Plumbing Questions? Ask a Plumber for Answers ASAP
Hi Noel . Yes you will have to drain down. If you had a boiler that used a header tank you could have bunged it to create a vacuum .Put some inhibitor in the top of the towel rail before filling back up.
What I normally do is turn the new rad valves off , fill up , check the pipework for leaks , then bleed the towel rail