Thermostatic radiator valves are often known by the initials "TRV".
A TRV works by sensing the surrounding air temperature. It progressively
closes down the flow through the radiator as the air temperature rises, thereby
cooling the radiator and regulating the room temperature.
They seem like a good idea in principle but the Great British Public finds
them particularly difficult to understand. Most people who have them don't seem
to recognise them as a heating control at all and keep them set to 'maximum'.
There seems to be no way around this. People seem to like their radiators HOT
when they are on, and TRVs just don't do this!
Another (technical) drawback is that they need the boiler to be on all the
time making hot water available for the TRV to draw as and when it is needed.
This rather defeats the fuel efficiency claimed for them in my opinion.
I have to say that, like combi boilers, they have their applications but I
just don't like them for general use. Sorry. I have them in my own house and
they are not very effective. We tend to have them all set to 'max' too. QED?!
The best application for TRVs is in a room which gets particularly warm.
Perhaps the radiator is too big, or it is south-facing and getting a lot of sun.
The reason doesn't matter. A TRV will generally keep it cool when the rest of
the house needs heating.