In reply to gethin_allen:
> (In reply to Hugh J)
> Could it just be a balancing issue? If you turn off the new rad you recently replaced does the problem rad get hot?
> If so just close off the lock shield valve (the one on the other end of the rad from the thermo valve) on the new rad just enough to get the problem one hot.
I agree with that, I tweaked all the hottest radiators upstream of the cold one that I had ,shutting the loch Shield valve a touch restricting the flow, and reducing the temperture of the over hot ones. The water started to flow through the cold one, but do it gradually and leave it for a while to settle, don't make big changes and expect immediate results.