Given we have UFH embedded in a passive slab with a huge mass, standard thermostatic control is not a feasible control approach given the large lag.  A correctly configured weather compensation approach would be suitable (and this is what I use in the winter), but the Vaillant controller AFAIK doesn't use weather compensation for cooling.  Given Vaillant doesn't support external "call for cooling" to date I've simply been manually controlling cooling in an attempt to keep a decent temperature in