In an earlier contribution to this discussion,
4square wrote:

In an open-vented system, anyone know how much water is used, by
evaporation from the tank, if any? level in my tank dropping slightly,
maybe slight lk somewhere I suspect.

At what rate are you using water - in litres per week, or whatever?

Is there not a ballvalve which automatically tops up the tank?
--
Cheers,
Roger
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