Thermostatic Valves (TRVs)
The thermostatic valve allows you to automatically regulate the flow of hot water into the radiator. The control inside the valve head will increase or decrease the flow of hot water to the radiator depending on the ambient room temperature.
This allows you to set the valve to stay open until the temperature you want to reach is achieved, at which point the valve closes. If the temperature then drops the valve will reopen.
A manual valve maintains the temperature of the radiator regardless of how warm the room is.
The control of the radiator temperature can be adjusted by hand by turning the valve either up or down.