I have had to replace the motor on a 2-port sunvic type valve in my CH
system. I believe the reason for the motor failing is because the
valve is very stiff (cannot turn it by hand, I need to use pliers).

So a couple of questions: is it possible to get a valve stem
replacement, or is it necessary to buy the whole thing?
I'd like to avoid draining half my system to effect this
repair/replacement. Has anyone used those small freezing kits (as sold
by Screwfix for about 9 quid) on pretty big pipes (about 28mm)? Anyone
got any better suggestions?