Radiator valves control the flow of water through your radiators. They are situated at the bottom, where the pipe feeds into the unit.
There are three different types of valve:
Using air temperature sensors, Thermostatic radiator valves (TRVs) automatically regulate the hot water flow. You can adjust the temperature through a series of settings from 1 (the coolest at around 10°C) to 5 (the hottest at around 23°C), or switch them off using the * setting, which will stop the radiator from heating up, but allow enough warmth to ensure the pipes can’t freeze.
A more basic version of the modern TRV, manual valves are generally found on older heating systems. Rather than being self-regulating, they operate on an open/closed basis, either allowing hot water to flow through and heat the radiator, or stopping the flow to allow the radiator to cool.
Located at the foot of your radiators, on the opposite side to the TRV or manual valve, lockshield valves are covered by a removable plastic cap and require a spanner (or similar tool) to open (turn anti-clockwise) or close them (turn clockwise). They rarely play a part in controlling heating systems anymore, due to the introduction of TRVs.