FWIW, we have all our UFH as a single zone, as we don't like keeping doors closed all the time.  Flow temperature on our system is typically about 26°C to 28°C.  UFH heat output averages about 6 W/m² of heated floor area, with a maximum in extremely cold weather of about 22 W/m², but that's for a well-insulated house that only needs a small amount of heating.   Changing the flow temperature increases the heat output per m², by raising the floor surface temperature.  For a room temperat