a PI, many sensors and controls. the pi to do the things the pi always does. the sensors to sense room temp. in each room. And this is the hardest one, flow control per radiator.
I want that I can set room temp schedules, and can go off schedule using the app (manual intervention) When the current temp in any room is below the set temp for that room the heater system turns on The rooms that are already above their set temp have the radiators turned down.
Why? Because I don't like to waste heat to a room I don't enter 90% of the day. And when my living room has reached the target temp there is always a room that is either still stone cold or feels like sauna.
The hardest part about this is the controllable radiator valves, the rest already exists.