You really need to drain the system for this.
You can save a bit of hassle by turning off all the other radiators at both ends, and closing one of the pump service valves. Then water off to the header tank, and drain the pipes from a convenient drain valve.
If you don't have a drain valve - fit a radiator valve with one on a downstairs run.
Use a sealant paste when assembling to avoid doing it all again.
If you drain the system into a clean bucket - you can fill the header tank again with it and not have to use inhibitor again.
You may be able to keep the lower olive and nut depending on the new fitting - but apply paste or tape to avoid leaks.
A really quick way is to use a freezing kit - but expencive.
Available from Screwfix.
Richard., June 2009
