If you are extending the system by adding an extra radiator you will need to drain the system and run two pipes to it - supply and return.
If you are just replacing a radiator new for old you may be able to just drain the old radiator and put the new one in joining on to the existing pipes and valves.
The key thing being the dimensions of the old and new rads. If the size difference is too great then you will have to drain the system and adjust the pipework to suit.
Peccavi, December 2008
