Radiators

Thermostatic valves regulate the temperature
as follows:
when the air temperature is higher than
the setpoint, the bulb dilates and closes the
valve (partially or fully). This reduces the flow
of hot water to the radiators, and hence the
amount of heat delivered to the room;
when the temperature falls below the
setpoint, the bulb contracts and opens the
valve. This increases the flow of hot water
to the radiators, and hence the amount of
heat delivered to the room;