I have a radiator coupled into a central-heating system which seems to get hot...but less so than others in the system. The control valve on one end appears to rotate freely in either direction, but does not come to a 'stop'. Is it 'broken'? If a replacement is needed, would I have to drain the system?
They don't fail that often - most usually the plastic control knob breaks.
One valve will be the regular control valve that lets the hot water in and the other end will be what they call the lock shield valve that lets water out.
Take off the plastic lids/control knobs and open them both up with a pair of pliers (anticlockwise).
If a replacement valve is really needed you will have to drain the system so let's hope it's just the plastic knob that's broken.
See also Balancing Radiators here....
http://www.miketheboilerman.com/balancing.htm
Peccavi
February 2009
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.