On Wed, 22 Feb 2017 16:36:12 +0000, Roger Mills wrote:

Except that the OP wants to avoid any draining down - so that means that
the valves need to remain in their original positions.

If the system is open vented, block the feed and expansion pipes (you
can get tapered "bungs on strings" desigend for 15 and/or 22 mm
pipes). Shut both valves, drain the rad, you can then remove *ONE*
valve and very little water will escape as no air can get in. A
vertical up opening one won't glug but may have a little flow, A
horizontal or vertical opening down pipe might glug, and if one does
a fair bit of water may emerge...

In my previous post I had assumed that the OP wanted the temporary
replacement radiator in the middle of the gap left by the longer
original - probably because he mentioned pipes (in the plural) for
extending the reach of the valves.

Provided there is only ever one open to the air point in the system
at any one time you should be able to do one end, shut that, then do
the other.

In theory if you de-pressurise a sealed system the same should apply
but I've never had a pressurised system. I have succesfully used the
above technique on an open vented system though.

--
Cheers
Dave.