but that is too cold for us. LG Sabine