How do household radiator thermostatic valves work
I know it seems crazy but those thermostatic valves actually do detect room ambient temperature and increase or reduce the flow of water through the radiator based on that temperature.  It does not care about the temperature of the water flowing through the lines.  If you set it to '5' and the room is cool, it will allow a lot of the water circulating through the system into that radiator.  As the room warms, the valve starts to restrict the amount of water into the radiator.  When the room hits a predetermined level, the valve will shut all the water off flowing to the radiator.