Turn both radiator valves off. Undo the nut that holds the valves onto the rads. and collect the water that drains out. Open one valve at a time until no more water flows - you can then replace the ordinary valves with TRV types by undoing the pipe to valve nut but you'll need to take note of the pipe diameters - either 10 or 15mm usually. Your system will need repressurising afterwards, naturally. And that is that in a nutshell, as they say, but easier said than done! With a bit of luck you won't need to make pipework alterations, but you need to be prepared for this, in case.
John, January 2008
