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

Set Square wrote:
Is the water level in the small tank higher than that in the large
one? [The small tank is the fill & expansion tank for the
heating/hot water primary circuit]

Thanks Set Square,

No the water level (and overflow) of the small tank is below the water
level of the larger tank.

Cheers,

Andrew

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.
--
Cheers,
Set Square
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