On Thu, 06 Jun 2019 16:45:21 GMT, DerbyBorn wrote:

Why drain twice? If your radiators have valves on both ends
already
just close those and leave them attached to the pipe ends.

What I was thinking - even if the valves need moving when fitting the
new radiator - it is only one drain down.

May not even have to do that with an open vented system. When it
comes to the replumbing for the new rad block the feed and expansion
pipes at the header tank and ensure there is only ever *ONE* opening
into the system.

Air can't get in. water can't get out, at least for vertical pipes
open at the top. There might be some spillage so be prepared...
Horizontal may well "glug" and drain out and then any vertical pipes
leading up from that horizontal, especially with the vibration and
banging about cutting/making joints etc.

You can certainly change bottom fed radiator valves this way. The
absolute key is ensuring that there is only *ONE* opening into the
system at any time.

--
Cheers
Dave.