Cooling with heat pump via underfloor heating?

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%.

Radiators not, but large-area heating and cooling ceiling systems are, for example, almost standard in high-quality office construction. The whole thing then almost only works through radiation and not through convection. Therefore, I find "Nobody" not quite right!

Maybe the technology is more advanced than a few years ago. But I once had such an office and it ended with the ladies sitting under their desks in winter with an electric fan heater; the gentlemen were rather too proud ;-) In short: it didn’t work at all. The architectural firm was also sued because of these and other planning errors but then went bankrupt....

80% RH is already clearly too high. Mold can occur from 60%, and from 70% mold is to be expected on most substrates. A single sensor also brings little, as different values can occur in the rooms. Air conditioners cause more trouble than benefit if you want to maintain a seasonally sliding RH corridor of 50-70%. Even the expensive ones.

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 blocks the cooling function if a definable threshold is exceeded. For me, the value is set to 80% since mold growth can occur from this relative humidity.
You can also set the value higher, as the temperature of the floor will always be higher than that of the supply line - and thus high moisture on the pipe does not necessarily mean that there is also moisture on the floor. Since the effect is - as explained above - more than sufficient for me, I leave it at 80%.

Perhaps technology is more advanced than a few years ago. .... sitting in winter with an electric fan heater under the desk, .... In short: It didn’t work at all.

perhaps technology has advanced, but the physics has remained the same:

1. there is no such thing as "cold radiation." If you feel that a spot in the room feels colder, it is at most the lack of heat reflection. But this would come from below here, which does not help but only causes cold feet.

2. a large-area heat source on the ceiling (in principle inversion) would have practically no radiant heat effect at the typically low supply temperatures of heat pumps. For this, the supply temperature would have to be well above 60°. That is why almost no one does it this way.

therefore, the top design principle is always: never fight against thermodynamics, but always support it. (which is why in traditional house construction radiators are always placed under the windowsills)