in my house the kitchen tap and a few others
come direct from the water mains,
others come from a big water tank,
and the hot water comes from a third tank.

i suggest jim experiments with various stop taps,
perhaps wiring a float up and off,
to find where the shower water comes from...

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[george]

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shower and cold taps in room not actually fed from said tank, but from
mains which is reduced pressure as is dug up down road due to a leak?

the setup you describe would enable a night-time glass of water to be
somewhat "unfresh" having been stored in roof for ? days...

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