In an earlier contribution to this discussion,
wrote:

Hi,
The ball valve in our cold water header tank seems to be causing some
knocking and
generally seems to be due for a change (intermittent overflow for
example). In my local
B&Q there are two varieties of valve: high and low pressure. My
Collin's DIY manual gives
no guidance on this subject so I'm wondering if there is some decisive
factor in which to use.
The house is a normal two storey semi with a good mains pressure.
Thanks in advance for any advice,
Peter.

High pressure are for mains input - as your header tank will be. Low
pressure are for toilets etc. fed by gravity from header tanks rather than
being directly connected to the mains.

Many modern torbeck-type valves are suitable for either - and are supplied
with 2 or more flow restrictors. You fit the appropriate restrictor
depending on the supply pressure.
--
Cheers,
Roger
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