Arthur2 presented the following explanation :
Hi all.
Sorry. Sort of a re-post here.
If there are typical characteristics that a
bar shower thermostat requires replacing,
what are they ?

Not sure you mean by 'bar' shower?

For a basic shower....
Water flows and is heated by the element, water temperature is governed
by the rate of water flowing - the faster the flow, the colder the
output. The switching on of the element is by a flow pressure switch,
no flow and this turns itself off. It addition there is a temperature
switch, which turns of the element if the water temperature rises too
high - which could occur if the water outlet is obstructed.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk