I Need to change old radiator valves to modern thermostatic / temprature controlled valves. is it as simple as buying the type required and swapping the 'tops' or does the job entail draining down etc

Dave, January 2010

if you are very careful you can fit the TMV without having to drain down the entire system.
The back nut on the existing valve must be exactly the same size/type as the one thats on on the TMV to do this.
The system water needs to be cool so it can't be done with the heating up and working

garryk, January 2010

You will have to drain down and change the valve complete,you can't put a new hat on an old head.

Rich, January 2010