John Rumm wrote:
If the rads do not have TRVs then they will need replacing to come into
line with Part L of the building regs. You may also need to replace the
lockshield valves if when you convert to a sealed system since the
higher pressure can cause older ones to seep a little.

Or even the rads themselves, if they are as old and crabby as two of
ours were when we had a sealed system fitted a few years ago: both of
them 'blew' without warning a couple of days later; ie a hole appeared -
in the middle of a panel - and just about flooded out the house.

Actually it was particularly galling as after the first rad went, and we
replaced it, we decided that it would be sensible to replace the second
one urgently as well: however, before we had a chance, that one blew too
(on the very next day), with identical results (fortunately in the room
below the original, which was already totally trashed...)

David