Your radiators may be fitted with a Thermostatic Radiator Valve. These work by sensing the air temperature in the room and adjusting the heat accordingly.

A TRV is different to a room thermostat. A room thermostat determines the maximum temperature for the house and the TRV lets you vary this temperature room by room, meaning you don't have to waste money warming rooms you seldom use. The number on the valve corresponds to a specific air temperature.  Once you have selected a number, the thermostatic valve will maintain this temperature

Some things to remember when using TRVs:

If you turn a TRV down to low in a room, keep the door to that room closed. This will reduce condensation and prevent the radiators in other rooms from working overtime trying to heat the air in there too.

The radiator will not be hot all the time. The purpose of the valve is to allow you to control the temperature in individual rooms. It turns the hot water flow off when the room reaches the set temperature. Before you ring to report a radiator that is not working, check that the TRV isn’t set too low.