In an earlier contribution to this discussion,
fred wrote:

If installing a combi,
this uses mains pressure.
older radiator valves where not designed for this, expect leaks!

Well, there *some* truth buried in there somewhere - but you're mixing up
two things.

A combi uses mains pressure for domestic hot water - but that doesn't go
anywhere near the radiators.

Most combis use unvented (pressurised) systems - and thus run the heating
circuit at a higher pressure than vented systems - but still not usually as
high as mains pressure.
--
Cheers,
Roger
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