Radiator valves work by controlling the amount of hot water and steam that flow through the pipes of a hot water central heating system to each radiator in the system. They do this by means of plugs which stop the hot water and steam from flowing when the preferred temperature is reached.
Each radiator valve is fixed to a mechanical actuator, which is a plastic case that contains a wax plug attached to a metal pin. Controlled by the settings on the radiator, the valve reacts to the temperature in the room. When the preferred temperature is reached, the actuator wax plug closes the connection between the central boiler and the valve. When the wax plug closes, the hot water and steam are unable to flow through the valve, so no more heat is generated in the radiator.
Radiators are typically placed in each room of the house that requires a different temperature. Each unit has separate settings to control how much steam is allowed to flow into that particular unit to heat the room. High settings allow more steam into each unit, creating warmer temperatures. Low settings allow the room to become cooler and are more energy efficient. The temperature of the water coming from the boiler into the system as a whole is controlled by a separate thermostat.