In an earlier contribution to this discussion,
Andrew L com wrote:

Set Square wrote:
And it's the larger tank which continues to fill, and overflows -
even with the water supply turned off?

Then I'm flummoxed!

The suggestion made by someone regarding some sort of cross-coupling
between the mains cold water system and low pressure hot water
system seems worthy of investigation.

Downstairs we have a gravity fed mixer shower. Water comes direct from
the hot water tank into one side, and from the mains feed into
another. The temperature control is separate from the on-off control.
Normally we keep the temparature near maximum hot. Overnight last
night as a test I set it to cold...and this morning the level in the
feeder tank hadn't risen a millimetre.

This looks like it could be my chief suspect although quite exactly
what is happening I'm not sure.

Thanks,

Andrew

It does indeed! It sounds as if somehow, even with the shower turned off
(with nothing coming out of the shower-head), high pressure cold water can
flow across to the hot water side - and push hot water back up into the
header tank.

I guess that the mixer valve wasn't designed to cope with unequal hot and
cold pressures - and does the mixing *upstream* of the on/off valve.
--
Cheers,
Set Square
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