Are you talking about a vertical tube or column rad?
They will never heat the room as effectively as a convection radiator will unless they are seriously oversized. They heat the space differently.
Convection rads work 90% convection, 10% radiation, as such are the most effective at space heating. Their design actively encourages convection and channels cold air up up through the bottom through the finned space and exit hot out the top.
Column/tube rads work the opposite way, more like 80% radiation, 20% convection. They do create some convection, especially if they have more than one row of columns but not to any great degree as the air isn't 'channelled' as such and rely more on radiation.
This is the primary reason why column radiators of old were so large, they were seriously oversized to enable them to heat large spaces effectively.
I have had to change column radiators that have been installed back over to convection rads numerous times, especially in the older Glasgow tenements. They have large bay windowed 3.5m ceilings and the new 'designer' rads couldn't heat the space properly, some customers wont listen though.
The only time it ever really worked was a flat in the West End that had a large 5mx6m living room with a panelled domed ceiling 4m at it's peak, beautiful space but it took 8 x 700mm by 1400mm 10 column rads to heat it, even then it had a LDFE gas fire to augment the heat in winter and it certainly needed it.
They do look lovely but are really quite crap at heating larger spaces.
