Cooling via underfloor heating with brine heat pump

It is rather to be understood as a placebo. A noticeable effect cannot be achieved (except cold feet) because the effect does not take place where it is needed to enable effective cooling.

The effect is not huge, but definitely present. I would use it again and again, because it brings the temperature down by 1 to 2 degrees (and 25 or 23 degrees in the bedroom can already make a big difference), practically without ongoing costs. You permanently draw heat from the large heat buffers of the house (screed, concrete ceiling, etc.). Not an unlimited amount at once, but 24 hours a day, thus ensuring that serious overheating cannot occur in the first place.

Of course, you cannot cool significantly against a huge unshaded south-facing facade, but it is a great addition that works excellently in combination with good shading and can keep a house cool.

do you have a proper BKA in the ceiling or are you just running cool temperatures through the underfloor heating? If the latter, how many degrees to avoid problems with the dew point?

The effect is not huge, but definitely present. I would always use it again, because it lowers the temperature by 1 to 2 degrees (and 25 or 23 degrees in the bedroom can make a big difference), practically without ongoing costs. You are permanently drawing heat from the house’s large thermal buffers (screed, concrete ceiling, etc.). Not arbitrarily much at once, but 24 hours a day, thus preventing excessive overheating from happening in the first place. You obviously can’t cool against a huge unshaded south-facing front, but it’s a great addition that works super well in combination with good shading and can keep a house cool.

Without costs? And the pump(s)?

For cooling via the underfloor heating, you don't necessarily need more pumps, right? And in summer, there should be enough photovoltaic yield to use it directly for the pump. So almost (without additional pipes in the ceiling as BKA or without extra air conditioning units).

It still is lost revenue and thus costs that you otherwise would not have.