Cooling with heat pump via underfloor heating?

We have both, an air-to-water heat pump for heating and an air conditioning system. The climate specialist also explained to us during the briefing that it is basically a heat pump, like the heating system. But with 3 split units mounted on the walls that exchange air in the room, and not through piping in the floor. We chose air conditioning and against the special cooling function of the air-to-water heat pump. I have heard from most sources that on really hot days, when you want to cool, it is not enough to lower the temperature by 2-3°C as is possible with an air-to-water heat pump/floor pipes. It was important to us that especially on days above 35°C we have a functioning cooling system, and if it is 37°C outside, I don't want it to be 34°C inside the house.

Absolute priority number 1 should be to design a house in such a way that overheating does not occur in the first place. It's crazy to knowingly stumble into this problem and then install an air conditioner. If a heat pump can "only" cool 2-3° and that is not sufficient, in my view this represents a serious planning error.

You mean the popular large window fronts, which are then darkened most of the time with [Rollläden], [Raffstores] or similar?

I'd rather use an air conditioner and leave the light and view as they are intended.

If a heat pump can "only" cool 2-3° and that is not enough, in my view there is a serious planning error.

Objection: If I look back at the development of recent years, the planning error lies in building in Germany. In Sweden, for example, summers are not that hot ;-) Eventually, the heat gets into every house – planning or not. And then you are stuck, because not only the cold is kept outside, but also the heat inside. Then only an air conditioner helps.

Since the attic sometimes reaches sauna-like temperatures in summer, the COP drops to rock bottom and the system unnecessarily consumes more electricity than needed. So if you want an efficient system, it must be placed outside or the heat exchanger must be freely ventilated with outside air in the attic.

Yes, sounds plausible. Since we still plan to renovate our facade, it might be sensible to have empty conduits installed, to at least keep the option open later. The bedrooms are on the upper floor. The final solution is not yet decided. How big should the empty conduits be? It could also be a ventilation system installed in the attic to help in summer. Should I create a new thread for this?

The only right decision. With today's (intentional) construction methods without relying on special gimmicks.

Absolute priority number 1 should be to design a house in such a way that overheating doesn't occur in the first place. It's completely crazy to knowingly stumble into this problem only to then install an air conditioner.

Not everyone wants to live in a bunker. The focus of all the regulations (Energy Saving Ordinance and co.) is on the winter months in order to save heating energy and avoid that oh-so-evil CO2. In summer, however, the effect does not reverse (how should it), so the heat gains just remain inside.

If a heat pump can only cool 2-3° and that's not enough, in my view, there is a gross planning error.

No, there is no planning error here. This is what is possible without additional measures and only with the advertised underfloor heating cooling, no matter what the general contractor/developer/broker/manufacturer says. Purely from a physics point of view.

How big should the empty conduits probably be?

DN50 is sufficient per indoor unit. But you also have to think about the condensate drain. So if you are already planning, you need to provide a little more than just empty conduits.