I have replaced an existing radiator (fitting a new Danfoss TRV in place of the standard valves). I drained down the whole system and now have a system where all the rads function except for the new one! There is water in the rad - bleed valve emits water not air - but nither feed, nor return pipes show any sign of heating up, not even evidence of conduction from the nearest upstairs rad. I don't believe it!