Cooling with heat pump via underfloor heating?

The limiting factor is the dew point.

That is exactly why the effect is even less the more humid the air is, therefore: An air conditioner is not only about cooling but also about dehumidifying. This should not be overlooked.

Exactly for that reason, the effect is all the less the more humid the air is, therefore: an air conditioner is not just about cooling but also about dehumidifying. That should not be overlooked.

Absolutely correct.

As I said, it is important to me to distinguish between opinions and real EXPERIENCES, since I have read the argument "humidity stays the same anyway" several times in this thread.
Concrete experience in my house:
- without cooling function: approx. 28 degrees at 60 percent relative humidity
- with air-to-water heat pump cooling function: approx. 24 degrees at 72 percent relative humidity
The latter feels (despite higher relative humidity) SIGNIFICANTLY more pleasant.

Experience in my parents' house:
With air conditioning it goes down to 24 degrees or less at 30 percent humidity. Clearly fresher, but in the long run (for some people) also more unpleasant due to very dry air, keyword "I get sick from air conditioning."

If both cost the same, I would choose the air conditioner. Cooling 200 sqm with air conditioning costs, however, in my case, five times as much as the heat pump cooling function.

So, my experiences with both (air conditioning + air-to-water heat pump cooling) are on the table. What is better individually is now up to everyone to decide.

In my opinion, the crucial answer is still missing here:

THERMAL CONVECTION

As is well known, cold air wants to go down and warm air rises.
No one would come up with the idea to install radiators on the ceiling.

Using cooling surfaces at the bottom therefore brings little benefit, because you want to cool down the warm room air above.
Instead, you only get cold feet and keep a hot head.
For this, you would have to operate at least a solid fan.

Therefore, the answer to the original question is:
yes, but it works more poorly than well.

The poor cooling efficiency could still be tolerated if this
cooling is only used a few days per year.

But this solution does not make you happy.

No one would think of mounting radiators on the ceiling.

Not radiators, but large-area heating and cooling ceiling systems are, for example, almost standard in high-end office construction. The whole thing then works almost exclusively via radiation and not convection. Therefore, I don't find "No one" quite correct! I agree with you about cooling. On the ceiling, of course, is ideal.

below that, the pipes (which must be significantly cooler than the target temperature) start to sweat.

Which pipes are you talking about here? The 50 cm per heating circuit in the HKV? Others are not exposed. :confused: My cold water pipe in the heating room sweats when I fill our "paddling pool" over many minutes...

Which pipes are you talking about here? The 50 cm per heating circuit in the HKV? Others are not exposed. :confused: The cold water pipe in the heating room sweats for me when I fill our "paddling pool" for many minutes...

A dew point sensor is installed on the supply line between the heat pump and the HKV. It measures the relative humidity directly on the pipe and disables the cooling function if a defined threshold is exceeded. For me, the value is set to 80% because mold can occur from this relative humidity onward. You can also set the value higher, since the temperature of the floor will always be higher than that of the supply line—and thus high humidity on the pipe does not necessarily mean that there is also moisture on the floor. Since the effect is more than sufficient for me—as explained above—I leave it at 80%.